Article

Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit

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Abstract

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.

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... In nature, most living species rely evidently on condensed atmospheric vapor. Moreover, some plants and animals get their share of fresh water by evolutionary modified surfaces that enhance the condensation process, such as the Darkling beetles [1], and Sequoia Sempervirens [2]. Utilizing the phenomenon in numerous applications has been a course of scientific curiosity for a very long time, dating back to Aristotle (300 BC). ...
... In order to take care of turbulence, standard k − ω model was implemented. Adding perturbed state variables to equations (1)(2)(3)(4), yields the extra term of Reynolds stress (ρu i u j ). ...
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Directing a jet of humid air to impinge on a surface that is cooled below the dew point results in micro-sized water droplets. Lord Rayleigh discussed the phenomenon called such behavior Breath Figures (BF). Historically, utilizing dew as a water source was investigated by several scientists dating back to Aristotle. However, due to the degrading effects of air as a non-condensable gas (NCG) such efforts are limited to small scale water production systems. Recently, the concept of BF has been utilized extensively in the generation of micro-scale polymer patterns as a self-assembly process. However, the generation of BF on surfaces while being impinged by a humid air jet has not been quantified. In this work, we illustrate that a BF spot generated on a cooled surface is a manifestation of a recovery concentration. The concept is analogous to the concept of adiabatic-wall temperature defined for heat transfer applications. Upon closer examination of the vapor concentration distribution on a cooled impinged surface, we found that the distribution exhibits distinct regimes depending on the radial location from the center of the impingement region. The first regime is confined within the impingement region, whereas the second regime lies beyond this radial location including the wall jet region. Scaling analysis as well as numerical solution of the former regime shows that the maximum concentration on the surface is equivalent to its counterpart of a free unbounded jet with similar geometrical conditions. Additionally, the scaling analysis of the latter regime reveals that the jet speed and standoff distance are not important in determining the recovery concentration. However, the recovery concentration is found to vary monotonically with the radial location. Our conclusions are of great importance in optimizing jet impingement where condensation phase change is prevalent.
... Transpiration may decrease due to a lower vapour pressure deficit. In Abies fraseri, photosynthetic activity and water relations were strongly related with cloud conditions (Reinhardt & Smith, 2008) and fog interception resulted in better leaf water potential and photosynthesis for Sequoia sempervirens (Simonin, Santiago, & Dawson, 2009). ...
... Plants were allowed to dry, as in Boucher et al. 1995, to avoid an indirect effect of the canopy wetting such as reduced transpiration. An increase in gas exchange was also to be expected after leaf drying (Gouvra & Grammatikopoulos, 2003;Simonin et al., 2009). ...
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Foliar absorption is a known water acquisition mechanism in many species and ecosystems. Experiments in the field showed that mature black spruce [Picea mariana (Mill.) BSP] can surprisingly sustain periods of summer drought. A possible explanation for this phenomenon is that this species is able to rehydrate via its needles. In this study, we explored if black spruce saplings are able to absorb water via the needles or to increase their water potential and photosynthesis after needle wetting. Forty saplings were used, of which half were excluded from irrigation until water potential of -2.70 MPa. For the first part of the experiment, the saplings were sprayed at night with a colorant solution and water potential was measured the following day. No colorant was absorbed by the saplings and no difference in water potential was found between irrigated and non-irrigated individuals. The experiment was then repeated, spraying saplings with normal water and measuring water potential and photosynthesis. Once again there was no increase in water potential or photosynthesis following the canopy spraying. The results of this study show no evidence of foliar absorption in black spruce saplings. However this does not exclude the occurrence of foliar absorption via passive or active mechanisms in mature trees, which grow under different circumstances.
... The majority of root water uptake during these periods is from the upper soil layer where rooting density is high. As soils dry, equivalent drops in soil water potential yield smaller and smaller volumes of water since soil moisture release curves relating water potential to water content are non-linear (Selker et al. 1999). Root water uptake shifts to deeper layers as the soil moisture content in the upper soils decreases (Warren et al. 2005;Brooks et al. 2006). ...
... Foliar uptake is an alternative uptake pathway that bypasses root and soil processes. The uptake of water on leaf surfaces, typically occurring under foggy, humid conditions (Simonin et al. 2009;Gerlein-Safdi et al. 2018) when plants have lower water potentials (Goldsmith et al. 2013), represents a potentially different isotopic signature. These are generally small supplements of water (Gotsch et al. 2014), and identifying their magnitude is difficult because the isotopic effects of foliar uptake are also not always distinguishable from the back diffusion into stomata that occurs with transpiration Lehmann et al. 2017; also see Chaps. 10 and 11). ...
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The water present within trees when sugars and cellulose are formed is the source of hydrogen and oxygen atoms that are incorporated into tree-ring cellulose (see Chaps. 10.1007/978-3-030-92698-4_10 and 10.1007/978-3-030-92698-4_11 ). However, the isotope composition of relevant water pools is often unknown when trying to interpret δ ¹⁸ O and δ ² H isotopic records in tree rings. This chapter focuses on the factors that can influence the O and H isotope ratios of source waters for trees. Trees generally use water that originated as precipitation, but this does not mean that the isotope ratios of water used by trees—predominantly taken up by roots from soils—and incorporated in cellulose exactly matches precipitation isotope ratios. Precipitation isotope ratios vary in space and time, and only a fraction of all precipitation infiltrates soils, reaches roots, and is ultimately taken up by trees. Considering species, soils, and climates may allow for predicting which fraction of water resides in the root-zone during the growing seasons, and how its isotope ratios deviate from that of average precipitation. Here we provide an overview of the terrestrial water cycle and the associated transport and fractionation processes that influence the stable isotope ratios of water used by trees. We highlight obstacles and opportunities to be considered, towards more accurately interpreting the records of O and H isotope ratios in tree cellulose.
... studies on the mechanisms and pathways of FWU have become more numerous in the last 10 years (Burgess and Dawson, 2004;Anderson, 2005;Lai et al., 2007;Liang et al., 2009;Yang et al., 2010;Goldsmith, 2013;Eller et al., 2016;Berry et al., 2018). Several studies have revealed physiological advantages of FWU, such as increased water status and benefits for gas exchange (Grammatikopoulos and Mannetas, 1994;Martin and von Willert, 2000;Simonin et al., 2009), but the importance of FWU for gas exchange and leaf water status has not yet been described for any shrub and woody species in the Brazilian ferruginous rupestrian fields. The water acquired by FWU is considered ecologically relevant in dry or seasonally dry environments, where fog events are frequent (Oliveira et al., 2014). ...
... Stomata may also be clogged by water droplets without any of this water being absorbed, so that CO 2 cannot enter (Gerlein-Safdi et al., 2018). In contrast, water absorption by leaves might vary positively in relation to gas exchange in different species of different ecosystems (Grammatikopoulos and Mannetas, 1994;Martin and von Willert, 2000;Simonin et al., 2009;Burkhardt et al., 2012;Eller et al., 2013;Berry et al., 2014;Rosado et al., 2018). Leaf wetting changes the leaf energy balance, decreasing temperature and consequently stomatal closure (Dawson and Goldsmith, 2018;Gerlein-Safdi et al., 2018). ...
... Nevertheless, dew can have significant effects on crop production through leaf wetting and subsequent effects on plant physiological functions. Direct water absorption from the leaf surface, known as foliar water uptake, is one of the important effects of leaf wetting on plant physiological functions (Martin and von Willert, 2000;Simonin et al., 2009;Eller et al., 2013). Subsidiary to root water uptake, foliar water uptake can be a significant water acquisition pathway that contributes to improvements in plant-water relations and consequently mitigates decrease in photosynthetic rate, increasing a plant's chances of survival under drought conditions (Martin and von Willert, 2000;Simonin et al., 2009;Eller et al., 2013). ...
... Direct water absorption from the leaf surface, known as foliar water uptake, is one of the important effects of leaf wetting on plant physiological functions (Martin and von Willert, 2000;Simonin et al., 2009;Eller et al., 2013). Subsidiary to root water uptake, foliar water uptake can be a significant water acquisition pathway that contributes to improvements in plant-water relations and consequently mitigates decrease in photosynthetic rate, increasing a plant's chances of survival under drought conditions (Martin and von Willert, 2000;Simonin et al., 2009;Eller et al., 2013). Leaf wetting can also improve plant-water relations by mitigating excessive transpirational water loss. ...
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Dew is a frequently observed meteorological phenomenon and its importance to the water balance in arid and semi-arid regions has been recognized. The focus of our study was to evaluate the potential significance of leaf wetting by dew on semi-arid crop production in terms of its effects on plant physiological functions. We conducted a field experiment in a maizefield in northwest China. Leaf wetting by dew occurred on 45% of days during cultivation periods, and leaf water potential was higher for leaves wetted by dew than for those not wetted during the morning. In addition, we conducted a potted maize control experiment consisting of four treatments (two different soil water treatments: water-stressed and well-watered, and two different leaf wetting treatments: with and without leaf wetting). The water-stressed treatment strongly inhibited plant physiological functions (decrease in leaf water potential, stomatal closure, decrease in photosynthetic rate); therefore, growth rate decreased. However, in the water-stressed with leaf wetting plot, the maize leaves absorbed water from their surfaces during nighttime, which significantly increased pre-dawn leaf water potential. Consequently, this plot showed a higher photosynthetic rate than did the water-stressed without leaf wetting plot during daytime. The positive effects of leaf wetting on plant physiological functions were not explicit in the well-watered plots. We assumed that the positive effects of leaf wetting were mainly manifest as effects on plant–water relations; such effects may be limited when plants have sufficient water. As a result, water-stressed plants with leaf wetting showed greater water use efficiency. Our results indicate the importance of leaf wetting by dew as a water resource in semi-arid crop production owing its effects on plant physiological functions.
... Canopy water uptake occurs when water absorbed by leaves during rain, fog or dew events is transported from the wet foliage to the dryer stem, root or soil compartments (Figure 1.8a). This foliar absorption of water has shown to play a significant role in the water balance of many ecosystems during drought events (Yates & Hutley, 1995;Burgess & Dawson, 2004;Breshears et al., 2008;Limm et al., 2009;Simonin et al., 2009). Next to water uptake, water can also be redistributed within the tree. ...
... Foliar water uptake has been described for many species, especially in drought stress conditions, improving photosynthetic performance and growth and is now considered to be a widespread phenomenon (Dawson, 1998;Burgess & Dawson, 2004;e.g. Breshears et al., 2008;Limm et al., 2009;Simonin et al., 2009;Goldsmith et al., 2012). While the pathways for foliar uptake are not entirely unravelled, the cuticle (Yates & Hutley, 1995;Limm & Dawson, 2010), trichomes (Franke, 1967) and ...
... As noted by Zhang et al. (2019), low-intensity rain cannot be disregarded, as it commonly infiltrates to recharge soil moisture rather than running off, and may cumulatively account for a significant fraction of total annual rainfall. Moreover, ecologically, light rain and drizzle may have effects comparable to those of fog deposition on leaves: supporting plant metabolism and partially offsetting soil water deficits (Simonin et al., 2009). Persistent leaf wetness that might result from low-intensity rain is also pertinent to the occurrence of leaf diseases of various kinds (Ishibashi andTerashima, 1995, Morales et al., 2018). ...
Article
Tipping-bucket rain gauges (TBRGs) are an established and proven means of recording rainfall amount, but are not well-suited to the estimation of rainfall duration, owing to lag times arising from bucket filling. TBRG data are unable to reveal aspects of rainfall arrival such as short-term rainfall intermittency; nor can they adequately capture the start and end times of low intensity rain. These limitations pose challenges for the estimation of rainfall rates, for which accurate rain duration must be known. Here, a new approach to this problem is explored: the use of acoustic recording apparatus co-located with a TBRG for the principal purpose of identifying true raining time. From an Australian wet tropical ground observing station, TBRG data were processed to yield 5 min, 15 min, and 1 h accumulated rainfall amounts, and corresponding estimates of raining time. These were compared with raining time estimated from high-precision. WAV recordings of raindrop arrival sensed by a responsive drum skin. The acoustic recording allowed true raining time to be measured, and compared with the TBRG data. Only rarely did the TBRG rain durations provide acceptable accuracy; generally, they were severely biased. Moreover, the 5 min data consistently provided estimates with larger bias than did 15 min or 1 h data. In general, none of the TBRG rainfall data provided acceptable estimates of rain duration, and hence derived intensity data were also badly biased. These findings have clear implications for the widespread use of time-aggregated TBRG data, such as hourly rainfall amounts. These may yield little or no accurate information on rainfall intensities; the marked bias of rain duration estimated from TBRG data may hamper attempts to detect and quantify secular change in rainfall frequency, duration, and intensity. Acoustic methods provide an economical means to provide less biased data.
... Although the main pathway of water uptake is from soil to plant to atmosphere, some species are also able to absorb water directly through their leaves Alpha et al., 1996;Goldsmith et al., 2013;Eller et al., 2013;Fernandez et al., 2014;Berry et al., 2018;Schreel and Steppe, 2018) confirming that water movement is multi-directional. It has been shown that water absorption from leaf surface can contribute up to 42 % of the total leaf water content (Eller et al., 2013) and could significantly maintain or improve water status (Limm et al., 2009;Simonin et al., 2009;Eller et al., 2013) via an increase in water storage or relaxing xylem tension. Foliar water uptake (FWU) is a common water acquisition strategy plants in several arid and humid ecosystems (Berry et al., 2018), however its functional significance is still poorly understood. ...
Article
Foliar water uptake (FWU) could be relevant for plants, allowing them to use alternative water sources than the primary one that is soil water uptake and thus prevent dehydration especially during the dry season in arid/semiarid ecosystems characterized by small and erratic water pulses. The main objective of this study was to evaluate the effects of FWU on leaf water status and its contribution to daily transpiration in an arid ecosystem in southern Argentina. Eight dominant species, including shrubs and grasses, with different rooting depth (<0.5, <1, <2 and >2 m) and thus different soil water access, were selected. We hypothesized that FWU is higher in species with shallow roots than in species with permanent water sources and thus FWU enhances plant water availability. We determined FWU capacity by changes in leaf mass after spraying with deionized water. The water potential (ΨLeaf) was determined in the field before and after small water experimental application. Leaf transpiration (E) was measured using a portable photosynthesis system. All study species exhibited leaf water uptake capacity, and was lower in species with deep roots than in the species with shallow roots during the growing season. However, the effects on ΨLeaf were different. During the dry season, only grasses had a substantial enhancement of water status after an experimental water application. A linear negative relationship between FWU and the changes in ΨLeaf after an experimental water pulse was observed during the spring season and an inverse relationship during summer. Species with higher FWU were those with higher transpiration rate suggesting that FWU contribute to plant water balance allowing stomata to remain open, thus enhancing carbon assimilation during the growing season.
... The habitat of this monotypic genus is experiencing a major climate change impact: the level of summer coastal fog, essential for the maintenance of the IUCN red-listed (endangered) coast redwood forests (Simonin et al. 2009, Farjon & Schmid 2013, is being reduced (Johnstone & Dawson 2010). Their massive tree canopies capture moisture at a critical time during annual droughts, allowing the conservation and stability of moist forest characteristics. ...
Article
Various undescribed Cybaeina Chamberlin & Ivie (Araneae: Dictynoidea: Cybaeidae) and Cybaeina-like taxa are known from forested habitats in the west coast of North America. Most have very restricted ranges within the northern portion of the Californian Floristic Province, a well-known biodiversity hotspot. Here we describe Allocybaeina Bennett gen. nov. and its single included species, Allocybaeina littlewalteri Bennett spec. nov. This infrequently collected species is restricted to forested habitat in a small area of the coastal watersheds of southern Humboldt and western Mendocino Counties in northwestern California, U.S.A. In addition to descriptions we provide diagnoses, illustrations, a distribution map, and discuss conservation implications for this distinctive new genus and species.
... This ability, commonly referred to as foliar water uptake (FWU) is shared across phylogeny, and may have profound implications for the water and carbon balance at both the plant and ecosystem level (Binks et al. 2019;Boanares et al. 2019;Hayes et al. 2020). Foliar water uptake can increase plant water status and primary productivity (Berry et al. 2014;Gouvra and Grammatikopoulos 2003;Eller et al. 2013;Fernández et al. 2014;Kerhoulas et al. 2020;Pina et al. 2016;Simonin et al. 2009), and enhance survival by potentially allowing the restoration of xylem transport capacity (Fuenzalida et al. 2019;Laur and Hacke 2014) or facilitating xylem/phloem transport during crucial phenological stages (e.g. fruit development or leaf senescence; Guzmán-Delgado et al. 2017. ...
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Plants can absorb water through their leaf surfaces, a phenomenon commonly referred to as foliar water uptake (FWU). Despite the physiological importance of FWU, the pathways and mechanisms underlying the process are not well known. Using a novel experimental approach, we parsed out the contribution of the stomata and the cuticle to FWU in two species with Mediterranean (Prunus dulcis) and temperate (Pyrus communis) origin. The hydraulic parameters of FWU were derived by analyzing mass and water potential changes of leaves placed in a fog chamber. Leaves were previously treated with abscisic acid to force stomata to remain closed, with fusicoccin to remain open, and with water (control). Leaves with open stomata rehydrated two times faster than leaves with closed stomata and attained three to four times higher maximum fluxes and hydraulic conductance. Based on FWU rates, we propose that rehydration through stomata occurs primarily via diffusion of water vapor rather than in liquid form even when leaf surfaces are covered with a water film. We discuss the potential mechanisms of FWU and the significance of both stomatal and cuticular pathways for plant productivity and survival.
... This ability, commonly referred to as foliar water uptake (FWU) is shared across phylogeny, and may have profound implications for the water and carbon balance at both the plant and ecosystem level (Binks et al. 2019;Boanares et al. 2019;Hayes et al. 2020). Foliar water uptake can increase plant water status and primary productivity (Berry et al. 2014;Gouvra and Grammatikopoulos 2003;Eller et al. 2013;Fernández et al. 2014;Kerhoulas et al. 2020;Pina et al. 2016;Simonin et al. 2009), and enhance survival by potentially allowing the restoration of xylem transport capacity (Fuenzalida et al. 2019;Laur and Hacke 2014) or facilitating xylem/phloem transport during crucial phenological stages (e.g. fruit development or leaf senescence; Guzmán-Delgado et al. 2017. ...
... It has been shown (e.g., Limm et al. 2009;Simonin et al. 2009;Eller et al. 2013;Holanda et al. 2019) that FWU improves plant water status, decreasing stomatal conductance, recovering from xylem cavitation, increasing shoot water potential and positively affecting plant survivorship and growth. However, in this study, we observed that higher FWU was not necessarily related to higher increase in leaf water potential. ...
Article
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Foliar water uptake (FWU) has been reported for different species across several ecosystems types. However, little attention has been given to arid ecosystems, where FWU during dew formation or small rain events could ameliorate water deficits. FWU and their effects on leaf water potential (ΨLeaf) were evaluated in grasses and shrubs exploring different soil water sources in a Patagonian steppe. Also, seasonal variability in FWU and the role of cell wall elasticity in determining the effects on ΨLeaf were assessed. Eleven small rain events (< 8 mm) and 45 days with dew formation were recorded during the study period. All species exhibited FWU after experimental wetting. There was a large variability in FWU across species, from 0.04 mmol m−2 s−1 in species with deep roots to 0.75 mmol m−2 s−1 in species with shallow roots. Species-specific mean FWU rates were positively correlated with mean transpiration rates. The increase in ΨLeaf after leaf wetting varied between 0.65 MPa and 1.67 MPa across species and seasons. The effects of FWU on ΨLeaf were inversely correlated with cell wall elasticity. FWU integrated over both seasons varied between 28 mol m−2 in species with deep roots to 361 mol m−2 in species with shallow roots. Taking into account the percentage of coverage of each species, accumulated FWU represented 1.6% of the total annual transpiration of grasses and shrubs in this ecosystem. Despite this low FWU integrated over time compared to transpiration, wetting leaves surfaces can help to avoid larger water deficit during the dry season.
... This means that the 18 O enrichment of bleeding sap and stem water cannot be the effect of phloem water. (4) Foliar water uptake and back diffusion were only reported when plants were exposed to high air humidity and dry soil simultaneously (Limm et al., 2009;Simonin et al., 2009;Eller et al., 2013). ...
Article
Determining whether isotope fractionation occurs during root water uptake is a prerequisite for using stem or xylem water isotopes to trace water sources. However, it is unclear whether isotope fractionation occurs during root water uptake in gramineous crops. We conducted prevalidation experiments to estimate the isotope measurement bias associated with cryogenic vacuum distillation (CVD). Next, we assessed isotope fractionation during root water uptake in two common agronomic crops, wheat (Triticum aestivum L.) and maize (Zea mays L.), under flooding after postdrought stress conditions. CVD caused significant depletion of 2H but negligible effects on 18O for both soil and stem water. Surprisingly CVD caused depletion of 2H and enrichment of 18O in root water. Stem and root water δ18O were higher than soil water δ18O, even considering the uncertainty of CVD. Soil water 18O was depleted compared with irrigation water 18O in the pots with plants but enriched relative to irrigation water 18O in the pots without plants. These results indicate that isotope fractionation occurred during wheat and maize root water uptake after full irrigation and led to a heavy isotope enrichment in stem water. Therefore, the xylem/stem water isotope approach widely used to trace water sources should be carefully evaluated.
... This soil water recharge may be sufficient to prevent permanent damage to root structures by drought [11] and can increase survival rates [85,86]. Hence, plants can partially decouple their water status from the soil water availability [14] thus improving the entire plant water status [29]. ...
Article
Foliar water uptake (FWU) has been identified as a mechanism commonly used by trees and other plants originating from various biomes. However, many questions regarding the pathways and the implications of FWU remain, including its ability to mitigate climate change-driven drought. Therefore, answering these questions is of primary importance to adequately address and comprehend drought stress responses and associated growth. In this review, we discuss the occurrence, pathways, and consequences of FWU, with a focus predominantly on tree species. Subsequently, we highlight the tight coupling between FWU and foliar fertilizer applications, discuss FWU in a changing climate, and conclude with the importance of including FWU in mechanistic vegetation models.
... Different colored rectangles and circles show the progress of dewdrop absorption in leaf trichomes. Photographs were taken by digital microscopy (Leica DVM6) increasing number of species, for example, conifers (Breshears et al., 2008;Simonin et al., 2009), herbaceous vegetation (Zhuang & Zhao, 2010), broadleaf trees (Yan et al., 2015), and diverse species in multiple biomes, from tropical montane cloud forests to mangrove forests (Steppe et al., 2018) and desert ecosystems (Yan et al., 2015). ...
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Investigating plant morphological traits can provide insights into plant drought tolerance. To date, many papers have focused on plant hydraulic responses to drought during dehydration, but atmospheric water absorption by trichomes to mitigate drought stress by influencing leaf hydraulics in plant species that inhabit arid environments has been largely ignored. The experiment in this study was designed to assess how dew absorbed by leaf trichomes helps Caragana korshinskii withstand drought. The results showed that under a drought stress and dew (DS & D) treatment, C. korshinskii displayed a strong capacity to absorb dew with trichomes; exhibited slow decreases in leaf water potential (Ψleaf), leaf hydraulic conductivity (Kleaf), and gas exchange; experienced 50% Kleaf and gas exchange losses at lower relative soil water content levels than plants treated with drought stress and no dew (DS & ND); and experienced 50% Kleaf loss (Kleaf P50) at similar Ψleaf levels as DS & ND plants. Its congener C. sinica, which does not have leaf trichomes, displayed little ability to absorb dew under drought stress and did not show any remarkable improvement in the above parameters under the DS & D treatment. Our results indicated that leaf trichomes are important epidermal dew‐uptake structures that assist in partially sustaining the leaf hydraulic assimilation system, mitigate the adverse effects of drought stress and contribute to the distribution of C. korshinskii in arid environments. This article is protected by copyright. All rights reserved.
... The low water retention of sandy soils can create elevated stress through lack of water accessibility during summer drought. Water accessibility is a significant factor affecting coast redwood growth [23], and fog is an important source of moisture for naturally occurring US coast redwood forest stands [20,56]. Additionally, the fog has been identified as a significant direct and indirect source of nitrogen to coast redwood trees [57]. ...
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Background Effective matching of genotypes and environments is required for the species to reach optimal productivity and act effectively for carbon sequestration. A common garden experiment across five different environments was undertaken to assess genotype x environment interaction (GxE) of coast redwood in order to understand the performance of genotypes across environments. Results The quantitative genetic analysis discovered no GxE between investigated environments for diameter at breast height (DBH). However, no genetic component was detected at one environment possibly due to stressful conditions. The implementation of universal response function allowed for the identification of important environmental factors affecting species productivity. Additionally, this approach enabled us to predict the performance of species across the New Zealand environmental conditions. Conclusions In combination with quantitative genetic analysis which identified genetically superior material, the URF model can directly identify the optimal geographical regions to maximize productivity. However, the finding of ideally uncorrelated climatic variables for species with narrow ecological amplitude is rather challenging, which complicates construction of informative URF model. This, along with a small number of tested environments, tended to overfit a prediction model which resulted in extreme predictions in untested environments.
... CWD calculations use monthly precipitation data to determine moisture availabilitybut since redwoods use summer fog, in particular, as a significant source of moisture (Burgess and Dawson, 2004;Limm et al., 2009) traditional calculations of CWD based on precipitation totals will not work. VI. Simonin et al. (2009) found that because of fog effects, coast redwood water stress level is uncoupled to some extent from soil moisture availability -and thus models based on CWD may over-estimate redwood water deficit effects. VII. ...
... Evidence suggests that FWU may result in significant fluxes of water at the ecosystem scale (Binks et al. 2019), and could play a fundamental role in determining the hydraulic vulnerability of plants both in terms of partially decoupling canopies from the soil water status (Binks et al., 2019;Schreel & Steppe, 2019;Simonin, Santiago, & Dawson, 2009) and of the potential of branch-level uptake to refill embolised conduits (Mayr et al., 2014). The mechanism of water ingress into leaves is of wide interest and various pathways have been proposed. ...
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Foliar water uptake, the uptake of atmospheric water directly into leaves, has been reported to occur in nearly 200 species spanning a wide range of ecosystems distributed globally. Until recently, this flux has not been taken into consideration in land–surface models representing global fluxes of water, in interpreting plant hydraulic status or in the determination of species’ vulnerability to drought. A key trait required to represent foliar uptake at canopy to ecosystem scales is conductance to foliar uptake, KFWU, which is the flux of water into the leaf normalised by the water potential difference between the leaf and water source. This trait is biophysically equivalent to stomatal conductance, gs; however, the two variables are typically normalised by different measures of water ‘concentration’. Here we show that when converted to the same units, the typical ranges of gs overlap with the few published values of KFWU suggesting that, theoretically, water vapour moving in through the stomata could partially, or even wholly, account for the fluxes attributed to foliar water uptake in some species. Establishing the extent to which such ‘reverse transpiration’ contributes to foliar uptake may be key to incorporating foliar water uptake into our existing understanding of plant‐atmosphere interactions.
... In the present context, Ryan et al. (2006) referred to reports of decreases with tree height in the ratio of the leaf area held above some height to the sapwood area in the stem at that height, sapwood through which water must pass to supply those leaves; such a decrease implies that the conductivity of sapwood to water movement has increased so affording some alleviation of water stress in leaves above that height. However, other reports have shown leaf area/ sapwood area ratios increasing with height (Phillips et al. 2003;Buckley and Roberts 2006;Simonin et al. 2006;Ambrose et al. 2010). The present discussion has suggested that sapwood area at any height in the tree stem reflects a compromise between water conductivity and stem strength requirements. ...
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Once forests have achieved a full canopy, their growth rate declines progressively with age. This work used a global data set with estimates from a wide range of forest types, aged 20‒795 years, of their annual photosynthetic production (gross primary production, GPP) and subsequent above- plus below-ground biomass production (net primary production, NPP). Both GPP and NPP increased with increasing mean annual temperature and precipitation. GPP was then unrelated to forest age whilst NPP declined progressively with increasing age. These results implied that autotrophic respiration increases with age. It has been proposed that GPP should decline in response to increasing water stress in leaves as water is raised to greater heights as trees grow taller with age. However, trees may make substantial plastic adjustment in morphology and anatomy of newly developing leaves, xylem and fine roots to compensate for this stress and maintain GPP with age. This work reviews the possibilities that NPP declines with age as respiratory costs increase progressively in, any or all of, the construction and maintenance of more complex tissues, the maintenance of increasing amounts of live tissue within the sapwood of stems and coarse roots, the conversion of sapwood to heartwood, the increasing distance of phloem transport, increased turnover rates of fine roots, cost of supporting very tall trees that are unable to compensate fully for increased water stress in their canopies or maintaining alive competitively unsuccessful small trees.
... This has led to speculations about an increased importance of FWU as climate change progresses [13]. On the one hand, a decrease in leaf water potential during drought results in a higher water potential difference between leaf and atmospheric water, which is the driving force for FWU [19]. On the other hand, the pathway used for FWU might be blocked during drought, e.g., if stomata are used in FWU, and these close during drought [3], leaves will absorb less water [4]. ...
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Foliar water uptake (FWU) has been investigated in an increasing number of species from a variety of areas but has remained largely understudied in deciduous, temperate tree species from non-foggy regions. As leaf wetting events frequently occur in temperate regions, FWU might be more important than previously thought and should be investigated. As climate change progresses, the number of drought events is expected to increase, basically resulting in a decreasing number of leaf wetting events, which might make FWU a seemingly less important mechanism. However, the impact of drought on FWU might not be that unidirectional because drought will also cause a more negative tree water potential, which is expected to result in more FWU. It yet remains unclear whether drought results in a general increase or decrease in the amount of water absorbed by leaves. The main objectives of this study are, therefore: (i) to assess FWU-capacity in nine widely distributed key tree species from temperate regions, and (ii) to investigate the effect of drought on FWU in these species. Based on measurements of leaf and soil water potential and FWU-capacity, the effect of drought on FWU in temperate tree species was assessed. Eight out of nine temperate tree species were able to absorb water via their leaves. The amount of water absorbed by leaves and the response of this plant trait to drought were species-dependent, with a general increase in the amount of water absorbed as leaf water potential decreased. This relationship was less pronounced when using soil water potential as an independent variable. We were able to classify species according to their response in FWU to drought at the leaf level, but this classification changed when using drought at the soil level, and was driven by iso- and anisohydric behavior. FWU hence occurred in several key tree species from temperate regions, be it with some variability, which potentially allows these species to partly reduce the effects of drought stress. We recommend including this mechanism in future research regarding plant–water relations and to investigate the impact of different pathways used for FWU.
... However, the "LMWL departure" distances reflect, in relative terms, how much of the water available to each tree has undergone post-precipitation isotopic modification; effectively the amount of source water fractionation has occurred because of evaporative processes that lead to isotope enrichment of the residual water that trees might take up. This metric therefore measures how much trees rely on water stored in shallow layers, largely soils (which is influenced to a greater extent by evaporative processes that cause enrichment, rendering these shallow waters less similar to the original precipitation inputs), relative to water stored at greater depths (less exposed to evaporative enrichment and thus more similar to precipitation) (Benettin et al., 2018, Oshun et al., 2016, Simonin et al., 2009. The smaller the LMWL departure, the greater the tree's use of deeper subsurface water. ...
Article
Drought extent and severity have increased and are predicted to continue to increase in many parts of the world. Understanding tree vulnerability to drought at both individual and species levels is key to ongoing forest management and preparation for future transitions in community composition. The influence of subsurface hydrologic processes is particularly important in water‐limited ecosystems, and is an under‐studied aspect of tree drought vulnerability. With California’s 2013‐2016 extraordinary drought as a natural experiment, we studied four co‐occurring woodland tree species, blue oak (Quercus douglasii), valley oak (Quercus lobata), grey pine (Pinus sabiniana), and California juniper (Juniperus californica), examining drought vulnerability as a function of climate, lithology and hydrology using regional aerial dieback surveys and site scale field surveys. We found that in addition to climatic drought severity (i.e. rainfall), subsurface processes explained variation in drought vulnerability within and across species at both scales. Regionally for blue oak, severity of dieback was related to the bedrock lithology, with higher mortality on igneous and metamorphic substrates, and to regional reductions in groundwater. At the site scale, access to deep subsurface water, evidenced by stemwater stable isotope composition, was related to canopy condition across all species. Along hillslope gradients, channel locations supported similar environments in terms of water stress across a wide climatic gradient, indicating that subsurface hydrology mediates species’ experience of drought, and that areas associated with persistent access to subsurface hydrologic resources may provide important refugia at species’ xeric range edges. Despite this persistent overall influence of the subsurface environment, individual species showed markedly different response patterns. We argue that hydrologic niche segregation can be a useful lens through which to interpret these differences in vulnerability to climatic drought and climate change.
... Calculations of the upper limit of leaf water potential can thus be modified to Ψ t_max = Ψ soil -ρgh + ΔΨ FU , where ΔΨ FU = dt F FU / C leaf , and F FU is the flux into the leaf via FU; dt is the duration over which the flux occurs and C leaf is the hydraulic capacitance of the leaf. This equation relates to the relationship set out in Simonin et al. (2009) describing a modified version of the soil-plantatmosphere-continuum model which includes parameters for foliar water uptake. ...
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The absorption of atmospheric water directly into leaves enables plants to alleviate the water stress caused by low soil moisture, hydraulic resistance in the xylem and the effect of gravity on the water column, whilst enabling plants to scavenge small inputs of water from leaf wetting events. By increasing the availability of water, and supplying it from the top of the canopy (in a direction facilitated by gravity), foliar uptake (FU) may be a significant process in determining how forests interact with climate, and could alter our interpretation of current metrics for hydraulic stress and sensitivity. FU has not been reported for lowland tropical rain forests; we test whether FU occurs in six common Amazonian tree genera in lowland Amazônia, and make a first estimation of its contribution to canopy‐atmosphere water exchange. We demonstrate that FU occurs in all six genera and that dew‐derived water may therefore be used to ‘pay’ for some morning transpiration in the dry season. Using meteorological and canopy wetness data, coupled with empirically‐derived estimates of leaf conductance to FU (kfu), we estimate that the contribution by FU to annual transpiration at this site has a median value of 8.2% (103 mm yr⁻¹) and an interquartile range of 3.4 to 15.3%, with the biggest sources of uncertainty being kfu and the proportion of time the canopy is wet. Our results indicate that FU is likely to be a common strategy and may have significant implications for the Amazon carbon budget. The process of foliar water uptake may also have a profound impact on the drought tolerance of individual Amazonian trees and tree species, and on the cycling of water and carbon, regionally and globally. This article is protected by copyright. All rights reserved.
... In foliar uptake of water, fog plays a greater role in improving the daily water stress and an important role in suppressing leaf water loss. Studies in Sequoia sempervirens demonstrated that the plant can direct absorb the fog water and this helps in preventing dehydration of leaves, improving the carbon fixation, and contributes to the faster growth rates and greater size of trees of same species (Burgess et al. 2004;Simonin et al. 2009). Foliar uptake of fog water in the forest tree species D. brasiliensis improve stomatal conductance, leaf water potential, photosynthesis, and growth related to plants shielded by fog (Eller et al. 2013). ...
Article
Aquaporins (AQPs) are the membrane water channel proteins present in all organisms. Plants have the highest amount and the largest variety of aquaporin homologs with diverse subcellular localization patterns, solute specificity, and gating properties. As aquaporins regulate plant water relations, they are alleged to play a pivotal role in the defense response of plants against biotic and abiotic stress. In particular, aquaporins play a crucial role in defense response against drought stress. Plants in arid and semi-arid regions which are severely affected by drought, overcome this by absorbing water from external sources besides precipitation such as fog, dew and cloud mist through leaves by a process known as foliar water uptake. Studies had shown aquaporin's role in foliar water uptake, which regulates the transpiration rate and hydraulic conductivity in plants. However, the molecular mechanisms behind it are still up in the air. The current review emphasizes on the aquaporins and relation to foliar water uptake through its regulatory role of water status in plants under stress. Also this review underlines the functions of aquaporin genes in stomatal regulation, transpiration and its response to drought stress.
... The trees in particular significantly influence the magnitude of fog water input to the ecosystems (Dawson, 1998). Foliar uptake of fog water comprises an important mechanism for water gain that can mitigate the deleterious effects of soil water deficit (Breshears et al., 2008;Simonin et al., 2009;Eller et al., 2013). Fog is of special importance for high species diversity in tropical forests, both montane and lowland (Hamilton et al., 1995;Obregon et al., 2011). ...
Article
Fog is influenced by numerous factors, including forests. The aim of our study was to examine in detail the extent to which forests affect fog. We hypothesised that: (i) fog incidence is affected by forests, (ii) the forested area in the station’s neighbourhood is a factor influencing fog incidence, (iii) the influence on fog differs between coniferous and broad-leaved forests and (iv) the effect of forests on fog occurrence differs with altitude. For this, we used long-term records of fog incidence measured in 1981–2017 at 56 professional meteorological stations in Romania, GIS-derived information on forested areas in the neighbourhood of these stations, and land-use data on the types of these forests. The analyses are based on a semiparametric generalised additive logistic model for the probability of fog occurrence with potentially nonlinear, smooth effects modelled via penalised splines. Our results indicated that forests do affect fog incidence significantly, though their effect is considerably lower than the effect of dominant factors we studied previously, such as seasonality and altitude. It was indicated that forested areas in the neighbourhood of these stations are a factor significantly influencing fog incidence, even when forest is not the dominant land-use factor. In this respect, a radius of 3 km was the most effective when considering the forested area in a circle around the meteorological station. Our model showed that the influence on fog occurrence differs between coniferous and broad-leaved forests, and the effect of forests on fog occurrence is modified by altitude. The hypotheses propounded were confirmed and the hypothesised effects were quantified. Our findings, relevant at least for temperate forests, will enhance further considerations related to fog formation and wet atmospheric deposition. Moreover, our study opens a new challenge for further research of water balance as related to deforestation in catchment areas.
... Fog harvesting is considered as a promising method to improve freshwater supply in dry areas and there are ongoing efforts to improve artificial fog collectors, including biomimetic approaches (Azad et al. 2015;Gurera and Bhushan 2019;Ju et al. 2013;Klemm et al. 2012;Shigezawa et al. 2016). Fog (and dew) are recognized as potentially important water sources for plants (Dawson 1998;Eller et al. 2013;Simonin 2009) and this motivates the search for special plant traits that might improve fog collection (Andrews et al. 2011;Azad et al. 2015;Gurera and Bhushan 2019). Expectedly, selective pressure for traits that improve fog collection should be particularly high in areas showing low precipitation but frequent fog events. ...
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Bio-inspired design (BID) means the concept of transferring functional principles from biology to technology. The core idea driving BID-related work is that evolution has shaped functional attributes, which are termed “adaptations” in biology, to a high functional performance by relentless selective pressure. For current methods and tools, such as data bases, it is implicitly supposed that the considered biological models are adaptations and their functions already clarified. Often, however, the identification of adaptations and their functional features is a difficult task which is not yet accomplished for numerous biological structures, as happens to be the case also for various organismic features from which successful BID developments were derived. This appears to question the relevance of the much stressed importance of evolution for BID. While it is obviously possible to derive an attractive technical principle from an observed biological effect without knowing its original functionality, this kind of BID (“analog BID”) has no further ties to biology. In contrast, a BID based on an adaptation and its function (“homolog BID”) is deeply embedded in biology. It is suggested that a serious and honest clarification of the functional background of a biological structure is an essential first step in devising a BID project, to recognize possible problems and pitfalls as well as to evaluate the need for further biological analysis.
... Several studies have also demonstrated that transpiration rates decline during fog events across a wide-variety of plant species in natural ecosystems (Burgess and Dawson, 2004;Ritter et al., 2009;Berry and Smith, 2013;Alvarado-Barrientos et al., 2014;Gotsch et al., 2016) due to lower vapor pressure deficit and leaf-wetting events. Direct foliar absorption of fog water can reduce leaf water deficit and increase leaf gas-exchange rates (Burgess and Dawson 2004;Simonin et al., 2009;Limm et al., 2009;Goldsmith et al., 2013;Berry et al., 2014;Baguskas et al., 2016) and contribute to whole-plant rehydration (Eller et al., 2013). There are also potential tradeoffs between reduced plant water stress and reduced solar radiation on foggy days (Bai et al., 2012). ...
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In coastal California, the peak growing season of economically important crops is concurrent with fog events, which buffer drought stress during the dry season. Coastal fog patterns are changing, so we quantified its effects on the energy, water, and carbon fluxes of a strawberry farm located in the fog‐belt of the Salinas Valley, California. We used Geostationary Operational Environmental Satellite (GOES) total albedo to detect and quantify large scale patterns of coastal fog. We used eddy covariance (EC) to quantify actual evapotranspiration and gross primary productivity (GPP) at the field scale (approximately 0.5–3 hectares) from June to September 2016. We measured canopy‐scale (approximately 0.6 m²) strawberry physiology on foggy and non‐foggy days within the measurement footprint of the EC tower. Downwelling longwave radiation (L↓), observed by a surface‐mounted pyrgeometer, was consistently higher on foggy compared to clear‐sky days (regardless of fog‐drip), indicating that emission of longwave radiation was derived almost entirely from the cloud base. L↓ and total GOES albedo were positively and strongly correlated (R² = 0.68, P < 0.01). For both field‐ and canopy‐scales, water‐use and light‐use efficiency increased by as much as 50% and 70%, respectively, during foggy compared to non‐foggy conditions. The initial slope of the curvilinear relationship fit between GPP and photosynthetically active radiation was twice as steep during foggy (α = 0.0395) than non‐foggy (α = 0.0210) conditions, suggesting that the scattering of light during fog events enhances photosynthetic output of whole‐plants. Our results suggest that irrigation for these fields could be rescheduled during foggy periods without sacrificing plant productivity.
... Such reversal of water movement can occur when water vapour concentrations in leaf intercellular gas spaces are less than those in the atmosphere (Vesala et al., 2017;Cernusak et al., 2018) or when plant surfaces become wet under saturated atmospheres by immersion in fog (Burgess & Dawson, 2004;Simonin et al., 2009) or cloud (Oliveira et al., 2014), interception of rainfall (Breshears et al., 2008), or temperature-dependent condensation of dew (Munne-Bosch et al., 1999). Much less attention has been given to understanding how leaf wetting events that are promoted under unsaturated atmospheres by the structure and chemical characteristics of leaf surfaces, such as capillary condensation among dense trichomes (Konrad et al., 2015) or deliquescence of hygrophilic substances (Burkhardt, 2010), can also improve plant water balances in water-limiting environments. ...
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The mangrove Avicennia marina adjusts internal salt concentrations by foliar salt secretion. Deliquescence of accumulated salt causes leaf wetting which may provide a water source for salt‐secreting plants in arid coastal wetlands where high nocturnal humidity can usually support deliquescence whereas rainfall events are rare. We tested the hypotheses that salt deliquescence on leaf surfaces can drive top‐down rehydration, and that such absorption of moisture from unsaturated atmospheres makes a functional contribution to dry season shoot water balances. Sap‐flow and water relations were monitored to assess uptake of atmospheric water by branches during shoot wetting events under natural and manipulated microclimatic conditions. Reverse sap flow rates increased with increasing relative humidity from 70 to 89%, consistent with function of salt deliquescence in harvesting moisture from unsaturated atmospheres. Top‐down rehydration elevated branch water potentials above those possible from root water uptake, subsidising transpiration rates and reducing branch vulnerability to hydraulic failure in the subsequent photoperiod. Absorption of atmospheric moisture harvested through deliquescence of salt on leaf surfaces enhances water balances of Avicennia marina growing in hypersaline wetlands under arid climatic conditions. Top‐down rehydration from these frequent, low intensity wetting events contributes to prevention of carbon starvation and hydraulic failure during drought.
... This finding was consistent with another study (Brandes et al. 2007) showing foliar water uptake in pine trees. While these studies investigated water sources in plants, others further claimed that fog and dew can enhance plant growth and performance (Boucher et al. 1995;Limm et al. 2009;Simonin et al. 2009), and contribute to plant survival (Stone 1963). Foliar uptake of dew was found to alleviate drought stress and improve survival (Munné-Bosch 2010;Eller et al. 2013;Pina et al. 2016;Nadezhdina and Nadezhdin 2017;Schreel and Steppe 2020), However, the question still remains as to how dew can contribute to plant water uptake and by which pathway-through roots or leaves and stems (Breshears et al. 2008;Atkin et al. 2015;Aparecido et al. 2016)? ...
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Dew is an important water resource for plants in most deserts. The mechanism that allows desert plants to use dew water was studied using an isotopic water tracer approach. Most plants use water directly from the soil; the roots transfer the water to the rest of the plant, where it is required for all metabolic functions. However, many plants can also take up water into their leaves and stems. Examining the dew water uptake pathways in desert plants can lend insight on another all water-use pathways examination. We determined where and how dew water enters plants in the water limited Negev desert. Highly depleted isotopic water was sprayed on three different dominant plant species of the Negev desert—Artemesia sieberi, Salsola inermis and Haloxylon scoparium—and its entry into the plant was followed. Water was sprayed onto the soil only, or on the leaves/stems only (with soil covered to prevent water entry via root uptake). Thereafter, the isotopic composition of water in the roots and stems were measured at various time points. The results show that each plant species used the dew water to a different extent, and we obtained evidence of foliar uptake capacity of dew water that varied depending on the microenvironmental conditions. A. sieberi took up the greatest amount of dew water through both stems and roots, S. inermis took up dew water mainly from the roots, and H. scoparium showed the least dew capture overall.
... Fog can increase the leaf water potential and maintain turgor of some TMCF species, which is essential to promote growth and plant productivity Eller, Lima, & Oliveira, 2013Eller et al., 2018). Similar drought-alleviation effects by fog have been observed during the driest periods of the year for temperate species (Eller et al., 2013;Goldsmith et al., 2013;Limm et al., 2009;Simonin et al., 2009), reducing the risk of hydraulic failure and maintaining their productivity during the daytime. However, even though they had lower changes in water potential, the hydraulic safety margin was still lower than evergreen tropical species. ...
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Tropical montane cloud forests (TMCF) have unique climatic conditions, which allow the coexistence of plant lineages with different phytogeographical origins from tropical versus temperate climates. Future climate projections suggest TMCFs will be subjected to increasing drought stress due to fog uplift and higher temperatures, possibly leading to tree mortality and local extinctions, and consequently changes in forest composition and functioning. Characterizing community functional composition, trade‐offs among traits and the drivers of community assembly is of utmost importance to improve our capacity to predict the response of montane plant communities to forecast climate change. Here, we aimed to test whether species from different phytogeographical origins (i.e. tropical – evergreen × deciduous − and temperate) differ in drought vulnerability and how the coexistence of these groups change the hydraulic composition of TMCFs. We used a framework based on measurements of key hydraulic traits (i.e. xylem embolism resistance, hydraulic safety margin, stomata control, turgor loss point, minimum water potential) of 16 dominant species (>70% of the forest basal area) within a TMCF in the Atlantic Rain Forest Domain in southeast Brazil. We used community‐weighted means to model whether removing each species group would change the community hydraulic functional composition. Temperate, tropical deciduous and tropical evergreen groups differ in their hydraulic functioning and these differences explain forest functional composition and taxa dominance. Temperate and tropical deciduous taxa were consistently more vulnerable hydraulically (i.e. lower safety margins and embolism resistance). The coexistence of different phytogeographical lineages is a key determinant of TMCF hydraulic composition. We also used models including phylogeny to evaluate the variation of hydraulic traits across phytogeographical groups, and the results suggest some niche conservatism associated with plant hydraulic functioning. Our results provide evidence of the importance of species phytogeographical origin on TMCF functioning, and niche conservatism in the evolution of hydraulic traits. The higher drought vulnerability observed in temperate group might be a mechanistic explanation for the restriction of temperate taxa distribution to wetter places during past colder and drier climate. Thus, we suggest hydraulic functional traits may be useful to predict future dynamics of TMCFs under changing climatic conditions. A free Plain Language Summary can be found within the Supporting Information of this article. Florestas nebulares tropicais (TMCF) têm condições climáticas únicas, que permitem a coexistência de linhagens de plantas com diferentes origens fitogeográficas em climas tropicais e temperados. Projeções climáticas futuras sugerem que Florestas Nebulares estarão sujeitas a crescente estresse hídrico, devido a temperaturas mais elevadas e a mudança na zona de ocorrência de neblina, possivelmente levando à mortalidade de árvores e extinções locais e, consequentemente, mudanças na composição e no funcionamento dessas floresta. Caracterizar a composição funcional dessas comunidades, entender os trade‐offs entre atributos e os fatores envolvidos na montagem de comunidades são de extrema importância para melhorar nossa capacidade de prever respostas das comunidades de plantas montanas diante às mudanças climáticas previstas. Neste estudo, nosso objetivo foi testar se espécies de diferentes origens fitogeográficas (ou seja, tropicais ‐ perenes × decíduas ‐ e temperadas) diferem na vulnerabilidade à seca e como a coexistência desses grupos altera a composição hidráulica de TMCFs. Para isso, avaliamos atributos hidráulicos essenciais (dentre eles a resistência do xilema ao embolismo, a margem de segurança hidráulica, controle estômático, ponto de perda de turgor, potencial hídrico mínimo) de 16 espécies dominantes (>70% da área basal da floresta) dentro de uma TMCF, na Mata Atlântica no sudeste do Brasil. Usamos a média ponderada pela comunidade para modelar se a remoção de cada grupo de espécies mudaria a composição funcional hidráulica da comunidade. Grupos temperados, tropicais decíduos e tropicais perenes diferem em seu funcionamento hidráulico e essas diferenças explicam a composição funcional da floresta e a dominância dos táxons. Táxons temperados e tropicais decíduos foram consistentemente mais vulneráveis hidraulicamente (ou seja, margem de segurança mais baixa e menor resistência ao embolismo). A coexistência de diferentes linhagens fitogeográficas é um fator determinante da composição hidráulica de TMCF. Também usamos modelos incluindo filogenia para avaliar a variação de características hidráulicas entre os grupos fitogeográficos, e os resultados sugerem algum conservadorismo de nicho associado ao funcionamento hidráulico das plantas. Nossos resultados fornecem evidência da importância da origem fitogeográfica das espécies no funcionamento de TMCF e do conservadorismo de nicho na evolução das características hidráulicas. A maior vulnerabilidade à seca observada no grupo temperado pode ser uma explicação mecanicista para a distribuição dos táxons temperados se restringir a locais mais úmidos durante o clima mais frio e seco do passado. Assim, sugerimos que as características funcionais hidráulicas podem ser úteis para prever a dinâmica futura de TMCFs sob mudanças nas condições climáticas. A free Plain Language Summary can be found within the Supporting Information of this article.
... Some plant species can reverse the soil-plant-atmosphere continuum through foliar water uptake (FWU) when water moves from high to low water potential (Burgess and Dawson, 2004;Simonin et al., 2009;Eller et al., 2013;Berry et al., 2019). Studies on FWU have been reported for tropical mountain forests, where fog is one of the most critical water sources for plants (Limm et al., 2009;Limm and Dawson, 2010;Goldsmith et al., 2013;Oliveira et al., 2014;Baguskas et al., 2016). ...
... This ability, commonly referred to as foliar water uptake (FWU), is shared across phylogeny and may have profound implications for the water and carbon balance at both the plant and ecosystem level (Binks et al. 2019;Boanares et al. 2019;Hayes et al. 2020). Foliar water uptake can increase plant water status and primary productivity (Berry et al. 2014;Gouvra and Grammatikopoulos 2003;Eller et al. 2013;Fernández et al. 2014;Kerhoulas et al. 2020;Pina et al. 2016;Simonin et al. 2009), and enhance survival by potentially allowing the restoration of xylem transport capacity (Fuenzalida et al. 2019;Laur and Hacke 2014) or facilitating xylem/phloem transport during crucial phenological stages (e.g. fruit development or leaf senescence; Guzmán-Delgado et al. 2017. ...
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Plants can absorb water through their leaf surfaces, a phenomenon commonly referred to as foliar water uptake (FWU). Despite the physiological importance of FWU, the pathways and mechanisms underlying the process are not well known. Using a novel experimental approach, we parsed out the contribution of the stomata and the cuticle to FWU in two species with Mediterranean (Prunus dulcis) and temperate (Pyrus communis) origin. The hydraulic parameters of FWU were derived by analyzing mass and water potential changes of leaves placed in a fog chamber. Leaves were previously treated with abscisic acid to force stomata to remain closed, with fusicoccin to remain open, and with water (control). Leaves with open stomata rehydrated two times faster than leaves with closed stomata and attained three to four times higher maximum fluxes and hydraulic conductance. Based on FWU rates, we propose that rehydration through stomata occurs primarily via diffusion of water vapor rather than in liquid form even when leaf surfaces are covered with a water film. We discuss the potential mechanisms of FWU and the significance of both stomatal and cuticular pathways for plant productivity and survival. This article is protected by copyright. All rights reserved.
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Biological factors impacting hydrological processes. The effects of biological factors on the water cycle and subsequently on hydrological processes have wide and profound consequences on ecosystem structure, function and management. A profound understanding of their effects is of prime importance especially in light of climate change projection.
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Foliar water uptake (FWU) is a mechanism that enables plants to acquire water from the atmosphere through their leaves. As mangroves live in a saline sediment water environment, the mechanism of FWU might be of vital importance to acquire freshwater and grow. The goal of this study was to assess the FWU capacity of six different mangrove species belonging to four genera using a series of submersion experiments in which the leaf mass increase was measured and expressed per unit leaf area. The foliar water uptake capacity differed between species with the highest and lowest average water uptake in Avicennia marina (Forssk.) Vierh. (1.52 ± 0.48 mg H2O cm−2) and Bruguiera gymnorhiza (L.) Lam. (0.13 ± 0.06 mg H2O cm−2), respectively. Salt-excreting species showed a higher FWU capacity than non-excreting species. Moreover, A. marina, a salt-excreting species, showed a distinct leaf anatomical trait, i.e., trichomes, which were not observed in the other species and might be involved in the water absorption process. The storage of leaves in moist Ziplock bags prior to measurement caused leaf water uptake to already occur during transport to the field station, which proportionately increased the leaf water potential (A. marina: −0.31 ± 0.13 MPa and B. gymnorhiza: −2.70 ± 0.27 MPa). This increase should be considered when performing best practice leaf water potential measurements but did not affect the quantification of FWU capacity because of the water potential gradient between a leaf and the surrounding water during submersion. Our results highlight the differences that exist in FWU capacity between species residing in the same area and growing under the same environmental conditions. This comparative study therefore enhances our understanding of mangrove species’ functioning.
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Fog and low-lying cloud (fog) play a significant role in the maintenance of ecosystems, from desert to alpine and from coastal to inland systems. Our central thesis is that fog provides ecosystems with critical water and nutrient subsidies, and also delivers pollutants, that often control ecosystem function. Fog is a medium, vector, and connector. In this mini-review, we synthesize recent research advances that reveal the diverse ways that fog shapes ecosystem processes. Crown wetting, elemental deposition, and light scattering and absorption are fundamental mechanisms by which fog has been shown to influence water fluxes, productivity, and decomposition in hyper-arid to ever-wet regions. These impacts are ultimately mediated by the structure and composition of biological systems that allow fog capture and utilization of resource subsidies. Climate change, and changes in land use, ocean circulation, and atmospheric pollution are simultaneously altering the nature of fog itself, and the architecture of the ecosystems adapted to capture it. The coupling between atmosphere and biosphere in fog-en-shrouded areas raises new questions about past and future fog-dominated ecosystems, and their maintenance and diversity, in the face of global change.
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We studied the effects of leaf wetting on midday depression of photosynthesis regarding plant water balance and leaf morphological traits. The plants without leaf wetting showed a significant reduction in midday photosynthesis with a concomitant decrease with leaf conductance, because of lower leaf water potential (-1.3 MPa) due to excessive transpiration water loss. However, midday depression was not observed in the plants with leaf wetting. Lower contact angle between leaf surface and water droplet showed that tomato leaves have lower water repellency. However, water on the leaf surface completely dried within 20 min indicating that effect of water coverage on stomata for CO2 uptake was small. In addition, leaf wetting significantly decreased evaporative demands, which contributed to maintaining appropriate water balance and avoided midday stomatal closure, and it contributed to mitigation of midday depression of photosynthesis. Additional key words: plant water relations; photosynthetic rate; stomatal conductance; transpiration rate; whole-plant chamber. temperature; VPD-vapor pressure deficit; VPDair-to-leaf-vapor pressure deficit between air and leaf; Ψw-leaf water potential.
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Background and aims: The eco-hydrological significance of leaf wetting due to atmospheric water in arid and semiarid ecosystems is not well understood. In these environments, the inputs of precipitation or dew formation resulting in leaf wetting have positive effects on plant functioning. However, its impact on plant water relations may depend on the degree of leaf surface wettability. In this study we evaluated leaf wettability and other leaf traits and its effects on foliar water uptake and canopy interception in plant species of a Patagonian steppe. We also studied how leaf traits affecting wettability vary seasonally from growing to dry season. Method: Contact angle of a water droplet with the leaf surface, water adhesion, droplet retention angle, stomatal density, cuticular conductance, canopy interception and maximum foliar water uptake were determined in six dominant shrub species. Key results: All species increased its wettability during the dry season and most species were considered highly wettable. The leaf surface had very high capacity to store and retain water. We found a negative correlation between foliar water uptake and leaf hydrophilia. Conclusions: Despite the diversity of life forms including cushion shrubs and tall shrubs, as well as phenological variability, all species converged in similar seasonal changes in leaf traits which favor its wettability. Intercepted water by crowns and the extremely high capacity of retention of droplets on leaf surfaces can have a significant impact on eco-hydrological process in water limited ecosystems where most of the precipitation during the growing and the dry season may be dew or small rainfall events, which do not always increase soil water availability.
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Premise of the study: Trees in wet forests often have features that prevent water films from covering stomata and inhibiting gas exchange, while many trees in drier environments use foliar water uptake to reduce water stress. In forests with both wet and dry seasons, evergreen trees would benefit from producing leaves capable of balancing rainy-season photosynthesis with summertime water absorption. Methods: Using samples collected from across the vertical gradient in tall redwood (Sequoia sempervirens) crowns, we estimate tree-level foliar water uptake and employ physics-based causative modeling to identify key functional traits that determine uptake potential by setting hydraulic resistance. Key results: We show that Sequoia has two functionally distinct shoot morphotypes. While most shoots specialize in photosynthesis, the axial shoot type is capable of much greater foliar water uptake and its within-crown distribution varies with latitude. A suite of leaf surface traits cause hydraulic resistance, leading to variation in uptake capacity among samples. Conclusions: Shoot dimorphism gives tall Sequoia trees the capacity to absorb up to 48 kg H2 O hr-1 during the first hour of leaf wetting, ameliorating water stress while presumably maintaining high photosynthetic capacity year-round. Geographic variation in shoot dimorphism suggests that plasticity in shoot-type distribution and leaf surface traits helps Sequoia maintain a dominate presence in both wet and dry forests. This article is protected by copyright. All rights reserved.
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In tall conifers, leaf structure can vary dramatically with height due to decreasing water potential (Ψ) and increasing light availability. This variation in leaf structure can have physiological consequences such as increased respiratory costs (Rm), reduced internal CO2 conductance rates (gi), and ultimately reduced maximum photosynthetic rates (Amax). In Picea sitchensis (Bong.) Carrière, leaf structure varies along the vertical gradient in ways that suggest compensatory changes to enhance photosynthesis, and this variation seems to be driven largely by light availability rather than by Ψ. These trends in leaf structure coupled with remarkably fast growth rates and dependence on moist environments inspire two important questions about P. sitchensis: 1) does foliar water uptake minimize the adverse effects of decreasing Ψ with height on leaf structure, and 2) do trends in leaf structure increase photosynthetic rates despite increasing height? To answer these questions, we measured foliar water uptake capacity, predawn (Ψpd) and midday (Ψmd) water potential, and gas-exchange rates as they varied between 25 and 89 m heights in 300-year-old P. sitchensis trees in northwestern California. Our major findings for P. sitchensis include: 1) foliar water uptake capacity was quite high relative to published values for other woody species, 2) foliar water uptake capacity increased between the crown base and treetop, 3) wet season Ψpd was higher than predicted by the gravitational potential gradient, indicating foliar water uptake, and 4) maximum photosynthetic rate increased with height, presumably due to shifts in leaf structure between the crown base and treetop mitigating height-related decreases in Amax. These findings suggest that together, the use of fog, dew, and rain deposits on leaves and shifts in leaf structure to conserve and possibly enhance photosynthetic capacity likely contribute to the rapid growth rates measured in this species.
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Ecohydrological monitoring technology is experiencing unprecedented expansion in capacity at ever lower costs. This allows for monitoring of systems at new scales spatially and allows for completely new strategies in observation. To represent the scale of this transformation, we present the framework for establishing a novel ecohydrological observation platform across the African continent (addressing the transformative opportunities made possible by wide-scale GPRS communication systems combined with solid-state sensing technology), as well as a strategy to leverage newly available accelerometer systems to monitor the dynamics of aboveground tree mass. The African observations are organized under the Trans-African Hydrometeorological Observatory (TAHMO.org), currently with about 500 installed stations across 20 African countries. Specific sensor technologies also open completely new approaches to measure key environmental variables. Aboveground mass of trees reflects, among other processes, the interception of rain, fog and snow, delivery of sap, addition of leaves, and loss of stem water. We demonstrate that passive sensing of tree acceleration due to wind can be used to evaluate change in mass caused by events such as leafing out or loss of leaves. We conclude by exploring the implications of ecohydrological observation at ever greater resolution and richness of variables.
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The Tibetan Plateau is considered as one of most sensitive region to global change. Nutrient (N and P) availability is an important limiting ecological factor in cold terrestrial ecosystems such as the alpine belt of the Tibetan Plateau. We focused on Corydallis hendersonii, an endemic alpine species of the Tibetan Plateau. Exploring the N and P below- and above-ground responses of C. hendersonii to climatic factors is crucial for biodiversity conservation of the alpine Tibetan plateau under global change. We used the Outlying Mean Index and regression analyses to assess N and P stoichiometry and biomass responses in leaves and roots of C. hendersonii along climatic gradients. We found that investment and allocation of nutrient and biomass in C. hendersonii were mainly driven by rainfall continentality. In the eastern less-continental wet area of the Tibetan plateau, C. hendersonii had higher biomass in leaf, and lower N and P investment in roots than in the western more continental dry part. Specifically, 300 mm year⁻¹ Mean annual precipitation (MAP) and ca. 80° Rainfall continentality index (GAMS) were threshold values of climate stress inducing strong nutrient limitation for C. hendersonii across the Tibetan Plateau. Our results suggest that rainfall continentality is the primary climatic driver of variation in biomass and nutrients allocation of C. hendersonii on the Tibetan Plateau. Thus, global warming and drying should induce a decrease in total biomass, a reduction in leaf N and P concentrations and an increase in root/shoot ratio in the alpine region of the Tibetan Plateau.
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The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day‐night conditions on tree growth remained uncertain. Here we present the first comprehensive field study of hourly‐resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 years. We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Species‐specific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close.
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The soil-water-plant-atmosphere system (SWPAS) is a “physically integrated, dynamic system in which interacting processes of mass and energy are performed.” The SWPAS system is comprised of four different components with varying physical and chemical properties which ultimately poses a complex mechanism. Water stress is primarily caused due to non-uniform precipitation. The exhaustion of this reservoir by a crop requires its artificial reloading, which is the case of irrigation. Soil moisture has been shown to have major implications for carbon storage and related climatic feedbacks. Therefore it is more important than ever to understand how the flow of water interacts with ecosystem health and the mechanisms controlling water fluxes at the land-atmosphere interface. Atmosphere acts as an upper buffer which takes up, transforms, and protects water, as a substance, in the climatic system. The SPAC is the pathway for water moving from soil through plants to the atmosphere. Movement of water occurs in response to differences in the potential energy of water. The flow path of water through SPAC is complex with a series of resistances offered by different components of the system. Different atmospheric, plant canopy and soil factors affects the water flow through SPAC. With increasing water scarcity, improvement in crop water productivity will be vital in terms of food security for the future generation. As a result, the effect of soil-plant-atmosphere (SPA) interactions on how ecosystems respond to and exert influence on, the global environment remains difficult to predict.
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This paper presents a numerical model to study the process of vapor condensation on surfaces characterized by film-wise condensation with the presence of Non-condensable gases (NCG). State variables in both the condensate film and the diffusion layer were solved separately and the condensation interface was used to couple the two solutions. The solution of the condensate film was obtained using well-established solutions of laminar film condensation of pure vapor. In contrast to other models surveyed, this work provides a inexpensive and accurate predictions of heat and mass transfer characteristics. We validated the work against two classical condensation problems. The model was first validated against empirical correlations and experimental work, resulting in a very good agreement. We then assessed the applicability of ignoring the condensate film effect, as performed in previous models, on the condensation processes by observing the thermal resistances of both the condensate film and diffusion layer. Results indicated that for the studied cases of NCG mass fractions above 20%, the condensate thermal resistance was at least an order of magnitude lower than that of the diffusion layer. However, the two thermal resistances seem to approach each other as NCG mass fraction becomes smaller. On another front, we observed that models that ignore the condensate film thermal resistance underestimate the interfacial temperature albeit accurately predicting the overall heat transfer rate. To simulate even lower NCG mass fractions, we validated our model to the classical analytical work of Sparrow and co-workers. Results showed a striking agreement between the two solutions at different NCG mass fractions (0.5%–10%) and subcooling degrees (5∘F–40∘F). Finally, we found a good agreement between results of our model and the heat/mass transfer analogy. The heat/mass transfer analogy is a semi-empirical method therefore, is limited to the existing correlations and their uncertainties. On the other hand, our model does not use any empiricism and relies on the available solutions of laminar condensate film of pure vapor in predicting the liquid side heat transfer coefficient.
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The cool-temperate (nemoral) zone is currently the most densely populated zone on Earth; its vegetation has therefore been altered considerably over the past centuries. Particularly in Europe and East Asia, anthropogenic land use (forest plantations, agricultural and settlement areas, including many neobiota) has replaced the summergreen broad-leaved forest, which mostly constitutes the natural zonal vegetation. Furthermore, under extremely oceanic conditions, there are evergreen laurel and Nothofagus forests on the western side of the Andes and the Southern Alps of New Zealand, and tall nemoral coniferous forests on the Pacific side of North America. A distinctive feature of the summergreen broad-leaved forests is the seasonal rhythm of the vegetation of the tree and field layers, from leaf development in spring to leaf shedding in fall, with abundant geophytes in spring.
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Experimental drought has been shown to delay the development of the root microbiome and increase the relative abundance of Actinobacteria, however, the generalizability of these findings to natural systems or other diverse plant hosts remains unknown. Bacterial cell wall thickness and growth morphology (e.g., filamentous or unicellular) have been proposed as traits that may mediate bacterial responses to environmental drivers. Leveraging a natural gradient of water‐availability across the coast redwood (Sequoia sempervirens) range, we tested three hypotheses: (a) that site‐specific water‐availability is an important predictor of bacterial community composition for redwood roots and rhizosphere soils; (b) that there is relative enrichment of Actinobacteria and other monoderm bacterial groups within the redwood microbiome in response to drier conditions; and (c) that bacterial growth morphology is an important predictor of bacteria response to water‐availability, where filamentous taxa will become more dominant at drier sites compared to unicellular bacteria. We find that both α‐ and β‐diversity of redwood bacterial communities is partially explained by water‐availability and that Actinobacterial enrichment is a conserved response of land plants to water‐deficit. Further, we highlight how the trend of Actinobacterial enrichment in the redwood system is largely driven by the Actinomycetales. We propose bacterial growth morphology (filamentous vs. unicellular) as an additional mechanism behind the increase in Actinomycetales with increasing aridity. A trait‐based approach including cell‐wall thickness and growth morphology may explain the distribution of bacterial taxa across environmental gradients and help to predict patterns of bacterial community composition for a wide range of host plants.
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Aim Drought stress has focused on water availability during the growing season, thus primarily on summer. However, variation in rainfall continentality can produce striking vegetation differences. We aim to disentangle summer water balance from winter rainfall continentality, to better understand how climate regulates the distributions of woody plants in the western USA. Location Western USA. Time period Actual. Major taxa studied Angiosperms and conifers. Methods We used redundancy analysis (RDA) to investigate correlations between rainfall continentality, summer water balance, minimum winter temperature and length of growing season on the distributions of 130 tree and shrub species in 467 plots. Rainfall continentality was calculated using the Gams index, modified for winter precipitation, and summer water balance with the ratio of summer precipitation to temperature. We estimated actual evapotranspiration (AET), deficit (DEF), mean annual temperature and rainfall from global gridded data sets and correlated them with RDA axes. Results Rainfall continentality measured with the Gams index and minimum temperatures best explained the contrast between oceanic vegetation in the Pacific Coast Ranges and continental vegetation in the Intermountain Region and Rocky Mountains. Growing season length (GSL) was the second strongest factor correlated with vegetation distributions. Summer water balance, despite being the most widely used climatic factor to assess drought stress in biogeography, was the third strongest factor correlating with vegetation classes of the western US. AET was equally correlated with RDA axes 1 and 3, and, thus, could not discriminate between the contrasts in the RDA. Main conclusions Rainfall continentality measured with the winter Gams index provides a more precise metric than summer water balance for understanding the biogeography of woody plants in the western USA. Broadly integrating the Gams index of continentality into plant distributions may improve our understanding of biogeographical distributions and predictions of responses to climate change.
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There is growing evidence that foliar water uptake (FWU) positively contributes to the water status of plants. However, the morpho-anatomical traits and the mechanisms responsible for variations in FWU of mangrove leaves are poorly understood. We evaluated the relationship between FWU and morpho-anatomical traits in eight mangrove species (Avicennia germinans, A. marina, A. schaueriana, Bruguiera gymnorhiza, Conocarpus erectus, Laguncularia racemosa, Rhizophora mangle, R. mucronata). FWU was measured in leaves experimentally submerged in distilled water and was related to morphological and anatomical traits using a Principal Component Analysis (PCA). There were inter-species differences in FWU, with L. racemosa and A. schaueriana having the highest values, and B. gymnorhiza and R. mucronata having the lowest. Three groups associated at the family level were found based on morpho-anatomical attributes: (1) greater thickness of the abaxial epidermis and specific leaf area (L. racemosa and C. erectus; Combretaceae); (2) presence of trichomes and salt glands and thicker palisade parenchyma (A. germinans, A. marina, and A. schaueriana; Acanthaceae), both families with relatively high FWU; and (3) higher leaf dry mass, leaf area, and thicker hypodermis (B. gymnorhiza and R. mucronata; Rhizophoraceae), but lower FWU. Our results suggest that mangrove species take up atmospheric water at different rates and strategies that depend on morpho-anatomical traits, suggesting its differential use as a supplementary resource.
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Directing a jet of humid air to impinge on a surface that is cooled below the dew point results in micro-sized water droplets. Lord Rayleigh discussed the phenomenon by contrasting clean to flame-exposed glass and called such behaviour Breath Figures (BF). Historically, utilizing dew as a water source was investigated by several scientists dating back to Aristotle. However, due to the degrading effects of air as a non-condensable gas (NCG) such efforts are limited to small scale water production systems and exhaled breath condensate (EBC) technology, to name a few. Recently, the concept of BF has been utilized extensively in the generation of micro-scale polymer patterns as a self-assembly process. However, the generation of BF on surfaces while being impinged by a humid air jet has not been quantified. In this work, we illustrate that a BF spot generated on a cooled surface is a manifestation of a recovery concentration. The concept is analogous to the concept of adiabatic-wall temperature defined for heat transfer applications. Upon closer examination of the vapor concentration distribution on a cooled impinged surface, we found that the distribution exhibits distinct regimes depending on the radial location from the center of the impingement region. The first regime is confined within the impingement region, whereas the second regime lies beyond this radial location including the wall jet region. Scaling analysis as well as numerical solution of the former regime shows that the maximum concentration on the surface is equivalent to its counterpart of a free unbounded jet with similar geometrical conditions. Additionally, the scaling analysis of the latter regime reveals that the jet speed and standoff distance are not important in determining the recovery concentration. However, the recovery concentration is found to vary monotonically with the radial location. Our conclusions are of great importance in optimizing jet impingement where condensation phase change is prevalent.
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How plants thrive in a cloud immersed environment where leaves are intermittently wet and the environmental conditions that drive transpiration and growth are limited, raises a relevant question in fog affected ecosystems. In order to provide insight into how cloud immersion and fog interception may affect Macaronesian laurisilva forests, micrometeorological variables, artificial fog water collection, throughfall, soil water content and the altitude of the trade wind inversion layer, together with the hourly sap flow, Qt, of a dominant tree species, Myrica faya, were measured at an exposed site of the Anaga Massif Rural Park Biosphere Reserve (Tenerife, Canary Islands) over a year period. Foggy conditions led to a 45.1% reduction in global radiation and a more than a 10-fold decrease in sap flow, throughout all day hours. M. faya showed a weak control of the transpiration rate and a profligate water use strategy, such that a substantial night time sap flow, Qtn, was observed under high nocturnal atmospheric evaporative demand, representing 23.3% of the total daily Qt, although fog was more frequent at night. Fog water interception resulted in canopy wetting and dripping for at least 55.0% of the time, and an associated downward xylematic sap transport in the most apical branches, i.e. in foliar water uptake. This represented 4.0% of the upward sap flow and was observed in 26.7% of the hourly Qt records. Nocturnal transpiration was also enhanced by the entry of previous foliar moisture. This general plant and climatic phenomenology was related at the mesoscale with the trade wind inversion height in the subtropical Macaronesia area.
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We investigated the effects of foliar absorption of dew by eastern white pine (Pinus strobus L.) seedlings on midday shoot water potential, as well as on other water relations variables and growth. Two-year-old container-grown eastern white pine seedlings were subjected to contrasting watering regimes (normal and deficient) and three frequencies of artificial dew (none, once and three times per week) for 10 weeks in a greenhouse. Midday shoot water potential was measured on four occasions during the study. Other water relations variables (relative water content, stomatal conductance, pressure-volume curves) and growth (hypocotyl diameter, aboveground dry mass, root dry mass) were also measured. Artificial dew significantly increased shoot water potential, stomatal conductance and seedling root growth, with greater responses observed for seedlings subjected to a deficient watering regime than for well-watered seedlings. Because dew can be a frequent microclimatic event in some areas, this finding has practical implications for field studies of water relations of eastern white pine and possibly of other coniferous species.
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We investigated the impact of drought on the physiology of 41-year-old Scots pine (Pinus sylvestris L.) in central Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Comparison was made between drought-treated plots and those receiving average precipitation. In response to drought, transpiration rate declined once volu-metric water content (VWC) over the top 20 cm of soil reached a threshold value of 12%. Thereafter, transpiration was a near linear function of soil water content. As the soil water deficit developed, the hydraulic resistance between soil and needles increased by a factor of three as predawn needle water potential declined from −0.54 to −0.71 MPa. A small but significant increase in xylem embolism was detected in 1-year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below −1.5 MPa. Basal area, and shoot and needle growth were significantly reduced in the drought treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current-year needle extension recovered, but a significant reduction in basal area increment was evident one year after the drought. The results suggest that, in response to soil water deficit, mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. Reduced growth in the year after a severe soil water deficit is most likely to be the result of reduced assimilation in the year of the drought, rather than to any residual embolism carried over from one year to the next.
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At leaf water potentials (ψ) of around -2.5 MPa, detached leaves of Sloanea woollsii F.Muell., an Australian subtropical rainforest tree, were able to absorb small amounts of water vapour from a saturated atmosphere, but absorbed considerably more liquid water if their surfaces remained wet. When leaves attached to small branches exhibiting a ψ of -2 MPa were sprayed with water and maintained in a saturated atmosphere, leaf ψ returned to saturation values within about 6 h. In a further experiment, a ψ of -2 MPa was imposed on detached, forked branches. Branches were then exposed to a saturated atmosphere and leaves on one half of the fork were kept wet whilst the rest remained dry. Leaf ψ was measured periodically for both dry and wet leaves and in both cases was found to increase with time. This indicated that leaf surface water was imbibed by wet leaves and transported into the branch resulting in alleviation of low water potentials in the dry leaves. In the submontane rainforests in which S. woollsii occurs, extended periods with little or no rainfall occur regularly. Throughout the year, foliage is wet by fog or dew alone for about 25% of the time. It is suggested that the presence of leaf surface water during rainless periods when leaf Psi is low may be important for the survival of S. woollsii. The actual mechanism of foliar absorption is uncertain, but is likely to be direct imbibition through the cuticle.
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Leaf surface wetness, e.g., dewfall, has been shown to have a strong influence on photosynthetic CO2 exchange in native plants. The important influence of trichomes on leaf surface wettability has also been established. We evaluated the effect of leaf surface wetness on photosynthesis and yield in soybeans (Glycine max) for five isolines that varied in trichome density. Artificial misting was used to simulate the influence of natural dewfall as well as spray irrigation. Leaf trichomes had an important influence on droplet formation and the distribution and retention of liquid water on individual leaves, even though trichome densities were low compared with maximum values reported for native species. Greater water droplet formation and, thus, water repulsion occurred for isolines with greatest trichome density. Somewhat surprisingly, these isolines also have the greatest droplet retention. However, all isolines showed relatively low water repellency, along with reductions in CO2 assimilation that averaged about 15%. Isolines subjected to misting during the morning (simulated dewfall) also had lower aboveground(15%) and seed (19%) biomass, and total leaf area (14%) compared to control plants. Thus, surface wetting, either from natural events (e.g., dewfall) or spray irrigation, may lead to significant reductions in CO2 exchange and growth potential in agricultural species, as reported for native species.
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Above-ground parts of Phaseolus vulgaris L. plants were treated with artificial misty rain (‘rain’) in a growth chamber to investigate the effects of leaf wetness on photosynthetic performance. The following results were obtained. (1) Stomata closed completely within 2 min of the onset of continuous ‘rain’ application and gradually opened to half the original aperture by 60 min. The rate of CO2 exchange measured on such wet leaves changed in parallel with the changes in stomatal aperture and attained 60 to 70% of the control level by 1h. (2) The dependence of the rate of leaf photosynthesis, A, on the intercellular CO2 concentration, ci [A(ci) relationship], examined in thoroughly dried leaves which had been treated with ‘rain’ did not change until after 4 h of treatment. However, leaves treated for 6h showed discernible decreases in A at high ci (ci>500μmolmol −1). The photosynthetic rate of leaves treated with ‘rain’ for 24 h was reduced at all ci, and A at the ambient CO2 concentration of 350μmolmol−1 was 60 to 70% of that of the control level. The rate of photosynthesis did not recover even after 3 d of treatment of the plants in a dry environment. These results clearly indicate that leaf wetness causes not only instantaneous suppression of photosynthesis but also chronic damage to the photosynthetic apparatus. Potential effects of leaf wetness on photosynthetic performance in nature are also discussed.
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Fog is thought to influence ecological function in coastal forests worldwide, yet few data are available that illuminate the mechanisms underlying this influence. In a California redwood forest we measured water and nitrogen (N) fluxes from horizontally moving fog and vertically delivered rain as well as redwood tree function. The spatial heterogeneity of water and N fluxes, water availability, tree water use, and canopy N processing varied greatly across seasons. Water and N fluxes to soil (annual average of 98% and 89%, respectively) across the whole forest occurred primarily in the rain season and was relatively even across the whole forest. In contrast, below-canopy flux of fog water and N declined exponentially from the windward edge to the forest interior. Following large fog events, soil moisture was greater at the windward edge than anywhere else in the forest. Physiological activity in redwoods reflected these differences in inputs across seasons: tree physiological responses did not vary spatially in the rain season, but in the fog season, water use was greater, yet water stress was less, in trees at the windward edge of the forest versus the interior. In both seasons, vertical passage through the forest changed the amount of water and form and concentration of N, revealing the role of the tree canopy in processing atmospheric inputs. Although total fog water inputs were comparatively small, they may have important ecosystem functions, including relief of canopy water stress and, where there is fog drip, functional coupling of above- and belowground processes.
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The semiarid regions of northwestern Venezuela have extremely low and highly unpredictable precipitation, yet these conditions support species with contrasting phenology and leaf longevity. Episodic rains significantly increased leaf water potential (from –5 to –2.5MPa) in several species and, in some cases, triggered flowering, leading us to hypothesize that the coexistence of species with contrasting phenology is due to differences in their ability to utilize small rainfall events. Irrigation treatments were used to simulate brief rainfall events, and the response of three species (Erythrina velutina [deciduous], Croton heliaster [semideciduous], and Capparis odoratissima [evergreen]) was monitored over a period of 14 months. To partition the effects of water reaching the canopy versus the soil, irrigation was supplied either in the form of mist to the canopy or by minisprinklers near the base of the trees. Nonirrigated trees were used as controls. Productivity (estimated as aboveground litter production) and water potential were enhanced by soil irrigation in two species. However, in the evergreen species canopy irrigation had a greater effect on water relations and productivity than soil irrigation, as indicated by higher predawn water potential, higher total annual flower (40gm–2year–1) and fruit (5gm–2year–1) production, and longer leaf longevity (410 days in control trees versus 520 days in canopy-irrigated trees). Canopy irrigation augmented flower and fruit production in all three species. Our findings suggest that reproductive phenology in these species is driven by episodic rains and that evergreen species may sustain productivity by their ability to make use of water deposited on leaf surfaces.
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Changes in leaf physiology with tree age and size could alter forest growth, water yield, and carbon fluxes. We measured tree water flux (Q) for 14 ponderosa pine trees in two size classes (12 m tall and ∼40 years old, and 36 m tall and ∼ 290 years old) to determine if transpiration (E) and whole-tree conductance (g t) differed between the two sizes of trees. For both size classes, E was approximately equal to Q measured 2 m above the ground: Q was most highly correlated with current, not lagged, water vapor pressure deficit, and night Q was <12% of total daily flux. E for days 165–195 and 240–260 averaged 0.97 mmol m–2 (leaf area, projected) s–1 for the 12-m trees and 0.57 mmol m–2 (leaf area) s–1 for the 36-m trees. When photosynthetically active radiation (I P) exceeded the light saturation for photosynthesis in ponderosa pine (900 µmol m–2 (ground) s–1), differences in E were more pronounced: 2.4 mmol m–2 (leaf area) s–1 for the 12-m trees and 1.2 mmol m–2 s–1 for the 36-m trees, yielding g t of 140 mmol m–2 (leaf area) s–1 for the 12-m trees and 72 mmol m–2 s–1 for the 36-m trees. Extrapolated to forests with leaf area index =1, the 36-m trees would transpire 117 mm between 1 June and 31 August compared to 170 mm for the 12-m trees, a difference of 15% of average annual precipitation. Lower g t in the taller trees also likely lowers photosynthesis during the growing season.
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Predawn leaf water potential (Yw) and xylem pressure potential (Yp) are expected to be in equilibrium with the soil water potential (soil Yw) around roots of well- watered plants. We surveyed 21 plant species (desert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil Yw, leaf Yw, and xylem Yp in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil- root hydraulic continuity. Most species failed to equilibrate fully with soil Yw, depending on whether leaf Yw or xylem Yp was used as the measure of predawn plant water potential. The contribution of night-time transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpiration) to plants with unbagged canopies. Nighttime transpiration significantly reduced predawn xylem Yp for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermiculatus. The contribution of nighttime transpiration to predawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not necessarily equilibrate with soil Yw. Plant xylem Yp or leaf Yw were significantly more negative than soil Yw for 15 of 15 species where soil Yw was measured. Predawn disequilibrium based on leaf Yw was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predominately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil Yw is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf Yw and xylem Yp may not reflect soil Yw, particularly for woody plants and halophytes, even under well-watered conditions.
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The discipline of environmental biophysics relates to the study of energy and mass exchange between living organisms and their environment. The study of environmental biophysics probably began earlier than that of any other science, since knowledge of organism-environment interaction provided a key to survival and progress. Systematic study of the science and recording of experimental results, however, goes back only a few hundred years. Recognition of environmental biophysics as a discipline has occurred just within the past few decades.
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The extensive early literature on foliar uptake of moisture, spanning nearly three centuries, has been reviewed by Stone (1957a, 1970) and Gessner (1956a). Early field studies by a number of workers demonstrated water uptake by leaves or stems of intact plants (Lloyd 1905; Wetzel 1924; Krause 1935). These and other laboratory experiments (see Stone 1957 a, 1970) led to a unanimity of opinion on the ability of leaves to absorb water, but a strong divergence of views on the ecological and physiological significance of this uptake. Remarkably, however, the flow of papers on the subject over the past 50 years has done little to resolve the questions of significance, and foliar water uptake remains as controversial a subject as ever.
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Coast redwood (Sequoia sempervirens) distribution and foggy conditions are causally linked by some authors. Others suggest fog is of little importance to the distribution of coast redwood because stratus, not fog, is more typical of weather conditions in these areas. Limited fog/stratus data from adjacent northern Californian areas with and without redwood are compared in this paper. Areas with redwood had more frequent early morning and early season stratus cover, but the frequency range within the redwood zone was substantial; fog was infrequent in both areas. This suggests that fog/stratus may not serve as a dominant factor for redwood distribution.
Book
1 Conducting Units: Tracheids and Vessels.- 2 The Vessel Network in the Stem.- 3 The Cohesion-Tension Theory of Sap Ascent.- 4 Xylem Dysfunction: When Cohesion Breaks Down.- 5 Hydraulic Architecture of Woody Shoots.- 6 Hydraulic Architecture of Whole Plants and Plant Performance.- 7 Other Functional Adaptations.- 8 Failure and "Senescence" of Xylem Function.- 9 Pathology of the Xylem.- References.
Article
The effect of drought and simulated daily dew on the water relations and photosynthetic capacity of lemon balm plants (Melissa officinalis L.) grown in Mediterranean field conditions was evaluated. Drought stress during the summer caused a large decrease in xilematic water potential (Ψw) and relative water content of leaves (RWC), and values around −3 MPa and 34%, respectively, were attained. Water-stressed plants could maintain these values for 2 weeks due to drastically decreased stomatal conductance. Different CO2 assimilation rate patterns were observed in M. officinalis plants depending on the water stress treatment studied and also the hourly distribution of radiation and vapour pressure deficit. During the summer, watered-plants showed a one-peaked photosynthetic daily pattern with a maximum peak during the morning. AB drought stress progressed the one-peaked photosynthetic daily pattern was maintained but the maximum photosynthetic daily peak observed earlier in the morning decreased by ca. 50%. Simulated daily dew applied to water-stressed plants caused a maintenance or even an improvement in CO2 assimilation rates depending on the time of day and also led to a complete recovery of plant water status and leaf pigment content. Dew may have an important role in rehydrating and reactivating the metabolism of water-stressed M. officinalis plants during the summer.
Article
Water was sprayed on the adaxial surfaces of hairy and nonhairy leaves to study the possible significance of trichomes in dew or rainwater absorption. Plant species adapted to Mediterranean climate, experiencing periodic water shortage, were used. Water retention was higher and its duration significantly longer on hairy leaves, confirming visual observations in the field. Gravimetric measurements and fluorescence microscopy with the apoplasmic indicator Calcofluor showed that surface water quickly penetrated into the mesophyll of hairy but not nonhairy leaves. The trichome did not participate in the entrance pathway. Direct absorption of water increased the water potential of water-stressed cut leaves, improving their photosynthetic performance through decreased abaxial stomatal resistance. No such effects were found in nonhairy leaves. In a long-term experiment with whole seedlings of the hairy Phlomis fruticosa L., growth rate and photosynthetic pigment content in plants receiving water only through the leaves were intermediate between those of well-watered and water-stressed plants. We conclude that leaf hairs, besides other functions, may also improve leaf water status by entrapping and retaining surface water, thus assisting in its final absorption into the mesophyll. Their contribution to drought avoidance may be critical under some circumstances. Key words: dew, drought, Phlomis fruticosa L., trichome.
Article
Seasonal and diurnal trends in carbon assimilation, stomatal conductance and leaf water potential were studied using 1–3 m tall saplings of Eucalyptus tetrodonta (F.Muell.). The study site was in an unburnt savanna near Darwin, where rainfall is strongly seasonal. Mean daily maximum assimilation rates ranged from 14.5 µmol m-2 s-1 in May to 4.8 µmol m-2 s-1 in October. There was a linear relationship between daily maximum assimilation rates and pre-dawn leaf water potential (r = 0.62, n = 508) and a log–log linear relationship between daily maximum stomatal conductance and pre-dawn leaf water potential (r = 0.68, n = 508). Assimilation rates and stomatal conductance were always higher in the morning than in the afternoon, irrespective of season. Stomatal conductance responded more strongly to leaf-to-air vapour pressure difference when pre-dawn leaf water potentials were moderately low (–0.5 to –1.5 MPa) than when they were very low (< –1.5 MPa) or high (> –0.5 MPa). Assimilation decreased sharply when temperature exceeded 35˚C. Seasonal trends in assimilation rate could be attributed primarily to stomatal closure, but diurnal trends could not. High leaf temperatures were a major cause of lower assimilation rates in the afternoon. Approximately 90% of leaves were lost by the end of the dry season, and above-ground growth was very slow. It is hypothesised that E. tetrodonta saplings allocate most photosynthate to root and lignotuber growth in order to tolerate seasonal drought and the high frequency of fire in northern Australian savannas.
Article
The forest water balance has never been studied in Reunion Island (Indian Ocean). This study focuses on the interception of fog water by Sophora denudata, an endemic tree, which provides an important water input into the hydrologic budget of the upper-montane forest. Canopy throughfall, rainfall and fog have been compared. The first data were obtained in 2001 in Nez de Bœuf, 2040 m asl, from manual rain gauges. The measurements were made during the day only. The aim was to propose a typology of events, to understand the spatial pattern of canopy throughfall, especially fogdrip, and their relation to the trade-wind direction. A second series of experiments, carried out in 2004 in Piton de Tangues, 2150 m asl, investigated how throughfall and atmospheric water varied with time, using automatic instruments such as the shielded Grunow-type fog collector. Here measurements were made continuously and night data were not excluded. Over a period of 8 months, the throughfall gauges, which were placed under the trees, indicated 1180 mm whereas the total amount of rainfall had reached only 948 mm. The difference (232 mm) is attributed to fog. Of 278 events, 234 showed fog contribution; fog occurred alone in 167 cases. The observations confirm what was found in Nez de Bœuf, namely that fog or rain can occur separately or together. The role of fog contribution to the forest water budget is significant: the spatial variation of canopy throughfall does not only depend on the type of event, but also on wind direction.
Article
: Epidermal hydathodes were found on leaves of 46 of 48 species of Crassula collected from the Namib Desert in southern Africa. The possibility that these structures might allow the absorption of surface water was investigated in 27 species (including subspecies). The presence of hydathodes on leaf epidermi correlated, in most cases, with increases in leaf thickness and enhanced rates of nocturnal, and sometimes diurnal, CO2 uptake following wetting of the leaves during the night. The precise nature of these responses varied depending on the species. In addition, wetting only the older leaves on the lower portion of the shoot of C. tetragona ssp. acutifolia not only resulted in increased thickness of these leaves, but also effected an increase in leaf thickness and stimulation of CO2 uptake rates in the distal, younger portion of the shoot that was not wetted. Overall, foliar hydathodes were implicated in the absorption of surface water in many species of Crassula such that the ecophysiology of these desert succulents was positively affected. Although rainfall in the Namib Desert is infrequent, surface wetting of the leaves is a more common occurrence as a result of nighttime dew or fog deposition. Presumably, species with hydathodes benefit directly from this source of moisture. These findings have important implications in understanding a relatively unexplored adaptation of some xerophytes to an extremely arid environment.
Article
Our common view on water uptake by terrestrial plants is that it occurs via absorption by roots from the soil substrate. However, it has long been known that plants exhibit alternative water-absorption strategies, particularly in drought-prone environments. Examples include many tropical epiphytic orchids which use a specialized structure called velamen radicum around their aerial roots for moisture absorption directly from the air (Capesius & Barthlott 1975), specialized trichomes in bromeliads (Andrade 2003, Benzing 1990), uptake by hydathodes into leaves of species inhabiting dry desert ecosystems of Namibia (Martin & von Willert 2000) and foliar absorption by coastal California redwoods during the summer fog season (Burgess & Dawson 2004). One of the most intriguing and yet, least-studied examples of adaptations to severe water limitation is found with desiccation-tolerant plants (also called resurrection plants). During drought periods, the water content of these plants can equilibrate with the low relative humidity of the air to the point that the plants appear dead. However, when water is supplied, these plants fully rehydrate (Alpert 2000, Bewley & Krochko 1982) and become physiologically active. Desiccation-tolerant vascular plants are rare in most ecosystems but diverse in tropical inselbergs (granitic outcrops; Porembski & Barthlott 2000). Relatively little is known about inselberg species particularly from an ecophysiological perspective (see Lüttge 1997 and Klüge & Brulfert 2000 for reviews).
Article
The effects of summer drought, dew deposition on leaves and autumn rainfall on plant water relations and diurnal variations of photosynthesis were measured in two evergreen shrubs, rosemary (Rosmarinus officinalis) and lavender (Lavandula stoechas), grown in Mediterranean field conditions. Withholding water for 40 d caused a similar decrease in predawn shoot water potential (ψpd) from c.−0.4 to c.−1.3 MPa in both species, but a 50% decrease in the relative leaf water content in L. stoechas compared with 22% in R. officinalis. A similar decrease in CO2 assimilation rates by c. 75% was observed in water-stressed plants of both species, although L. stoechas showed smaller photosynthesis: stomatal conductance ratio than R. officinalis (35 vs 45 μmol CO2:mol H2O). The relative quantum efficiency of photosystem II photochemistry also decreased by c. 45% at midday in water- stressed plants of both species. Nevertheless, neither L. stoechas nor R. officinalis suffered drought-induced damage to photosystem II, as indicated by the maintenance of the ratio Fv:Fm throughout the experiment, associated with an increase in the carotenoid content per unit of chlorophyll by c. 62% and c. 30%, respectively, in water-stressed plants. Only L. stoechas absorbed dew by leaves. In this species the occurrence of 6 d of dew over a 15-d period improved relative leaf water content by c. 72% and shoot water potential by c. 0.5 MPa throughout the day in water-stressed plants, although the photosynthetic capacity was not recovered until the occurrence of autumn rainfall. The ability of leaves to absorb dew allowed L. stoechas to restore plant water status, which is especially relevant in plants exposed to prolonged drought.
Article
Fog is a defining feature of the coastal California redwood forest and fog inputs via canopy drip in summer can constitute 30% or more of the total water input each year. A great deal of occult precipitation (fog and light rain) is retained in redwood canopies, which have some of the largest leaf area indices known (Westman & Whittaker, Journal of Ecology 63, 493–520, 1975). An investigation was carried out to determine whether some fraction of intercepted fog water might be directly absorbed through leaf surfaces and if so, the importance of this to the water relations physiology of coast redwood, Sequoia sempervirens. An array of complimentary techniques were adopted to demonstrate that fog is absorbed directly by S. sempervirens foliage. Xylem sap transport reversed direction during heavy fog, with instantaneous flow rates in the direction of the soil peaking at approximately 5–7% of maximum transpiration rate. Isotopic analyses showed that up to 6% of a leaf's water content could be traced to a previous night's fog deposition, but this amount varied considerably depending on the age and water status of the leaves. Old leaves, which appear most able to absorb fog water were able to absorb distilled water when fully submersed at an average rate of 0.90 mmol m2 s−1, or about 80% of transpiration rates measured at the leaf level in the field. Sequoia sempervirens has poor stomatal control in response to a drying atmosphere, with rates of water loss on very dry nights up to 40% of midday summer values and rates above 10% being extremely common. Owing to this profligate water use behaviour of S. sempervirens, it appears that fog has a greater role in suppressing water loss from leaves, and thereby ameliorating daily water stress, than in providing supplemental water to foliar tissues per se. Although direct foliar absorption from fog inputs represents only a small fraction of the water used each day, fog's in reducing transpiration and rehydrating leaf tissues during the most active growth periods in summer may allow for greater seasonal carbon fixation and thus contribute to the very fast growth rates and great size of this species.
Article
Summary 1. The ratio of leaf to sapwood area generally decreases with tree size, presumably to moderate hydraulic costs of tree height. This study assessed consequences of tree size and leaf area on water flux in Quercus garryana Dougl. ex. Hook (Oregon White Oak), a species in which leaf to sapwood area ratio increases with tree size. We tested hypo- theses that Q. garryana individuals of greater size and leaf area show reduced leaf- specific hydraulic conductance ( K L ), crown water flux per leaf area ( E L ), and carbon isotope discrimination ( ∆ ). 2. K L , E L and ∆ differed between trees of two size classes examined, with 25 m trees showing evidence of lower water flux and carbon isotopic discrimination compared to 10 m trees. Whole-tree water fluxes were smaller in 25 m than in 10 m trees both per unit sapwood and per leaf area, but more so per unit leaf area: 25 m trees had a leaf to sapwood area ratio 1·6 times greater than that of 10 m trees. 3. The findings from this study are unique in that increased leaf to sapwood area ratio of larger trees compounded hydraulic constraints on water transport due to tree height. These results provide further support and generality to the hypothesis of hydraulic limitations to tree-water flux, and show that limitations to water flux are not necessarily accompanied by the structural compensation of reduced crown leaf area in larger trees.
Article
ABSTRACTA model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO2 concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid-afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO2 assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θs, than at low atmospheric demand, but all curves of LE versus θs fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.
Article
ABSTRACTA mathematical model of stomatal conductance is presented. It is based on whole-plant and epidermal hydromechanics, and on two hypotheses: (1) the osmotic gradient across guard cell membranes is proportional to the concentration of ATP in the guard cells; and (2) the osmotic gradient that can be sustained per unit of ATP is proportional to the turgor pressure of adjacent epidermal cells. In the present study, guard cell [ATP] is calculated using a previously published model that is based on a widely used biochemical model of C3 mesophyll photosynthesis. The conductance model for Vicia faba L. is parameterized and tested As with most other stomatal models, the present model correctly predicts the stomatal responses to variations in transpiration rate, irradiance and intercellular CO2. Unlike most other models, however, this model can predict the transient stomatal opening often observed before conductance declines in response to decreases in humidity, soil water potential, or xylem conductance. The model also explicitly accommodates the mechanical advantage of the epidermis and correctly predicts that stomata are relatively insensitive to the ambient partial pressure of oxygen, as a result of the assumed dependence on ATP concentration.
Article
A new model of stomatal conductance is proposed which combines the essential features of the Ball–Berry–Leuning (BBL) and Tardieu–Davies (TD) models within a simple spatially aggregated picture of guard cell function. The model thus provides a coherent description of stomatal responses to both air and soil environments. The model also presents some novel features not included in either the BBL or TD models: stomatal sensing of intercellular (rather than leaf surface) CO2 concentration; an explanation of all three observed regimes (A, B and C) of the stomatal response to air humidity (Monteith Plant, Cell and Environment 18, 357–364, 1995); incorporation of xylem embolism; and maintenance of hydraulic homeostasis by combined hydraulic and chemical signalling in leaves (in which leaf epidermal hydraulic conductivity plays a key role). Significantly, maintenance of hydraulic homeostasis in the model does not require a direct feedback signal from xylem embolism, the predicted minimum leaf water potential being independent of xylem hydraulic conductivity. It is suggested that stomatal regulation through combined hydraulic and chemical signalling in leaves and/or roots provides a general mechanism enabling plants to maintain their water potentials above a minimum value. Natural selection of the key stomatal parameters would then set the minimum potential to a specific value determined by the most vulnerable plant process under water stress (e.g. cell growth, protein synthesis or xylem cavitation), depending on species and growth conditions.
Article
The gravitational component of water potential contributes a standing 0.01 MPa m−1 to the xylem tension gradient in plants. In tall trees, this contribution can significantly reduce the water potential near the tree tops. The turgor of cells in buds and leaves is expected to decrease in direct proportion with leaf water potential along a height gradient unless osmotic adjustment occurs. The pressure–volume technique was used to characterize height-dependent variation in leaf tissue water relations and shoot growth characteristics in young and old Douglas-fir trees to determine the extent to which growth limitation with increasing height may be linked to the influence of the gravitational water potential gradient on leaf turgor. Values of leaf water potential (Ψl), bulk osmotic potential at full and zero turgor, and other key tissue water relations characteristics were estimated on foliage obtained at 13.5 m near the tops of young (approximately 25-year-old) trees and at 34.7, 44.2 and 55.6 m in the crowns of old-growth (approximately 450-year-old) trees during portions of three consecutive growing seasons. The sampling periods coincided with bud swelling, expansion and maturation of new foliage. Vertical gradients of Ψl and pressure–volume analyses indicated that turgor decreased with increasing height, particularly during the late spring when vegetative buds began to swell. Vertical trends in branch elongation, leaf dimensions and leaf mass per area were consistent with increasing turgor limitation on shoot growth with increasing height. During the late spring (May), no osmotic adjustment to compensate for the gravitational gradient of Ψl was observed. By July, osmotic adjustment had occurred, but it was not sufficient to fully compensate for the vertical gradient of Ψl. In tall trees, the gravitational component of Ψl is superimposed on phenologically driven changes in leaf water relations characteristics, imposing potential constraints on turgor that may be indistinguishable from those associated with soil water deficits.
Article
Leaf surface wetness that occurs frequently in natural environments has a significant impact on leaf photosynthesis. However, the physiological mechanisms for the photosynthetic responses to wetness are not well understood. The responses of leaf CO2 assimilation rate (A) to 72 h of artificial mist of a wettable (bean; Phaseolus vulgaris) and a non-wettable species (pea; Pisum sativum) were compared. Stomatal and non-stomatal limitations to A were investigated. A 28% inhibition of A was observed in the bean leaves as a result of a 16% decrease in stomatal conductance and a 55% reduction in the amount of Rubisco. The decrease of Rubisco was mainly due to its partial degradation. In contrast to the bean leaves, a 22% stimulation of A was obtained in the 72 h mist-treated pea leaves. Mist treatment increased stomatal conductance by 12.5% and had no effect on the amount of Rubisco. These results indicated that a positive photosynthetic response to wetness occurred only in non-wettable species and is due to the change in stomatal regulation.