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Variation in monsoon precipitation drives spatial and temporal patterns of Larrea tridentata growth in the Sonoran Desert
Abstract
1. Broad-scale patterns of above-ground net primary production (ANPP) are closely coupled to climate features, particularly the distribution and magnitude of rainfall. In arid and semi-arid regions, however, the precipitation regime, together with local geomorphology and plant life history traits, combine to regulate soil water availability and patterns of growth, complicating simple correlations with climatic variables. 2. To better understand the drivers of plant growth in desert ecosystems, we characterized the rates and spatial heterogeneity of stem elongation by the dominant perennial shrub, creosote bush (Larrea tridentata) in the northern Sonoran Desert of Arizona (USA). Estimates of stem growth were made over a 5-year period (2006—2010) from 60 plots at 15 sites spanning c. 120 km across the Central Arizona—Phoenix (CAP) Long-term Ecological Research (LTER) area. 3. We observed both the highest and lowest rates of stem growth during summer, and these patterns were related to the amount of monsoon rainfall and local rates of water infiltration. The relationship between growth and precipitation in the summer was nonlinear, with rates increasing more than eightfold at plots receiving more than 100 mm of seasonal rainfall. Conversely, growth during the winter/spring was intermediate in magnitude, similar among years and poorly correlated with seasonal precipitation. 4. The spatial heterogeneity of stem growth also differed markedly between seasons and was greater both within and among sites during summer compared to winter/spring growing periods. At broad scales, spatial heterogeneity of shrub growth was correlated with seasonal changes in the spatial variability of rainfall across the study area. At small spatial scales, greater heterogeneity during the summer appears linked to local soil properties that influence infiltration and water availability following monsoon storms. 5. Overall, the strong, nonlinear growth response by L. tridentata to monsoon rainfall suggests that the recurrence interval of wet summer seasons is an important driver of ANPP for this long-lived shrub. More generally, our results illustrate how regional climate variability and local soil properties can interact to influence the rates and heterogeneity of desert plant growth at multiple scales.
... Results from many studies that tested enhanced precipitation or nutrient effects on the growth of shrub twigs/branches were inconsistent, and the interactive effects of water and nitrogen addition on twig size are seriously scarce. Many field experiments found that shrub twig/branch elongation was unresponsive (Lajtha and Whitford, 1989; Yahdjian and Sala, 2006) or responded positively (Fisher et al., 1988; Sponseller et al., 2012; Zhang et al., 2015) to increased precipitation. Similarly, nutrient enrichment resulted in either non-significant effects (Lajtha and Whitford, 1989; Hall et al., 2011) or positive effects (Fisher et al., 1988; Drenovsky and Richards, 2005) on shrub twig/branch elongation. ...
... Similarly, nutrient enrichment resulted in either non-significant effects (Lajtha and Whitford, 1989; Hall et al., 2011) or positive effects (Fisher et al., 1988; Drenovsky and Richards, 2005) on shrub twig/branch elongation. Twig elongation is an important aspect of shrub growth but not a complete surrogate of ANPP (Sponseller et al., 2012). However, few studies examined twig number response to precipitation and nitrogen addition. ...
... Our results are also in line with the second hypothesis that enhanced precipitation and nitrogen would increase desert shrubland productivity through altering shrub twig size and number. Our results support previous findings that water addition promotes shrub branch elongation (Fisher et al., 1988; Sponseller et al., 2012; Zhang et al., 2015). Unexpectedly, it was nitrogen addition that had significant effects on twig number, instead of water addition (Figure 1D ). ...
Shrublands are one of the major types of ecosystems in the desert regions of northern China, which is expected to be substantially more sensitive to global environmental changes, such as widespread nitrogen enrichment and precipitation changes, than other ecosystem types. However, the interactive effects of nitrogen and precipitation on them remain poorly understood. We conducted a fully factorial field experiment simulating three levels of precipitation (ambient, +20%, +40%) and with two levels of nitrogen deposition (ambient, 60 kg N ha⁻¹ yr⁻¹) in a desert shrubland in the Mu Us Desert of northern China. We used plant architectural traits (plant cover, volume, twig size and number) as proxies to predict aboveground net primary productivity (ANPP) of the dominant shrub (Artemisia ordosica Krasch), and assessed the responses of plant productivity and architectural traits to water and nitrogen addition. We found significant differences in twig size and number of A. ordosica under water and nitrogen treatments but not in shrub cover/volume, which suggest that twig size and number of the shrub species were more sensitive to environmental changes. The productivity of the overall community was sensitive to increased precipitation and nitrogen, and shrubs played a more important role than herbaceous plants in driving productivity in this ecosystem. Precipitation- and nitrogen-induced increases in vegetation production were positively associated with increases in twig size and number of the dominant shrub. Water addition enhanced the twig length of A. ordosica, while nitrogen addition resulted in increased twig density (the number of twigs per square meter). Water and nitrogen interacted to affect twig length, but not twig number and shrub ANPP. The trade-off, defined as negative covariance between twig size and number, was likely the mechanism underlying the responses of twig length and shrub ANPP to water and nitrogen interactions. Our results highlight the sensitivity of twig size and number as indicators to estimate shrub production and the mechanism underpinning desert shrub ANPP response to global environmental changes.
... Photosynthetic activity has been monitored following individual wetting events [Bamberg et al., 1976; Franco et al., 1994; Hamerlynck et al., 2000a Hamerlynck et al., , 2000b. Some studies considered winter rain most important for perennial shrubs [Beatley, 1974; Ehleringer et al., 1991; Snyder et al., 2004; Verburg et al., 2013] , while others found weaker or more complicated seasonal rela- tionships [Hamerlynck et al., 2000a; Reynolds et al., 2004; Barker et al., 2006; Sponseller et al., 2012] . In permanent plots, Hereford et al. [2006] found overall biomass changes at the Nevada Test Site (northern Mojave Desert) to correspond to multiyear wet and dry periods. ...
... and rain rates are steadier [Hereford et al., 2006]. However, in other studies, summer rains, which usually fall in convective storms [Hereford et al., 2006], were also observed to stimulate Larrea tridentata activity in the Mojave Desert [Hamerlynck et al., 2000a; Barker et al., 2006], as well as the Sonoran [Sponseller et al., 2012] and Chihuahuan Deserts [Reynolds et al., 1999]. Observed summertime responses were often restricted to soils with developed horizons [Hamerlynck et al., 2000a] or depended on seasonal precipitation conditions in the preceding winter [Reynolds et al., 1999] or summer [Sponseller et al., 2012]. ...
... However, in other studies, summer rains, which usually fall in convective storms [Hereford et al., 2006], were also observed to stimulate Larrea tridentata activity in the Mojave Desert [Hamerlynck et al., 2000a; Barker et al., 2006], as well as the Sonoran [Sponseller et al., 2012] and Chihuahuan Deserts [Reynolds et al., 1999]. Observed summertime responses were often restricted to soils with developed horizons [Hamerlynck et al., 2000a] or depended on seasonal precipitation conditions in the preceding winter [Reynolds et al., 1999] or summer [Sponseller et al., 2012]. Other investigations [e.g., Reynolds et al., 2004; McAuliffe and Hamerlynck, 2010] detected no correlations between Larrea tridentata LAI and subannual rainfall in the Mojave Desert. ...
The perennial shrub Larrea tridentata is widely successful in North American warm deserts but is also susceptible to climatic perturbations. Understanding its response to rainfall variability requires consideration of multiple time scales. We examine intra-annual to multiyear relationships using model simulations of soil moisture and vegetation growth over 50 years in the Mojave National Preserve in southeastern California (USA). Ecohydrological model parameters are conditioned on field and remote sensing data using an ensemble Kalman filter. Although no specific periodicities were detected in the rainfall record, simulated leaf-area-index exhibits multiyear dynamics that are driven by multiyear (∼3 years) rains, but with up to a 1 year delay in peak response. Within a multiyear period, Larrea tridentata is more sensitive to winter rains than summer. In the most active part of the root zone (above ∼80 cm), >1 year average soil moisture drives vegetation growth, but monthly average soil moisture is controlled by root uptake. Moisture inputs reach the lower part of the root zone (below ∼80 cm) infrequently, but once there they can persist over a year to help sustain plant growth. Parameter estimates highlight efficient plant physiological properties facilitating persistent growth and high soil hydraulic conductivity allowing deep soil moisture stores. We show that soil moisture as an ecological indicator is complicated by bidirectional interactions with vegetation that depend on time scale and depth. Under changing climate, Larrea tridentata will likely be relatively resilient to shorter-term moisture variability but will exhibit higher sensitivity to shifts in seasonal to multiyear moisture inputs.
... This framework has been verified in some grasslands (e.g., Heisler-White et al. 2008Thomey et al. 2011) but not others (e.g., Fang et al. 2005;Ross et al. 2012;Zhang et al. 2013). Studies in the Southwestern United States have focused on the effects of increased winter rainfall on increased deep soil water, thus favoring woody plants (deep rooted) over grasses (shallow rooted) (Brown et al. 1997;Gao and Reynolds 2003;Munson et al. 2012;Sponseller et al. 2012). However, little is known about the response of plants with crassulacean acid metabolism (CAM) and its competitive relationship with other functional groups (i.e., grasses) to changes in rainfall regime. ...
... Thus, a switch from drought tolerant species to drought intolerant species expands the domain in which plants respond negatively to a decrease in early growing season precipitation and increase in late growing season precipitation (Fig. 5b). Previous studies largely showed the competitive advantage of woody plants (deeprooted) over grasses (shallow-rooted) under conditions of increased winter rainfall and deep soil water (Brown et al. 1997;Gao and Reynolds 2003;Munson et al. 2012;Sponseller et al. 2012). This study investigates the effects of rainfall seasonality during the (summer) growing season when plants in drylands have high water stress (Schwinning and Sala 2004). ...
Background and aimsWe investigated the response of a model facultative CAM plant (Mesembryanthemum crystallinum) and its competition with a C3 grass (Bromus mollis) to changes in rainfall regime. Methods
Seedlings of M. crystallinum and B. mollis in both monoculture and mixtures growing in shallow and deep pots were subjected to three levels of intra-seasonal rainfall variability and rainfall seasonality in both high water and low water conditions. Response of plants were evaluated by nocturnal carboxylation and biomass. ResultsA high rate of water drainage beneath root zones in coarse soil led to a negative response of M. crystallinum and B. mollis in monoculture under increased intra-seasonal rainfall variability. Seasonal rainfall shifts to later dates during the growing season generally favored the growth of M. crystallinum and B. mollis in monoculture, with the exception of high water stress conditions whereby drought-intolerant species B. mollis was disfavored. Rainfall seasonality but not intra-seasonal rainfall variability affected nocturnal carboxylation by M. crystallinum in monoculture. Conclusions
We suggest that soil texture, root depth, and rainfall gradient are important mediators of plant growth under increased intra-seasonal rainfall variability. Drought severity and the ability of a plant to tolerate drought and can greatly affect its response to the seasonal timing of rainfall. Nocturnal carboxylation by M. crystallinum in response to rainfall variability depends on the timescale.
... Desert shrubland was insensitive to year-to-year variance in aridity ( Fig. 3G-I), and sensitivity did not increase when we integrated climate over the full year, suggesting that we did not miss a signal from winter precipitation. This result is consistent with prior work that identified monsoon precipitation as a key driver of creosotebush growth ( Sponseller et al. 2012). In contrast, Chihuahuan Desert grassland may benefit the most from CV in aridity when average conditions are arid ( Fig. 3B), as in the current climate window ( Fig. 2). ...
... Abundant fibrous roots, stoloniferous growth, and seed reproduction should give black grama a competitive advantage over creosotebush because all but the largest rain events increase water in shallow soil, where grass roots dominate over shrub roots (Pockman and Small 2010). The insensitivity of creosotebush to variance in aridity ( Fig. 3H) may occur because it can respond equally well to both winter/spring and summer precipitation events ( Allen et al. 2008, Sponseller et al. 2012). NPP-climate relationships under monsoon conditions could be additionally influenced by changes in the ratio of winter/ spring/monsoon precipitation, which were not captured by our monsoon SPEI metric, and likely deserve future investigation. ...
Understanding controls on net primary production (NPP) has been a long-standing goal in ecology. Climate is a well-known control on NPP, although the temporal differences among years within a site are often weaker than the spatial pattern of differences across sites. Climate sensitivity functions describe the relationship between an ecological response (e.g., NPP) and both the mean and variance of its climate driver (e.g., aridity index), providing a novel framework for understanding how climate trends in both mean and variance vary with NPP over time. Nonlinearities in these functions predict whether an increase in climate variance will have a positive effect (convex nonlinearity) or negative effect (concave nonlinearity) on NPP. The influence of climate variance may be particularly intense at ecosystem transition zones, if species reach physiological thresholds that create nonlinearities at these ecotones. Long-term data collected at the confluence of three dryland ecosystems in central New Mexico revealed that each ecosystem exhibited a unique climate sensitivity function that was consistent with long-term vegetation change occurring at their ecotones. Our analysis suggests that rising temperatures in drylands could alter the nonlinearities that determine the relative costs and benefits of variance in precipitation for primary production. This article is protected by copyright. All rights reserved.
... Thus, soil hydraulic characteristics intervene in the dynamics of water uptake by plants during the growing season . Recently, Sponseller et al. (2012) highlighted that assessing the responsiveness of dominant plant functional groups to variation in precipitation patterns and to local soil characteristics continues to be a research priority in arid and semiarid regions. ...
... The two communities are mixed in the landscape, they are subject to similar climatic conditions, and therefore, differences in growth among plants are more related to variation in water availability. Other authors highlighted that soil texture and local topography rules small-scale patterns of water availability to plants following precipitation events (McAuliffe 1994;Hamerlynck et al. 2004;Pockman & Small 2010;Sponseller et al. 2012). Similarly, soil characteristics can control the phenology of tropical trees (Cardoso et al. 2012), Larrea tridentata's physiological responses (Hamerlynck et al. 2000) and grass cover . ...
Arid land degradation diminishes the proportion of precipitation conducted to infiltration and increases the proportion lost to run-off and evaporation. Consequently, we expect that the effects of annual precipitation on shrub growth vary with land degradation as a result of changes in soil available water. Chuquiraga avellanedae is the dominant shrub and the main indicator of land degradation in semiarid rangelands of north-eastern Patagonia. We chose two communities with a different degree of land degradation: an herbaceous steppe with shrubs (HSS) and a degraded shrub steppe (SS). Vegetative growth of C. avellanedae was determined nondestructively using a double-sampling approach. Soil water content was estimated for the two communities using a soil water balance model. Linear regressions were used to evaluate the relationships between shrub growth and (i) annual precipitation and (ii) mean available water during the period of high vegetative growth in the soil layer that each plant community concentrates their roots. In SS, with elevated clay content, there were more roots of C. avellanedae in the upper layers of soil while in HSS, with coarse-textured soil, C. avellanedae had more roots in deeper layers. Vegetative growth of C. avellanedae, both in HSS and SS communities, was positively related to annual precipitation but, for a given precipitation, C. avellanedae presented higher vegetative growth in HSS than in SS. We also found a positive relationship between vegetative growth and soil available water, and this relationship did not differ between communities. SS presented lower water availability because of lower infiltration rates. Our results showed that, irrespective of the degree of land degradation, plants respond directly to water content of the soil layers where most roots are present at a specific window of time.
... However, such inferences have been challenged by recent observational and experimental results showing a surprising degree of variability in productivity responses to altered rainfall amounts and patterns across several grassland types (Knapp et al., 2002;Frank, 2007;Heisler-White et al., 2009;Cherwin & Knapp, 2012;Byrne et al., 2013;Zhang et al., 2013a,b). Much less is known about belowground net primary productivity (BNPP) responses to variations in precipitation amount (Frank, 2007;Byrne et al., 2013) and virtually all productivity responses to alterations in precipitation event size are limited to those aboveground (Knapp et al., 2002;Heisler-White et al., 2008Muldavin et al., 2008;Fay et al., 2011;Thomey et al., 2011;Cherwin & Knapp, 2012;Sponseller et al., 2012). While information about ANPP responses is integral for predictions of changes in key ecosystem services such as forage production, BNPP measures are critical for assessments of ecosystem carbon sequestration. ...
... In contrast to NMP, both TGP and SGP responded to watering pattern, but in opposing ways above-and belowground (Fig. 3a, b). A shift from the Many-Small to the Few-Large precipitation pattern had no effect on ANPP in TGP, contrary to previous studies in this grassland (Knapp et al., 2002;Heisler-White et al., 2009;Fay et al., 2011), whereas the Few-Large watering pattern significantly increased ANPP in SGP, as predicted by Knapp et al. (2008) and confirmed by several other studies (Heisler- White et al., 2009;Thomey et al., 2011;Sponseller et al., 2012). Watering pattern had less impact belowground for SGP, yet more for TGP. ...
Intensification of the global hydrological cycle with atmospheric warming is expected to increase inter-annual variation in precipitation amount and the frequency of extreme precipitation events. Although studies in grasslands have shown sensitivity of aboveground net primary productivity (ANPP) to both precipitation amount and event size, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and NPP. We conducted a two year experiment in three US Great Plains grasslands – the C4-dominated shortgrass prairie (SGP; low ANPP) and tallgrass prairie (TGP; high ANPP), and the C3-dominated northern mixed grass prairie (NMP; intermediate ANPP) – to test three predictions: (1) both ANPP and BNPP responses to increased precipitation amount would vary inversely with mean annual precipitation (MAP) and site productivity, (2) increased numbers of extreme rainfall events during high rainfall years would affect high and low MAP sites differently, and (3) responses belowground would mirror those aboveground. We increased growing season precipitation by as much as 50% by augmenting natural rainfall via (1) many (11-13) small or (2) fewer (3-5) large watering events, with the latter coinciding with naturally occurring large storms. Both ANPP and BNPP increased with water addition in the two C4 grasslands, with greater ANPP sensitivity in TGP, but greater BNPP and NPP sensitivity in SGP. ANPP and BNPP did not respond to any rainfall manipulations in the C3-dominated NMP. Consistent with previous studies, fewer larger (extreme) rainfall events increased ANPP relative to many small events in SGP, but event size had no effect in TGP. Neither system responded consistently above- and belowground to event size; consequently, total NPP was insensitive to event size. The diversity of responses observed in these three grassland types underscores the challenge of predicting responses relevant to C cycling to forecast changes in precipitation regimes even within relatively homogeneous biomes such as grasslands.This article is protected by copyright. All rights reserved.
... However, temporal variation in ANPP at the local scale is controlled by a variety of interacting factors, mainly the seasonal and spatial variability of precipitation event pulses [43]. In fact, research has shown that high rates of shrub production are triggered by water pulses during warm periods [44]. ...
... Compared with a 5 mm rain event, shrub species with deep roots would continue to take up water from the deeper soil after a large event [9,12], which could trigger a large productivity increase. For example, at the season scale, Sponseller et al. [44] indicated that rates of stem growth with a threshold of over 100 mm summer precipitation increased more than eightfold. Compared to previous research in grassland, this value is much higher than the one for a short-grass steppe, in which precipitation events of 15 mm to 30 mm contributed most of the effect of precipitation on ANPP [26]. ...
Rainfall events can be characterized as "pulses", which are discrete and variable episodes that can significantly influence the structure and function of desert ecosystems, including shifts in aboveground net primary productivity (ANPP). To determine the threshold and hierarchical response of rainfall event size on the Normalized Difference Vegetation Index (NDVI, a proxy for ANPP) and the difference across a desert area in northwestern China with two habitats - dune and desert - we selected 17 independent summer rainfall events from 2005 to 2012, and obtained a corresponding NDVI dataset extracted from MODIS images. Based on the threshold-delay model and statistical analysis, the results showed that the response of NDVI to rainfall pulses began at about a 5 mm event size. Furthermore, when the rainfall event size was more than 30 mm, NDVI rapidly increased 3- to 6-fold compared with the response to events of less than 30 mm, suggesting that 30 mm was the threshold for a large NDVI response. These results revealed the importance of the 5 mm and 30 mm rainfall events for plant survival and growth in desert regions. There was an 8- to 16-day lag time between the rainfall event and the NDVI response, and the response duration varied with rainfall event size, reaching a maximum of 32 days. Due to differences in soil physical and mineralogical properties, and to biodiversity structure and the root systems' abilities to exploit moisture, dune and desert areas differed in precipitation responses: dune habitats were characterized by a single, late summer productivity peak; in contrast, deserts showed a multi-peak pattern throughout the growing season.
... The response of B. eriopoda agrees with other drought and water addition experiments in mesic grasslands ( Zavaleta et al. 2003; Dukes et al. 2005; Suttle et al. 2007; but see Harpole et al. 2007), and confirms that drought can dramatically decrease production in desert grasslands (Knapp and Smith 2001; Gerten et al. 2008). The response by L. tridentata differs from that observed in the Sonoran Desert where above average monsoon rainfall lead to significant increases in production by L. tridentata (Sponseller et al. 2012). Nevertheless, our findings emphasize the variable importance of water availability for plant growth in arid ecosystems (Muldavin et al. 2008; Xia et al. 2010; Thomey et al. 2011). ...
... The mean and standard error on two axes are presented to compare drought (D), water addition (W), and ambient (A) precipitation plots. S shrubland, G grassland, M mixed vegetation where production during the summer monsoon can greatly exceed that of growth following more predictable winter rains (Sponseller et al. 2012). Nevertheless, abundance of L. tridentata at our site is likely to change only after prolonged ([5 years) severe drought, or in response to other drivers, such as extreme winter cold temperatures (Pockman and Sperry 1997; Medieros and Pockman 2011). ...
Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C(3) shrubs into C(4)-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass-shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C(4) grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C(3) shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland.
... Soil moisture carryover or legacy effects can be studied in North American deserts since water input occurs during both the winter and summer seasons (Sponseller et al., 2012). This bimodal regime is characterized by convective storms during the summer and widespread frontal storms during the winter (Scott et al., 2009;Scott & Biederman, 2019). ...
Net primary productivity in arid and semiarid regions is controlled by water availability for which rainfall has been a commonly used proxy at annual scales. However, the hydrological partitioning occurring through the water balance can also shape both seasonal and annual net ecosystem productivity. In this study, we used 10 years of water and carbon flux measurements in a mixed shrubland watershed of the Chihuahuan Desert to investigate the seasonal variability and controls on net ecosystem production. Over this period, the site exhibited a bimodal rainfall regime with an average of 211 and 67 mm for the wet (July-December) and dry (January-June) periods of the year, respectively. During the wet season, soil infiltration and channel transmission losses led to an average of 35.8 mm of water stored in the subsurface for subsequent dry periods. By contrast, dry seasons consumed 30.3 mm of stored sub-surface water to fulfill the ecosystem water demand, particularly during the springtime. In response, gross primary productivity occurred in equal amounts for both seasons, while ecosystem respiration was substantially higher during the wet season. This resulted in the mixed shrubland acting as an annual net sink of carbon (average of 153.2 g C m À2 year À1) of which 65% occurred during the dry season. We determined that the wet season provided water in excess of vegetation demands in that season, a portion of which was stored in the watershed subsurface for subsequent use, leading to a legacy effect. Gross primary productivity during the dry season was dependent on carryover soil moisture accessed by deep-rooted shrubs. Our coordinated observations of water-carbon dynamics revealed that water availability during the wet season can sustain the annual ecosystem productivity by impacting the subsequent dry season carbon uptake. K E Y W O R D S
... This result suggests that population variation in developmental patterns, for example in root:shoot ratios (Walters & Freeman, 1983), had a potentially stronger control on early growth and survivorship than resource availability. Several other studies documented relative insensitivity of L. tridentata stem growth to winter/spring precipitation (though not to summer precipitation) for Sonoran and Chihuahuan desert populations of L. tridentata (Muldavin et al., 2008;Reynolds et al., 1999;Sponseller et al., 2012). Still, reproductive cover was significantly lower in JT than in UT and FI (Figure 4) TA B L E 4 Climate regression models for stem growth of Ambrosia dumosa. ...
Widely distributed species are often locally adapted to climate gradients across their ranges. But little is known about the patterns of intraspecific adaptation in desert shrubs.
We examined the questions of local adaptation in multiple populations of two common shrub species of the winter‐wet Mojave Desert in North America in a multiple common garden experiment. Plants were raised in the greenhouse and transplanted at the age of 1 year. Ambrosia dumosa is a drought‐deciduous low shrub and Larrea tridentata is an exceptionally long‐lived evergreen. Over 4 years, we monitored growth, survivorship, leaf and reproductive cover and once measured leaf N content, δ¹³C and SLA. We hypothesized that populations of both species would be differentiated along a growth–survivorship trade‐off according to homesite aridity.
Both species exhibited previously undocumented population differences along gradients of winter precipitation and temperature. In general, populations from more winter‐mesic regions had faster growth in more mesic gardens and lower survivorship in the most arid garden. Homesites with more variable summer precipitation had greater growth for A. dumosa populations, but lower growth for L. tridentata. Among L. tridentata populations, leaf cover correlated positively with growth and negatively with survival time. For A. dumosa populations, growth and survival could not be attributed to specific traits across gardens. However, larger transplants had generally lower growth rates and higher survival rates across gardens, except in the driest garden, where the population averages of intrinsic water use efficiency (iWUE) and stem growth rate were positively correlated.
Synthesis. Two dominant species of the Mojave Desert adapted locally to variation in winter and summer precipitation and temperature. They did so in different ways, suggesting that L. tridentata mitigated the risk of hydraulic failure, while A. dumosa optimized carbon assimilation for growth.
... In mesic systems, addition of the common limiting nutrients, N and phosphorus (P), typically increases net primary production (NPP) while decreasing plant species diversity (Elser et al. 2007, LeBauer and Treseder 2008, Bobbink et al. 2010, Fay et al. 2015. In deserts, however, both water and nutrients limit NPP, and lack of water can diminish the effects of increased nutrient availability (Noy-Meir 1973, Hooper and Johnson 1999, Snyman 2002, Rao and Allen 2010, Yahdjian et al. 2011, Ladwig et al. 2012, Sponseller et al. 2012. These interactions between the effects of water and nutrients on NPP in arid and semiarid systems are also likely to affect the relationships among nutrient availability, species composition, and diversity. ...
Increased nitrogen (N) deposition threatens global biodiversity, but its effects in arid urban ecosystems are not well studied. In addition to altered N availability, urban environments also experience increases in other pollutants, decreased population connectivity, and altered biotic interactions, which can further impact biodiversity. In deserts, annual plant communities make up most of the plant diversity, support wildlife, and contribute to nutrient cycling and ecosystem processes. Functional trade‐offs allowing coexistence of a diversity of annual plant species are well established, but maintenance of diversity in urban conditions and with increased availability of limiting nutrients has not been explored. We conducted a 13‐yr N and phosphorus (P) addition experiment in Sonoran Desert preserves in and around Phoenix, Arizona, USA to test how nutrient availability interacts with growing season precipitation, urban location, and microhabitat to affect winter annual plant diversity. Using structural equation modeling and generalized linear mixed modeling, we found that annual plant taxonomic diversity was significantly reduced in N‐enriched and urban plots. Water availability in both current and previous growing seasons impacted annual plant diversity, with significant interaction effects showing increased diversity in wetter years and greater responsiveness of the community to water following a wet year. However, there were no significant interactions between N enrichment and water availability, urban location, or microhabitat. Lowered diversity in urban preserves may be partly attributable to increased urban N deposition. Changes in biodiversity of showy species like annual wildflowers in urban preserves can have important implications for connections between urban residents and nature, and reduced diversity and community restructuring with N enrichment represents a challenge for future preservation of aridland biodiversity.
... Because these parameters are often linked to plant physiology, wild and managed vegetation communities alike rely on the monsoon for cues associated with leaf out, growth, and senescence [2]. Mean and interannual variability of the monsoon ultimately controls overarching ecosystem parameters like nutrient pulses [3,4] and net primary production [5,6]. Where they are present, monsoons drive phenology. ...
In monsoonal ecosystems, vegetation phenology is generally modulated by the timing and intensity of seasonal precipitation. Seasonal precipitation is often characterized by substantial interannual variability in both space and time. A rigorous quantitative understanding of the ecology of the landscape requires spatially explicit information regarding the strength of the relationship between seasonal precipitation and vegetation phenology, as well as the interannual variability of the system. For this information to be accurately estimated, it must be based on spatially and temporally consistent measurements. The optical satellite image archive can provide these measurements. Satellite imagery offers observations of both a) atmospheric parameters such as the timing and spatial extent of monsoon cloud cover; and, b) phenological parameters, such as the timing and spatial extent of vegetation green-up and senescence. This work presents a method to capture both atmospheric and phenological parameters from an optical image time series. The method uses Empirical Orthogonal Function (EOF) analysis of a single spectral index for unified characterization of the spatiotemporal dynamics of both monsoon cloud cover and vegetation phenology. This is made possible by leveraging well-understood differences in the visible and near infrared reflectance of green vegetation, soil, and clouds. Image time series are transformed into a temporal feature space (TFS) that is comprised of low-order Principal Components. The structure of the temporal feature space reveals spatiotemporally distinct annual cycles of both cloud cover and vegetation phenology. In order to illustrate this technique, we apply it to the retrospective analysis of a seasonal cloud forest in the Dhofar Mountains of the southern Arabian Peninsula. Our results quantify known (but previously unmapped) local gradients in monsoon duration and vegetation community response. Individual ecological subsystems are also clearly distinguishable from each other, and consistent elevation gradients emerge within each subsystem. Novel observations also emerge, such as regreening/early greening events and spatial patterns in cloud duration. The method is conceptually straightforward and could be applied to characterize other monsoon environments anywhere on Earth.
... To simulate the annual growth cycles of creosotebush, the model simulates growth over 2 cycles each year: between October and April/May (winter/spring) and between April/May and October (summer) [82]. Due to the overlapping sequence of growth among years, we created a series of 2 year growth cycles for creosotebush (Figure 3). ...
... The study area has a semiarid climate and receives a bi-modal distribution of rainfall during the summer and winter with mean annual precipitation ranging from 403 to 472 mm (PRISM Climate Group, 2008). Spatial and temporal patterns of precipitation are largely controlled by the North American Monsoon system (Sheppard et al., 2002) resulting in unique responses of vegetation (Sponseller et al., 2012). Mean annual air temperature is approximately 16.5°C with a range from 9 to 25°C. ...
... Evapotranspiration demands decrease in the winter months when temperatures are cooler and plant transpiration is low, supporting maximum water flux, deep percolation, and the potential for a greater degree of mineral weathering. In comparison, the SCM sites are characterized by a bimodal precipitation regime, with ?50% of the annual rainfall delivered as short, intense summer rainfall events when evapotranspirative demand is high (Sala et al., 1992 ). Warmer summer temperatures drive increased evaporative rates, leading to substantial precipitation losses to evapotranspiration and limited fluxes of water through the soil profile (Cavanaugh et al., 2011; Sponseller et al., 2012). We speculate that the bimodal precipitation pattern in the SCM, specifically the high evaporative demands during summer rain events, leads to drier soil conditions and subsequently less mineral weathering compared with the Sierra Nevada soils. ...
Soils require study across semiarid ecosystems to better understand soil
organic C storage and landscape evolution in water-limited environments.
The objective of this research was to quantify soil morphologic development
in contrasting climate–vegetation zones and landscape positions along
a semiarid environmental gradient. Five ecosystems were examined across
the Santa Catalina Mountains, Arizona, that exhibit variation in precipitation
(45–95 cm yr−1), temperature (18–9°C), and vegetation (desert scrub to
mixed conifer). Granitic soil, saprock, and parent rock were sampled from
divergent summit and convergent footslope positions within each ecosystem.
Laser particle size analysis was combined with elemental analysis to
determine particle size distribution and total C for all soils. Harden’s profile
development index was applied to explore changes in soil development with
climate and landscape position. Soil organic C increased significantly from
0.37 to 1.1 kg m−3 in the transition from desert scrub to mixed conifer convergent
soils. Silt concentrations also increased significantly between the two
convergent field sites, with values increasing from 4.6 to 23 kg m−3. Profile
development indices more than doubled from the desert scrub to mixed conifer
sites. At the hillslope scale, indices were similar between desert scrub
divergent and convergent landscape positions. However, profile development
indices in mixed conifer convergent positions were twofold higher than those
of divergent sites, suggesting a stronger topographic control on soil development
in these forests. The results demonstrate links between water availability
and soil organic C accumulation, both regionally across climate–vegetation
zones and locally at the hillslope scale of study.
... In addition, soil resources show great variations in interplant spaces and beneath perennial plants (Sponseller, 2007;Cable et al., 2008;Su et al., 2013). Results from manipulative experiments show the rainfall pulses can exert different effects on plants, having no effect on deep-rooted plants and obvious influences on shallow-rooted plants Sponseller et al., 2012;Baez et al., 2013). Thus, vegetation structure may differ in interplant spaces and beneath shrubs, which can generate inconsistent GEP responses to environmental cues. ...
... The shrubland carbon sink in fall was nearly as large as in spring (Table 1), which has formerly been identified as the most productive period for creosotebush (Pennington & Collins, 2007;Muldavin et al., 2008). Although Sponseller et al. (2012) found that creosotebush production may be highest in response to wet summers, our results suggest that dry conditions may alternately favor spring and fall production (Table 1). During the dry years of our study, shrubland was a small carbon sink at every timescale of analysis, and the characteristics of shrubland carbon sequestration suggest this sink will persist in a more arid future climate. ...
The replacement of native C4-dominated grassland by C3-dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m−2 yr−1 on average, while nearby native C4 grassland was a net source of 31 g C m−2 yr−1 over this same period. Differences in C exchange between these ecosystems were pronounced - grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.
... For example, desert grassland plant productivity is strongly correlated with summer (August) precipitation, while desert shrubland plants may respond to both summer and winter precipitation. 10 This affects the balance between CO 2 uptake and loss in several ways, but generally, soil moisture and seasonal patterns of precipitation affect plant and microbial response and drive the variability in the balance between CO 2 uptake and loss. ...
Reducing atmospheric CO2 through enhanced
terrestrial carbon storage may help slow or reverse
the rate of global climate change. However,
information on how climate change in the Southwest
might affect the balance between CO2 uptake
and loss on semiarid rangelands is not easily
accessible to land managers.
• We summarize studies that focus on key components
of carbon exchange across the warm deserts
of North America to determine if common
trends exist that can be used in management.
• Management strategies that increase carbon sequestration
or decrease carbon loss are especially
important. Thus managers will need to know
what management practices are likely to promote
carbon storage or minimize losses during critical
time periods.
... Despite the results of other studies, summer precipitation comprises the largest proportion of total water input in the Chihuahuan Desert compared to other warm deserts, and L. tridentata has been shown to be physiologically responsive to summer precipitation (Reynolds et al. 1999). One study estimates that ;65% of the stem growth of L. tridentata in the Chihuahuan Desert is attributable to summer precipitation (Sponseller et al. 2012), in part due to its extensive system of fine roots in the top 30 cm of soil (Gibbens and Lenz 2001). ...
The performance of many desert plant species in North America may decline with the warmer and drier conditions predicted by climate change models, thereby accelerating land degradation and reducing ecosystem productivity. We paired repeat measurements of plant canopy cover with climate at multiple sites across the Chihuahuan Desert over the last century to determine which plant species and functional types may be the most sensitive to climate change. We found that the dominant perennial grass, Bouteloua eriopoda, and species richness had nonlinear responses to summer precipitation, decreasing more in dry summers than increasing with wet summers. Dominant shrub species responded differently to the seasonality of precipitation and drought, but winter precipitation best explained changes in the cover of woody vegetation in upland grasslands and may contribute to woody-plant encroachment that is widespread throughout the southwestern United States and northern Mexico. Temperature explained additional variability of changes in cover of dominant and subdominant plant species. Using a novel empirically based approach we identified "climate pivot points" that were indicative of shifts from increasing to decreasing plant cover over a range of climatic conditions. Reductions in cover of annual and several perennial plant species, in addition to declines in species richness below the long-term summer precipitation mean across plant communities, indicate a decrease in the productivity for all but the most drought-tolerant perennial grasses and shrubs in the Chihuahuan Desert. Overall, our regional synthesis of long-term data provides a robust foundation for forecasting future shifts in the composition and structure of plant assemblages in the largest North American warm desert.
... For example, differences in soil texture and structure (Sala et al. 1988, Loik et al. 2004), plant litter (Deutsch et al. 2010), or antecedent soil moisture conditions (Rawls et al. 1993) will influence the changes in soil moisture that result from a precipitation event. While beyond the scope of this study, research also indicates that landscape position (run-on/off) and soil properties may be a more important driver of ANPP in ecosystems like deserts than precipitation alone (Huenneke et al. 2002, Muldavin et al. 2008, Sponseller et al. 2012). While not as commonly available as long-term time series, soil moisture data can relate more directly to plant growth and help explain patterns in ANPP (Nippert et al. 2006, Muldavin et al. 2008). ...
Understanding drivers of aboveground net primary production (ANPP) has long been a goal of ecology. Decades of investigation have shown total annual precipitation to be an important determinant of ANPP within and across eco-systems. Recently a few studies at individual sites have shown precipitation during specific seasons of the year can more effectively predict ANPP. Here we determined whether seasonal or total precipitation better predicted ANPP across a range of terrestrial ecosystems, from deserts to forests, using long-term data from 36 plant communities. We also deter-mined whether ANPP responses were dependent on ecosystem type or plant functional group. We found that seasonal precipitation generally explained ANPP better than total precipitation. Precipitation in multiple parts of the growing season often correlated with ANPP, but rarely interacted with each other. Surprisingly, the amount of variation explained by seasonal precipitation was not correlated with ecosystem type or plant functional group. Overall, examining seasonal precipitation can significantly improve ANPP predictions across a broad range of ecosystems and plant types, with impli-cations for understanding current and future ANPP variation. Further work examining precipitation timing relative to species phenology may further improve our ability to predict ANPP, especially in response to climate change.
... For example, differences in soil texture and structure (Sala et al. 1988, Loik et al. 2004), plant litter (Deutsch et al. 2010), or antecedent soil moisture conditions (Rawls et al. 1993) will influence the changes in soil moisture that result from a precipitation event. While beyond the scope of this study, research also indicates that landscape position (run-on/off) and soil properties may be a more important driver of ANPP in ecosystems like deserts than precipitation alone (Huenneke et al. 2002, Muldavin et al. 2008, Sponseller et al. 2012). While not as commonly available as long-term time series, soil moisture data can relate more directly to plant growth and help explain patterns in ANPP (Nippert et al. 2006, Muldavin et al. 2008). ...
... Vegetation in the xeriscape sites included native trees such as palo verde (Parkinsonia spp.) and brittlebush (Encelia farinosa A. Gray ex Torr.). Plant growth patterns in the CAP LTER are regulated in part by the precipitation regime in the Sonoran Desert (Sponseller et al., 2012), with highly variable inter-annual and seasonal rainfall that averages 193 mm per year (NOAA, 2009). ...
Winter annual plants play an important role in arid and semiarid ecosystems because of their rapid response to resource pulses, which drive primary production that provides resources for herbivores and pollinators. Understanding the factors that control annual plant growth is key to predicting how arid and semiarid ecosystems will respond to changes in climate and resource availability from anthropogenic activities. We used a long-term nutrient enrichment experiment that spanned precipitation and urbanization gradients in central Arizona, USA, to examine the effects of climate, surface soil properties, soil nutrient availability and shrub cover on winter annual plant growth. At a landscape scale, aboveground net primary production (ANPP) of winter annual plants had a positive, nonlinear relationship to the amount of precipitation received from October through March of the current growing season. We found evidence for sequential resource limitation of ANPP initially from water then nitrogen and phosphorus. The resource limitation cascade was modified by surface soil properties and location relative to shrubs (under or between shrubs), highlighting the effect of small-scale factors on large-scale processes. Specifically, gravel cover had a negative effect on ANPP, and the effect of shrub cover on ANPP depended on nitrogen and current season rainfall. Our study emphasizes how small-scale factors, such as gravel cover, nutrient availability and presence of shrubs, can interact with large-scale drivers, such as seasonal precipitation, to affect interannual variation in winter annual plant production in the northern Sonoran Desert.
Graphical Abstract
Sonoran Desert landscape showing production of winter annuals 363 x 241mm (300 x 300 DPI)
The interactive relationships between ecological and hydrological processes drive plant performance, community structure, and community succession in arid areas. Yet the nature of potential hydrologic thresholds for responses of vegetation remains poorly understood. In this paper, we report on hydrologic thresholds associated with aboveground net primary production (ANPP) of Haloxylon ammodendron (HA) and sand-fixation region (SFR) between 1987 and 2012 in the ecotone of desert and oasis in the northwest China. In particular, we focused on precipitation and soil moisture dynamics. Our results showed that 1) ANPP and soil moisture of both HA and SFR decreased from 1987 to 2005, and then reached a stable state; 2) nonlinear models provided a much better fit to the data than linear models, highlighting the presence of a discontinuity in vegetation ANPP changes along precipitation and soil moisture gradients; 3) precipitation, accumulated between preceding-year June to current-year August, of <160 mm, or soil moisture at < 1.4–1.5% may decrease ANPP. Our results provide insights into the thresholds of precipitation and soil moisture in a long-term sand-fixing ecosystem, and highlight the importance of legacy precipitation for the recovery of the sand-fixing ecosystem. Consequently, the findings provide useful references for further understanding of the mechanisms of ANPP changes in a sand-fixing ecosystem with changes in precipitation and soil moisture.
The Central Arizona Project (CAP) is a large canal system that traverses hundreds of ephemeral stream channels in the Sonoran Desert. This longitudinal barrier alters flow during runoff events, causing water to pond behind the canal's wall. We asked: How has riparian vegetation of the ephemeral streams changed over the course of 35 years in response to canal construction? We compared field data (vegetation volume, woody plant stem density and size, and herbaceous cover) collected in distinct zones located upstream and downstream of the canal to unaltered controls.
By ponding water and inducing sedimentation of fine particles, the canal has created areas that support dense vegetation. The wettest zone has the greatest vegetation volume and plant height, and supports densities of Prosopis velutina that are six times greater than in the control zone. Larrea tridentata and other desert shrubs are displaced to the border of the wettest zone, but have greater height and stem diameter than typically occur in the desert owing to increased frequency of soil wetting in the canal-associated anthropogenic-floodplain. This research aids in understanding the impacts of canal barriers on desert riparian vegetation, and can be used to predict future outcomes of proposed canals in desert environments.
Rainfall is regarded as one of the important factors affecting productivity of grassland vegetation. To clarify the fluctuation of rainfall on different time scales and its consequential mechanism on steppe vegetation, a Hulunbeier meadow grassland was selected as a study object. Two parameters, rainfall concentration degree and deviated period, which can reflect the variations of rainfall annually, were weighed. A regression model was established based on net primary production (NPP), which was obtained from aboveground biomass spectral model, different periods of rainfall and its fluctuation factor. Subsequently, the effects of the annual rainfall variation on the primary productivity of the steppe vegetation were also analyzed. The results indicated that: 1) The period from April to July was the key factor, due to the great impact of accumulated precipitation on the meadow grassland primary productivity on Hulunbeier meadow grassland. The mean value of concentration degree (Cdk) was 0.439±0.182. Deviated period (dk) was 31.6 d, with a range from -3.6 to 94.2 days. 2) The prediction model of the meadow grassland productivity was set by precipitation amount during the key time, together with Cdk and dk, as y=-52.11+88.957Cdk+0.724dk+0.953Pk. It reflects the relationships between the NPP and the variance of precipitation amount with a prediction accuracy of 91.0%. Hence, the vegetation productivity model from remote sensing data was shown to have higher reliability, and its relevance to quadrat data was verified. ©, 2015, Editorial department of Molecular Catalysis. All right reserved.
The influences of previous-year precipitation and episodic rainfall events on dryland plants and communities are poorly quantified in the temperate desert region of Northwest China. To evaluate the thresholds and lags in the response of aboveground net primary productivity (ANPP) to variability in rainfall pulses and seasonal precipitation along the precipitation-productivity gradient in three desert ecosystems with different precipitation regimes, we collected precipitation data from 2000 to 2012 in Shandan (SD), Linze (LZ) and Jiuquan (JQ) in northwestern China. Further, we extracted the corresponding MODIS Normalized Difference Vegetation Index (NDVI, a proxy for ANPP) datasets at 250 m spatial resolution. We then evaluated different desert ecosystems responses using statistical analysis, and a threshold-delay model (TDM). TDM is an integrative framework for analysis of plant growth, precipitation thresholds, and plant functional type strategies that capture the nonlinear nature of plant responses to rainfall pulses. Our results showed that: (1) the growing season NDVIINT (INT stands for time-integrated) was largely correlated with the warm season (spring/summer) at our mildly-arid desert ecosystem (SD). The arid ecosystem (LZ) exhibited a different response, and the growing season NDVIINT depended highly on the previous year's fall/winter precipitation and ANPP. At the extremely arid site (JQ), the variability of growing season NDVIINT was equally correlated with the cool- and warm-season precipitation; (2) some parameters of threshold-delay differed among the three sites: while the response of NDVI to rainfall pulses began at about 5 mm for all the sites, the maximum thresholds in SD, LZ, and JQ were about 55, 35 and 30 mm respectively, increasing with an increase in mean annual precipitation. By and large, more previous year's fall/winter precipitation, and large rainfall events, significantly enhanced the growth of desert vegetation, and desert ecosystems should be much more adaptive under likely future scenarios of increasing fall/winter precipitation and large rainfall events. These results highlight the inherent complexity in predicting how desert ecosystems will respond to future fluctuations in precipitation.
A new analysis takes soil hydraulic properties, ring radius, depth of ring insertion, and depth of ponding into account. It also provides a means for determining the field-saturated hydraulic conducticity (Kfs) and the matric flux potential (φm). The analysis employs numerically determined shape factors (G) that are found to depend significantly on ring radius (a) and depth of ring insertion (d), but only slightly on depth of ponding (H) and soil hydraulic properties. Average G values (Ge) can be developed for specific d and a that apply to a wide range of ponded heads and soil types. Procedures for calculating Kfs and φm are based on G or Ge, and on the ponding of one, two, or multiple H levels in the ring. Test calculations based on Ge suggest that Kfs can be obtained with an accuracy of about ± 20% for H = 0.05 to 0.25 m and α = 1 to 36 m-1, where α is the soil parameter of the exponential hydraulic conductivity-pressure head relationship. -from Authors
1] This study investigates the causes for, and distribution of, unimodal versus bimodal seasonal cycle of vegetation greenness in the Southwest United States using extensive site observations, climate data, satellite data, and the Lund‐Potsdam‐Jena (LPJ) vegetation model. Peak vegetation greenness is achieved in a clockwise manner across the Southwest, beginning in spring in the Sonoran Desert following winter rains, then in Utah‐Colorado with snowmelt/summer rains, and finally in New Mexico–eastern Arizona with late summer monsoon rains. At high elevations, spring‐summer snowmelt is critical for supplying the necessary soil moisture to trigger vegetation growth. A bimodal seasonal cycle of vegetation greenness is evident in satellite data and LPJ simulations across eastern Arizona and western New Mexico, characterized by peaks during late spring–early summer and late summer–early autumn. This bimodal green‐up remains a pressing paradox for which many competing hypotheses exist. The mechanism for this seasonal pattern is demonstrated using LPJ and observational data and is found to deviate from the traditional pulse‐reserve paradigm. This paradigm states that rainfall events in arid lands produce nearly immediate pulses of vegetation growth and accumulation of reserves but does not consider cold dormancy, time‐lagged vegetation responses, or rainfall seasonality. The following soil moisture based mechanism for bimodal greening is proposed. The initial peak in vegetation greenness during late spring–early summer results from a break in cold dormancy and benefits from the gradual winter‐long accumulation of deep soil moisture from weak synoptic rain events and snowmelt in colder regions. Limited precipitation and ongoing transpiration, from the initial vegetation greening, trigger a midsummer drying of the soil and a consequential minimum in vegetation activity. Later, pulses of monsoon rainfall in late summer–early autumn support the secondary greening, although significant runoff of brief, intense rainstorms and substantial soil evaporation limit moisture to the upper soil layers.
A field experiment was conducted at the Jornada Long-Term Ecological Research (LTER) site in the Chihuahuan Desert of New Mexico to compare the rapidity with which the shrubs Larrea tridentata and Prosopis glandulosa utilized water, CO2 and nitrogen (N) following a simulated summer rainfall event. Selected plants growing in a roughly 50-m2 area were assigned to treatment and control groups. Treatment plants received the equivalent of 3cm of rain, while no supplemental water was added to the control plants. Xylem water potential (x) and net assimilation rate (Anet) were evaluated one day before and one and three days after watering. To monitor short-term N uptake, soils around each plant were labeled with eight equally distant patches of enriched 15N before watering. Each tracer patch contained 20ml of 20mM 15 NH4
15NO3 (99 atom%) solution applied to the soil at 20cm from the center of the plant at soil depths of 10 and 20cm. Nitrogen uptake, measured as leaf 15N, was evaluated at smaller time intervals and for a longer period than those used for x and Anet. Both Anet and x exhibited a significant recovery in watered vs. control Larrea plants within 3days after the imposition of treatment, but no such recovery was observed in Prosopis in that period. Larrea also exhibited a greater capacity for N uptake following the rain. Leaf 15N was five-fold greater in watered compared to unwatered Larrea plants within 2days after watering, while foliar 15N was not significantly different between the watered and unwatered Prosopis plants during the same period. Lack of a significant change in root 15 NO–
3 uptake kinetics of Larrea, even three days after watering, indicated that the response of Larrea to a wetting pulse may have been due to a greater capacity to produce new roots. The differential ability of these potential competitors in rapidly acquiring pulses of improved soil resources following individual summer rainfall events may have significant implications for the dynamic nature of resource use in desert ecosystems.
The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems.
Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition
are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting
infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured
soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15mm summer storm in the grassland–shrubland
ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (Ψs) increased to 30cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15cm of the
soil. The increase in Ψs was greater beneath grass canopies than beneath shrub canopies. Ψs under bare soil increased only to 5cm depth. The substantial redistribution of rainfall and different rooting depths of
the vegetation resulted in high Ψs throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the
shrub rooting volume. Consistent with this pattern, predawn water potential (ΨPD) of grasses increased more than 5MPa to greater than −1MPa whereas ΨPD of shrubs increased to −2.5MPa, a change of less than 2MPa. Transpiration increased roughly linearly with ΨPD in both grasses and shrubs. In grasses, assimilation was strongly correlated with ΨPD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture
following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and
assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during
periods without extreme drought.
Key wordsprecipitation pulses-surface hydrology-plant gas exchange-soil moisture-grass–shrub interactions-spatial pattern of soil moisture
Arid ecosystems experience prolonged dry periods, as well as storms that vary in size, intensity and frequency. As a result,
nitrogen (N) retention and export patterns may be a function of individual storm characteristics. Our objective was to determine
how seasonal patterns in rainfall as well as individual storm characteristics influence N transport and retention on terrestrial
hill slopes in a Sonoran Desert watershed. Regression models indicated that variation in runoff ammonium (NH4+) was best explained by antecedent conditions (cumulative seasonal rainfall, days since last storm) while variation in runoff
nitrate (NO3−) was best explained by single storm characteristics, primarily rain NO3−. Increases in runoff NO3− along overland surface flowpaths were balanced by decreases in NH4+ during summer, with no change in dissolved inorganic nitrogen (DIN) concentration; a pattern consistent with nitrification.
Nitrate increases along flowpaths were not as strong during winter storms. Results indicate that NH4+ is transported from hillslopes to other parts of the catchment, including streams, and that nitrification occurs along surface
flowpaths, particularly during summer storms. These findings suggest that the extent to which a receiving patch is supplied
with NH4+ or NO3− depends on the distance runoff has traveled (flowpath length) and the length of the antecedent dry period. The extent and
configuration of fluvial reconnection amongst patches in the landscape following long drought periods likely determines the
fate of available N, whether N is processed and retained in the terrestrial or in the aquatic component of the watershed,
and the mechanisms involved. The nature of this fluvial reconnection is driven by the size, intensity and sequence of storms
in space and time.
Terrestrial gross primary production (GPP) is the largest global CO2 flux driving several ecosystem functions. We provide an observation-based estimate of this flux at 123 ± 8 petagrams of carbon
per year (Pg C year−1) using eddy covariance flux data and various diagnostic models. Tropical forests and savannahs account for 60%. GPP over
40% of the vegetated land is associated with precipitation. State-of-the-art process-oriented biosphere models used for climate
predictions exhibit a large between-model variation of GPP’s latitudinal patterns and show higher spatial correlations between
GPP and precipitation, suggesting the existence of missing processes or feedback mechanisms which attenuate the vegetation
response to climate. Our estimates of spatially distributed GPP and its covariation with climate can help improve coupled
climate–carbon cycle process models.
Modeling has played a crucial role in understanding the structural and functional dynamics of forest and grassland ecosystems in the past decades, but relatively few ecosystem models have been developed for deserts. Adapting an existing desert ecosystem model to new regions with different community components and environmental settings may testify to the generality of model applicability, further verify model structure and formulations, and provide new insight into understanding desert ecosystem functioning. In this paper, we use a desert ecosystem model that was originally developed for the Chihuahuan Desert, Patch Arid Land Simulator-Functional Types (PALS-FT), to estimate the aboveground annual net primary productivity (ANPP) of a creosotebush (Larrea tridentata)-dominated Sonoran Desert ecosystem in the Phoenix metropolitan area, home to the Central Arizona-Phoenix Long-Term Ecological Research Project (CAP LTER). We modified and parameterized the model using meteorological data, ecophysiological parameters for different plant functional types, and site characteristic data from the CAP LTER study area and an independent test site in the San Simon Valley of southeastern Arizona. Model predictions were validated and calibrated using field observations from the San Simon Valley test site. The results showed that PALS-FT was able to simulate ANPP of this typical Sonoran Desert ecosystem reasonably well, with a relative error of ±2.4% at the ecosystem level and generally less than ±25% at the functional-type level. We then used the model to simulate ANPP and its seasonal and inter-annual dynamics for a similar ecosystem in the CAP LTER study area. The model predicted average annual ANPP of 72.3 g m−2 y−1, ranging from 11.3 g m−2 y−1 to 229.6 g m−2 y−1 in a 15-year simulation. The simulated average ANPP of the Sonoran Desert ecosystem is close to field observations in other areas of the Sonoran Desert, and the range of variation also is close to that reported by other researchers for arid and semiarid ecosystems. The dynamics of ecosystem ANPP in response to fluctuations in annual precipitation simulated by the model agreed well with the known relationship between ANPP and precipitation in arid and semiarid systems. A closer examination of this relationship at the level of plant functional types further revealed that seasonal distribution of rainfall significantly affected ANPP. A comparison between the PALS-FT model prediction and two regression models for North American warm deserts showed that both regression models underestimated the Larrea ecosystem ANPP, while the process-based PALS-FT model provided the most accurate prediction among the three models. This study provides a validation for use of the PALS-FT model to investigate Sonoran desert ecosystem responses to environmental changes.
Interannual variability in aboveground net primary production (ANPP) was assessed with long-term (mean = 12 years) data from
11 Long Term Ecological Research sites across North America. The greatest interannual variability in ANPP occurred in grasslands
and old fields, with forests the least variable. At a continental scale, ANPP was strongly correlated with annual precipitation.
However, interannual variability in ANPP was not related to variability in precipitation. Instead, maximum variability in
ANPP occurred in biomes where high potential growth rates of herbaceous vegetation were combined with moderate variability
in precipitation. In the most dynamic biomes, ANPP responded more strongly to wet than to dry years. Recognition of the fourfold
range in ANPP dynamics across biomes and of the factors that constrain this variability is critical for detecting the biotic
impacts of global change phenomena.
A common assumption historically in ecology is evident in the term Balance of nature. '' The phrase usually implies that undisturbed nature is ordered and harmonious, and that ecological systems return to a previous equilibrium after disturbances. The more recent concepts of point equilibrium and static stability, which characterize the classical equilibrium paradigm in ecology, are traceable to the assumptions implicit in ''balance of nature. '' The classical equilibrium view, however, has failed not only because equilibrium conditions are ran in nature, but also because of our part inability to incorporate heterogeneity and scale multiplicity into our quantitative expressions for stability. The theories and models built around these equilibrium and stability principles have misrepresented the foundations of resource management, nature conservation, and environmental protection. In this paper, we synthesize recent developments that advance our understandings of equilibrium vs. nonequilibrium, homogen
Two years of water and nitrogen augmentation experiments on Larrea tridentata(Creosotebush)were carried out in a southern Californian warm desert wash plant community. Treatments consisted of control(C), water(W), water and soil nitrogen(W + SN), and soil nitrogen(SN). Quantitative pheological data and microclimatic measurements were collected prior to the onset of and during the growth period and treatments. Predawn and midday water potentials were lower in non-irrigated than irrigated individuals. Leaf conductance was higher in irrigated than in non-irrigated shrubs, with a maximum difference of 1cm s-1 observed in July 1984 under relatively low vapour pressure deficit conditions. Leaf production rates were significantly higher in the irrigated (W and W + SN)treatments than in non-irrigated(C and SN) treatments in 1984. Addition of soil nitrogen caused no increase in vegetative growth rates in 1984. In 1985, a drier year, there was only minimal growth during the spring and summer growth periods in the non-irrigated treatments, while W and W + SN treatments resulted in significantly higher leaf and shoot growth rates. Growth rates in 1985 were significantly higher in the W + SN treatment than in the W treatment. Reproductive growth was higher in the non-irrigated than the irrigated treatments, with the lowest reproductive activity noted in the W treatment.
Rates of nitrogen (N) deposition have increased in arid and semiarid ecosystems, but few studies have examined the impacts of long-term N enrichment on ecological processes in deserts. We conducted a multiyear, nutrient-addition study within 15 Sonoran Desert sites across the rapidly growing metropolitan area of Phoenix, Arizona (USA). We hypothesized that desert plants and soils would be sensitive to N enrichment, but that these effects would vary among functional groups that differ in terms of physiological responsiveness, proximity to surface N sources, and magnitude of carbon (C) or water limitation. Inorganic N additions augmented net potential nitrification in soils, moreso than net potential N mineralization, highlighting the important role of nitrifying microorganisms in the nitrate economy of drylands. Winter annual plants were also responsive to nutrient additions, exhibiting a climate-driven cascade of resource limitation, from little to no production in seasons of low rainfall (winter 2006 and 2007), to moderate N limitation with average precipitation (winter 2009), to limitation by both N and P in a season of above-normal rainfall (winter 2008). Herbaceous production is a potentially important mechanism of N retention in arid ecosystems, capable of immobilizing an amount equal to or greater than that deposited annually to soils in this urban airshed. However, interannual variability in precipitation and abiotic processes that limit the incorporation of detrital organic matter into soil pools may limit this role over the long term. In contrast, despite large experimental additions of N and P over four years, growth of Larrea tridentata, the dominant perennial plant of the Sonoran Desert, was unresponsive to nutrient enrichment, even during wet years. Finally, there did not appear to be strong ecological interactions between nutrient addition and location relative to the city, despite the nearby activity of nearly four million people, perhaps due to loss or transfer pathways that limit long-term N enrichment of ecosystems by the urban atmosphere.
The multiyear, root zone soil moisture redistribution characteristics in a semiarid rangeland in southeastern Arizona were evaluated to determine the magnitude and variability of deep-profile, wintertime soil moisture recharge. Intermittent observations from 1990 to 1998 of average volumetric soil moisture under shrub and grass cover showed that significant recharge beyond 0.30 m principally occurs only in the wintertime when the vegetation is senescent and does not use the infiltrating water. Using the physically based, variably saturated flow model HYDRUS, wintertime observations were modeled to determine the recharge of soil moisture at different depth intervals in the vadose zone. Two approaches were carried out to estimate the soil model parameters. The first was to use basic soils data from detailed profile descriptions in conjunction with pedotransfer functions. The second parameter estimation strategy was to use an automatic parameter search algorithm to find the optimal soil parameters that minimize the error between the model-computed volumetric water content and observations. Automatic calibration of the model was performed using the shuffled complex evolution algorithm (SCE-UA), and it proved possible to satisfactorily describe the vadose zone observations using a simplified description of the soil profile with optimal model parameters. Simulations with the optimized model indicate that significant recharge of vadose zone does occur well beyond 0.30 m in winter but that such recharge is highly variable from year to year and appears correlated with El Nino episodes. This water could serve as a source of plant water for deeper-rooted plants that are active during the subsequent spring season, thereby exploiting a niche that the more abundant, shallower-rooted plants that are active during the summer rainy season do not. However, the year-to-year variability of the winter precipitation and consequent deep soil moisture recharge indicates that the deeper-rooted vegetation in this region must retain the ability to obtain moisture from the near surface in order to meet its water demands if necessary.
Water availability is the primary constraint to aboveground net primary productivity (ANPP) in many terrestrial biomes, and it is an ecosystem driver that will be strongly altered by future climate change. Global circulation models predict a shift in precipitation patterns to growing season rainfall events that are larger in size but fewer in number. This "repackaging" of rainfall into large events with long intervening dry intervals could be particularly important in semi-arid grasslands because it is in marked contrast to the frequent but small events that have historically defined this ecosystem. We investigated the effect of more extreme rainfall patterns on ANPP via the use of rainout shelters and paired this experimental manipulation with an investigation of long-term data for ANPP and precipitation. Experimental plots (n = 15) received the long-term (30-year) mean growing season precipitation quantity; however, this amount was distributed as 12, six, or four events applied manually according to seasonal patterns for May-September. The long-term mean (1940-2005) number of rain events in this shortgrass steppe was 14 events, with a minimum of nine events in years of average precipitation. Thus, our experimental treatments pushed this system beyond its recent historical range of variability. Plots receiving fewer, but larger rain events had the highest rates of ANPP (184 +/- 38 g m(-2)), compared to plots receiving more frequent rainfall (105 +/- 24 g m(-2)). ANPP in all experimental plots was greater than long-term mean ANPP for this system (97 g m(-2)), which may be explained in part by the more even distribution of applied rain events. Soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile. These results indicate that semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns.
We investigated the relationships between foliar stable carbon isotope discrimination (Δ), % foliar N, and predawn water potentials (ψpd) and midday stomatal conductance (g
s) of Larrea tridentata across five Mojave Desert soils with different age-specific surface and sub-surface horizon development and soil hydrologies. We wished to elucidate how this long-lived evergreen shrub optimizes leaf-level physiological performance across soils with physicochemical characteristics that affect the distribution of limiting water and nitrogen resources. We found that in young, coarse alluvial soils that permit water infiltration to deeper soil horizons, % foliar N was highest and Δ, g
s and ψpd were lowest, while %N was lowest and Δ, g
s and ψpd were highest in fine sandy soils; Larrea growing in older soils with well-developed surface and sub-surface horizons exhibited intermediate values for these parameters. Δ showed negative linear relationships with % N (R
2=0.54) and a positive relationship with ψpd (R
2=0.14). Multiple regression analyses showed a strong degree of multicolinearity of g
s and Δ with ψpd and N, suggesting that soil-mediated distribution of co-limiting water and nitrogen resources was the primary determinant of stomatal behavior, which is the primary limitation to productivity in this shrub. These findings show that subtle changes in the soil medium plays a strong role in the spatial and temporal distribution and utilization of limiting water and nitrogen resources by this long-lived desert evergreen, and that this role can be detected through carbon isotope ratios.
In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?
The 'pulse-reserve' conceptual model--arguably one of the most-cited paradigms in aridland ecology--depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of 'pulses', 'triggers' and 'reserves' are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse-reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant 'rainfall pulse', (2) how do rainfall pulses translate into usable 'soil moisture pulses', and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant 'pulses' of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October-March) or summer (July-September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.
In dryland ecosystems, the timing and magnitude of precipitation pulses drive many key ecological processes, notably soil water availability for plants and soil microbiota. Plant available water has frequently been viewed simply as incoming precipitation, yet processes at larger scales drive precipitation pulses, and the subsequent transformation of precipitation pulses to plant available water are complex. We provide an overview of the factors that influence the spatial and temporal availability of water to plants and soil biota using examples from western USA drylands. Large spatial- and temporal-scale drivers of regional precipitation patterns include the position of the jet streams and frontal boundaries, the North American Monsoon, El Niño Southern Oscillation events, and the Pacific Decadal Oscillation. Topography and orography modify the patterns set up by the larger-scale drivers, resulting in regional patterns (102–106 km2) of precipitation magnitude, timing, and variation. Together, the large-scale and regional drivers impose important pulsed patterns on long-term precipitation trends at landscape scales, in which most site precipitation is received as small events (
Water availability limits plant growth and production in almost all terrestrial ecosystems. However, biomes differ substantially in sensitivity of aboveground net primary production (ANPP) to between-year variation in precipitation. Average rain-use efficiency (RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints. Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUE(max)) that is typical of arid ecosystems. RUE(max) was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.
We find that extreme temperature and precipitation events are likely to respond substantially to anthropogenically enhanced greenhouse forcing and that fine-scale climate system modifiers are likely to play a critical role in the net response. At present, such events impact a wide variety of natural and human systems, and future changes in their frequency and/or magnitude could have dramatic ecological, economic, and sociological consequences. Our results indicate that fine-scale snow albedo effects influence the response of both hot and cold events and that peak increases in extreme hot events are amplified by surface moisture feedbacks. Likewise, we find that extreme precipitation is enhanced on the lee side of rain shadows and over coastal areas dominated by convective precipitation. We project substantial, spatially heterogeneous increases in both hot and wet events over the contiguous United States by the end of the next century, suggesting that consideration of fine-scale processes is critical for accurate assessment of local- and regional-scale vulnerability to climate change.
• extreme climate
• RegCM3
• regional climate model
• United States
• CO2
How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications
for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus
among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already
be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the
1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.
Two years of water and nitrogen augmentation experiments on Larrea tridentata (creosote bush) were carried out in a southern Californian warm desert wash plant community. Treatments consisted of control (C), water (W), water and soil nitrogen (W + SN), and soil nitrogen (SN). Quantitative phenological data and microclimatic measurements were collected prior to the onset of and during the growth period and treatments. Predawn and midday water potentials were lower in nonirrigated than irrigated individuals. Leaf conductance was higher in irrigated than in nonirrigated shrubs, with a maximum difference of 1 cm s⁻¹ observed in July 1984 under relatively low vapor pressure deficit conditions. Leaf production rates were significantly higher in the irrigated (W and W + SN) treatments than in the nonirrigated (C and SN) treatments in 1984. Addition of soil nitrogen caused no increased in vegetative growth rates in 1984. In 1985, a drier year, there was only minimal growth during the spring and summer growth periods in the nonirrigated treatments, while the W and W + SN treatments resulted in significantly higher leafand shoot growth rates. Growth rates in 1985 were significantly higher in the W + SN treatment than in the W treatment. Reproductive growth was higher in the nonirrigated than the irrigated treatments, with the lowest reproductive activity noted in the W treatment.
We tend to view arid environments as harsh and water-limited, due to low amounts of precipitation. In fact, many sites within desert regions receive considerable amounts of water due to run-on; for example, washes (arroyos, wadis). The productivity of these can be high. However, in reality, their extent is small. We often assume productivity of deserts to be only water-limited. Recent research in arid ecosystems indicates, however, that nutrients can be critical to production. We also think of desert precipitation as being unpredictable, in addition to low in quantity. Nevertheless, many of the major arid lands of the world are characterized by highly seasonal patterns of precipitation. Native plant and animal populations have evolved numerous tactics which capitalize on the predictable timing of precipitation. Humans have learned to exploit this predictability by growing crops and raising livestock that can gain from seasonal patterns. Humans also manipulate arid environments by removing the limits to productivity through irrigation and fertilization and, while some cases of agricultural production are noteworthy, the reality is that the cost/benefit ratio is not favorable. Furthermore, extensive arid and semi-arid areas are undergoing rapid desertification. Monitoring desertification is essential to our future understanding of arid and semi-arid land productivity.
Large areas of semiarid grasslands in the southwestern United States have been virtually replaced by shrubs during the past century. Understanding the causes and consequences of such vegetation dynamics requires that we elucidate the interplay between external forces of change (e.g., climate, human impacts) and the internal forces within these ecosystems that foster resilience and/or stability. Several conceptual models of arid ecosystems address this interplay by including the potential role of autogenic shrub effects on ecosystem processes, which lead to the formation of 'resource islands' and tend to promote shrub persistence. Specifically, during the process of shrub establishment and maturation, the cycling of nutrients is progressively confined to the zones of litter accumulation beneath shrubs, while bare intershrub spaces become increasingly nutrient poor. As shrub resource islands develop, there is increased interception and stemflow by shrub canopies, confining infiltration of nutrient-enriched rainfall directly beneath the shrubs; the barren intershrub spaces generate overland flow, soil erosion by wind and water, and nutrient losses. These islands are preferred sites for the regeneration of shrubs and herbaceous plants and are correlated with spatial variation in soil microbial populations and soil microfauna that promote nutrient cycling. If further changes in the transition between grassland and shrubland are to be correctly predicted - or if we wish to intervene and redirect transitions - we must develop a greater mechanistic understanding of the structural and functional relationships between shrubs and the resource islands associated with them. We conducted a 3-yr field study in the Jornada Basin of southern New Mexico to explore the relationships between seasonal manipulations of soil water and its impact on soil nutrient dynamics of resource islands and shrub growth and physiology. At our study site, where total annual precipitation is ~230 mm (~65% falls during the summer period), we simulated seasonal drought in summer (1 June-30 September) and winter/spring (1 October-31 May) by constructing large rainfall-exclusion shelters over shrub resource islands at different stages of development. Our experiment tests two principal hypotheses. The first is that the two major shrub species in the Jornada Basin, creosotebush (Larrea tridentata) and mesquite (Prosopis glandulosa), have different growth phenologies, rooting patterns, and physiological responses to resource availability (primarily water). The second is that different size classes of shrubs ('small' and 'large') represent distinct stages of resource island development (i.e., 'young' and 'mature,' respectively) and, hence, different stabilities - that is, as islands develop, their associated shrubs become less coupled to short-term fluctuations in precipitation and more resistant to long-term drought or climate shifts. With regard to the first hypothesis, we conclude that the two species are relatively similar in function despite the different phenological 'strategies' of Larrea (evergreen) and Prosopis (winter deciduous). In the absence of drought, both species exhibited maximal rates of shoot and root growth, as well as high photosynthesis and transpiration, in late spring. This remained as the period for maximal growth and physiological activity for Prosopis shrubs that experienced drought in either summer or winter/spring. On the other hand, Larrea shrubs that experienced drought in winter/spring had maximal growth and activity shifted to the summer period, and in the absence of drought, Larrea shrubs also exhibited high physiological activity during the summer (especially following high rainfall). Thus, Larrea appears to have a greater capacity for shifting its activity patterns to alternate periods to take advantage of changes in resource availability. Shrubs of both species appeared well adapted to withstand season-long droughts. Mechanisms for survival include the following capacities: (1) to shift growth and physiological activity to utilize different temporal moisture (Larrea); (2) to utilize different levels of soil water (both species); (3) to carry out limited physiological activity and growth during drought (especially Larrea); and (4) to compensate for some negative impacts of drought through enhanced physiology (especially Prosopis) and growth (especially Larrea) in the season following drought. With regard to the second hypothesis, we again found more similarities than differences between the different aged (young vs. mature) islands. The stage of maturity of a resource island complex did not seem to be a significant factor to the growth and physiological activity of the shrub.
The interaction of the North American Monsoon with watershed hydrology and landscape response is evaluated by observ- ing geomorphic characteristics of hillslopes, hydrology, and stream channels in two mountain ranges with contrasting intensity of precipitation. The study compares wa- tersheds in the Hualapai and Santa Cata- lina Mountains in Arizona, which are sim- ilar in lithology, elevation, tectonic setting, vegetation, and annual precipitation, but differ in the proportion of precipitation re- ceived in summer thunderstorms. The Hu- alapai Mountains receive most of their pre- cipitation in winter, whereas rainfall in the Santa Catalina Mountains occurs mostly in summer. Drainages in the Santa Catalinas are more variable in local relief and exhibit much more exposed bedrock and higher drainage density. The trunk channel of a major drainage in the Santa Catalina Mountains has measured discharges several orders of magnitude greater than a channel draining a comparable area in the Huala- pai Mountains. The alluvial segment of the channel in the Santa Catalina piedmont ad- ditionally displays greater concavity, small- er width-to-depth ratios, and a larger cali- ber of bed material. In contrast to conventional interpretation, summer mon- soonal precipitation is not primarily re- sponsible for generating most of the dis- charges that modify channels in large-scale drainages. In the monsoonal climate regime of the Santa Catalinas, small basins flood most often in summer, whereas larger drainages exhibit peak discharges in re- sponse to low-intensity winter precipitation. We attribute the paradoxical discharge re- sponse of larger drainages to the small spa- tial scale of summer thunderstorms, which fail to deliver enough precipitation to gen- erate floods in these larger basins, but do prime hillslopes by stripping colluvium and lowering hillslope-infiltration rates, making the large drainages more responsive to ar- eally extensive winter storms.
Onset and end of the wet season are examined using daily rainfall observations obtained from gauges. At most grid points, composite and interannual variations of onset and end are well-defined, although among individual stations that make up a grid average, variability is large. The wet season rain rate is a combination of the rainy day average, which decreases from the coast, and the frequency of daily precipitation, which is largest over the Sierra Madre Occidental. Thus the maximum total rate lies slightly to the west of the highest orography, as noted by Gochis et al. (2004). Although there are some exceptions, onset is not well-correlated with early season total precipitation, but in most areas at least 25% of the variance of late season total precipitation is explained by the end date. Correlations are small between the frequency of daily precipitation and the rainy day rate. The frequency is better correlated with the total rate than is the average amount per event. As noted in previous studies, summer rainfall in central to southern Mexico exhibits moderate negative correlations with sea surface temperature in an area that projects onto the equatorial Pacific pattern associated with El Niño. The correlation seems to be mainly through variability in the frequency of rainfall, rather than amounts per event.
This study provides an empirical description of intraseasonal rainfall variability within the North American monsoon (NAM) region. Applying particular definitions to historical daily rainfall observations, it demonstrates that distinct intraseasonal rainfall modes exist and that these modes differ considerably from the monsoon core region in northwest Sonora ( SON), California, to its northward extension in southeast Arizona (AZ). To characterize intraseasonal rainfall variability (ISV), separate P-mode principal component ( PC) analyses were performed for SON and AZ. The results indicate that in each area, much of the ISV in rainfall can be described by three orthogonal modes. The correlations between ISV modes and total seasonal rainfall reinforce the notion of differing behaviors between the monsoon's core and extension. For SON all three ISV modes exhibit significant correlation with seasonal rainfall, with the strongest relationship in evidence for the ISV mode, which is related to rainfall intensity. For AZ, total rainfall exhibits the strongest correlation with the ISV mode, which emphasizes season length and rainfall consistency. Examination of longer-period behavior in the ISV modes indicates that, for SON, there is a positive linear trend in intensity, but a countervailing trend toward a shorter monsoon season along with less consistent rainfall in the form of shorter wet spells. For AZ, the evidence for trend in the ISV modes is not nearly as compelling, though one of the modes appears to exhibit distinct multidecadal variability. This study also evaluates teleconnectivity between ENSO, the Pacific decadal oscillation (PDO), and the NAM's intraseasonal rainfall variability. Results indicate that part of the intraseasonal rainfall variability in both SON and AZ is connected to ENSO while only SON exhibits a teleconnection with the long-period fluctuations of the PDO.
Precipitation regimes are predicted to become more variable with more extreme rainfall events punctuated by longer intervening dry periods. Water-limited ecosystems are likely to be highly responsive to altered precipitation regimes. The bucket model predicts that increased precipitation variability will reduce soil moisture stress and increase primary productivity and soil respiration in aridland ecosystems. To test this hypothesis, we experimentally altered the size and frequency of precipitation events during the summer monsoon (July through September) in 2007 and 2008 in a northern Chihuahuan Desert grassland in central New Mexico, USA. Treatments included (1) ambient rain, (2) ambient rain plus one 20 mm rain event each month, and (3) ambient rain plus four 5 mm rain events each month. Throughout two monsoon seasons, we measured soil temperature, soil moisture content (y), soil respiration (R s), along with leaf-level photosynthesis (A net), predawn leaf water potential (C pd), and seasonal aboveground net primary productivity (ANPP) of the dominant C 4 grass, Bouteloua eriopoda. Treatment plots receiving a single large rainfall event each month maintained significantly higher seasonal soil y which corresponded with a significant increase in R s and ANPP of B. eriopoda when compared with plots receiving multiple small events. Because the strength of these patterns differed between years, we propose a modification of the bucket model in which both the mean and variance of soil water change as a consequence of interannual variability from 1 year to the next. Our results demonstrate that aridland ecosystems are highly sensitive to increased precipitation variability, and that more extreme precipitation events will likely have a positive impact on some aridland ecosystem processes important for the carbon cycle.
Aridland ecosystems cover about one‐third of terrestrial environments globally, yet the extent to which models of carbon (C) and nitrogen (N) cycling, developed largely from studies of mesic ecosystems, apply to aridland systems remains unclear.
Within aridland ecosystems, C and N dynamics are often described by a pulse‐reserve model in which episodic precipitation events stimulate biological activity that generate reserves of biomass, propagules and organic matter that prime the ecosystem to respond rapidly to subsequent precipitation events.
The role of microbial C and N processing within the pulse‐reserve paradigm has not received much study. We present evidence suggesting that fungi play a critical and underappreciated role in aridland soils, including efficient decomposition of recalcitrant C compounds, N‐transformations such as nitrification, and nutrient storage and translocation of C and N between plants and biotic soil crusts. While fungi may perform some of these functions in other ecosystems, this ‘fungal loop’ assumes particular importance in the N cycle in aridlands because water availability imposes even greater restrictions on bacterial activity and physicochemical processes limit accumulation of soil organic matter (SOM).
We incorporate these findings into a Threshold‐Delay Nutrient Dynamics (TDND) model for aridland ecosystems in which plant responses to pulsed precipitation events are mediated by a fungal loop that links C and N cycling, net primary production (NPP) and decomposition in aridland soils.
Synthesis . Arid ecosystems are highly sensitive to global environmental change including N deposition and altered precipitation patterns; yet, models from mesic ecosystems do not adequately apply to aridland environments. Our ‘fungal loop’ N cycle model integrates spatial structure with pulse dynamics and extends the pulse‐reserve paradigm to include the key role of microbial processes in aridland ecosystem dynamics.
1Quantifying the effects of individual- and population-level processes on plant-community structure is of fundamental importance for understanding how biota contribute to the flux, storage and turnover of matter and energy in ecosystems.2Here we synthesize plant-allometry theory with empirical data to evaluate the roles of individual metabolism and competition in structuring populations of the creosote Larrea tridentata, a dominant shrub in deserts of southwestern North America.3At the individual level, creosote data support theoretical predictions with regard to the size dependence of total leaf mass, short-term growth rates of leaves and long-term growth rates of entire plants. Data also support the prediction that root–shoot biomass allocation is independent of plant size.4At the population level, size–abundance relationships within creosote stands deviate strongly from patterns observed for steady-state closed-canopy forests due to episodic recruitment events. This finding highlights that carbon storage and turnover in water-limited ecosystems can be inherently less predictable than in mesic environments due to pronounced environmental forcing on demographic variables.5Nevertheless, broad-scale comparative analyses across ecosystems indicate that the relationship of total abundance to average size for creosote populations adhere to the thinning rule observed and predicted by allometry theory. This finding indicates that primary production in water-limited ecosystems can be independent of standing biomass due to competition among plants for resources.6Our synthesis of theory with empirical data quantifies the primary roles of individual-level metabolism and competition in controlling the dynamics of matter and energy in water-limited ecosystems.
Vegetation of dryland ecosystems is sensitive to precipitation pulses. Future climate scenarios suggest that the frequency and magnitude of precipitation events will change. How much and to what extent will these changes impact the hydrological cycle in creosotebush (Larrea tridentata) shrublands that dominate the three North American hot deserts? In this study, we examine the partitioning of precipitation inputs into bare soil evaporation (E) and transpiration (T) within creosotebush ecosystems at sites characterized by bimodal precipitation regimes: the Santa Rita Experimental Range (SRER) and the Walnut Gulch Experimental Watershed (WGEW). At both sites, during summer 2008, we measured evapotranspiration (ET) using eddy covariance, whole plant T using the heat-balance sap flow, and soil moisture at several depths. During the dry period preceding the summer monsoon, both ET and soil moisture were very low. With the onset of summer rains, E dominated ET; shrub transpiration did not respond to increases in soil moisture for approximately 3 more weeks. A series of large precipitation events increased moisture at deeper soil layers, and triggered T. Overall, ET was largely correlated to moisture levels in shallow soil layers typical of dryland ecosystems dominated by dry conditions, high evaporative demand, and poor soil infiltration. Under the current precipitation regime, characterized by many small storms and few large storms, soil moisture is low with most precipitation inputs lost as E. However, if climatic changes lead to less frequent but larger precipitation events, dryland communities could experience shifts in the partitioning of ET affecting the hydrologic budget of the ecosystem. Copyright © 2010 John Wiley & Sons, Ltd.
Climate change in the twenty-first century, projected by a large ensemble average of global coupled models forced by a mid-range
(A1B) radiative forcing scenario, is downscaled to Climate Divisions across the western United States. A simple empirical
downscaling technique is employed, involving model-projected linear trends in temperature or precipitation superimposed onto
a repetition of observed twentieth century interannual variability. This procedure allows the projected trends to be assessed
in terms of historical climate variability. The linear trend assumption provides a very close approximation to the time evolution
of the ensemble-average climate change, while the imposition of repeated interannual variability is probably conservative.
These assumptions are very transparent, so the scenario is simple to understand and can provide a useful baseline assumption
for other scenarios that may incorporate more sophisticated empirical or dynamical downscaling techniques. Projected temperature
trends in some areas of the western US extend beyond the twentieth century historical range of variability (HRV) of seasonal
averages, especially in summer, whereas precipitation trends are relatively much smaller, remaining within the HRV. Temperature
and precipitation scenarios are used to generate Division-scale projections of the monthly palmer drought severity index (PDSI)
across the western US through the twenty-first century, using the twentieth century as a baseline. The PDSI is a commonly
used metric designed to describe drought in terms of the local surface water balance. Consistent with previous studies, the
PDSI trends imply that the higher evaporation rates associated with positive temperature trends exacerbate the severity and
extent of drought in the semi-arid West. Comparison of twentieth century historical droughts with projected twenty-first century
droughts (based on the prescribed repetition of twentieth century interannual variability) shows that the projected trend
toward warmer temperatures inhibits recovery from droughts caused by decade-scale precipitation deficits.
Changes in the timing, frequency, and magnitude of precipitation events are projected for semiarid ecosystems worldwide. The ecological consequences associated with these precipitation changes will be better understood if the hydrological triggers of vegetation response can be better identified. Previous research has suggested that soil moisture, likely from large monsoon rainstorms, plays a critical role in triggering the phenological response of semiarid shrublands. Here we propose that the recent emergence of time-lapse repeat digital photography (pheno-cams) can play a role in further explaining the hydrological triggers of phenological response in semiarid shrublands. This study is focused on a creosotebush-dominated ecosystem of the Santa Rita Experimental Range, southeastern Arizona. In addition to typical eddy covariance instrumentation, this site offers continuous measurements of soil moisture in 6 one-meter profiles. Additionally, three pheno-cams have been installed in the footprint of the eddy covariance tower at the site. We demonstrate (1) that the green-up of evergreen creosotebush can be tracked using an average greenness index calculated from multiple pheno-cams within a tower footprint; (2) that the green-up of creosotebush is driven by deep soil moisture (e.g. > 30 cm); and (3) that carbon uptake can be predicted using image-derived green-up of creosotebush.
Confirms the overwhelming importance of water availability as a control on production. Lowest values of aboveground net primary production were observed in the west, highest values in the east. This spatial pattern was shifted eastward during unfavorable years, westward during favorable years. Variability in production among years was maximum in N New Mexico and SW Kansas and decreased towards the N and S. The regional pattern of production was largely accounted for by annual precipitation. Production at site level was explained by annual precipitation, soil water-holding capacity, and an interaction term. When precipitation is <370 mm/yr, sandy soils with low water-holding capacity are more productive than loamy soils with high water-holding capacity; the opposite pattern occurs when precipitation is >370 mm/yr. -from Authors
Metabolism provides a basis for using first principles of physics, chemistry, and biology to link the biology of individual organisms to the ecology of populations, communities, And ecosystems. Metabolic rate, the rate at which organisms take up, transform, and expend energy and materials, is the most fundamental biological rate. We have developed a quantitative theory for how metabolic rate varies with body size and temperature. Metabolic theory predicts how metabolic rate, by setting the rates of resource uptake from the environment and resource allocation to survival, growth, and reproduction, controls ecological processes at all levels of organization from individuals to the biosphere. Examples include: (1) life history attributes, including development rate, mortality rate, age at maturity, life span, and population growth rate; (2) population interactions, including carrying capacity, rates of competition and predation, and patterns of species diversity; and (3) ecosystem processes, including rat
Large areas of semi-arid grasslands in the southwestern United States have virtually been replaced by shrubs during the past century. Understanding the causes and consequences of such vegetation dynamics requires that we elucidate the interplay between external forces of change (e.g. climate, human impacts) and the internal forces within these ecosystems that foster resilence and/or stability. Several conceptual models of arid ecosystems address this interplay by including the potential role of autogenic shrub effects on ecosystem processes, which lead to the formation of "resource islands" and tend to promote shrub persistence. Specifically, during the process of shrub establishment and maturation, the cycling of nutrients is progressively confined to zones of litter accumulation beneath shrubs, while bare intershrub spaces become increasingly nutrient poor. As shrub resource islands develop, there is increased interception and stemflow by shrub canopies, confining infiltration of nutrient enriched rainfall directly beneath shrubs; the barren intershrub spaces generate overland flow, soil erosion by wind and water, and nutrient losses. These islands are preferred sites for regeneration of shrubs and herbaceous plants and are correlated with spatial variation in soil microbial populations and soil microfauna that promote nutrient recycling. If further changes in the transition between grassland are to be correctly predicted - or if we wish to intervene and redirect transitions - we must develop a greater mechanistic understanding of the structural and functional relationships between shrubs and the resource islands associated with them.
We conducted a 3 yr field study in the Jornada Basin of southern New Mexico to explore the relationships between seasonal manipulations of soil water and its impact on soil nutrient dynamics of resource islands and shrub growth and physiology. At our study site, where total annual precipitation is ~230mm (~65% falls during the summer period) , we simulated seasonal drought in summer (1 June - 30 September) and winter/spring (1 October - 31st May) by constructing large-rainfall exclusion shelters over shrub resource islands at different stages of development. Our experiment tests two principal hypotheses. The first is that the two major shrub species in the Jornada Basin, creosote bush (Larrea tridentata) and mesquite (Prosopis glandulosa), have different growth phenologies, rooting patterns, and physiological responses to resource availability (primarily water). The second is that different size classes of shrubs ("small" and "large") represent distinct stages of resource island development (i.e. "young" and "mature" respectively) and, hence, different stabilities - that is , as islands develop, their associated shrubs become less coupled to short-term fluctuations in precipitation and more resistant to long-term drought or climate shifts.
With regard to the first hypothesis, we conclude that the two species are relatively similar in function despite different phenological "strategies" of Larrea (evergreen) and Prosopis (winter deciduous). In the absence of drought, both species exhibited maximal rates of shoot and root growth, as well as high photosysthesis and transpiration, in late spring. This remained as the period for maximal growth and physiological activity for Prosopis shrubs that experienced drought in either summer or winter/spring. On the other hand, Larrea shrubs that experienced drought in the winter/spring had maximal growth and activity shifted to the summer period, and in the absence of drought, Larrea shrubs also exhibited high physiological activity during summer (especially following high rainfall). Thus, Larrea appears to have a greater capacity for shifting its activity patterns to alternate periods to take advantage of changes in resource availability. Shrubs of both species appeared well adapted to withstand season-long droughts. Mechanisms for survival include the following capacities:
1) to shift growth and physiological activity to utilise different temporal moisture(Larrea)
2) to utilise different levels of soil moisture (both species)
3) to carry out limited physiological activity and growth during drought (especially Larrea)
4) to compensate for some negative impacts of drought through enhanced physiology (especially Prosopis) and growth (especially Larrea)in the season following drought
With regard to the second hypothesis, we again found more similarities than differences between the different aged (young vs. mature) islands. The stage of maturity of a resource island complex did not seem to be a significant factor to the growth and physiological activity of the shrub.
Three alluvial pediments(bajadas of alluvial fans) studied in the Sonoran desert near Tucson, Arizona are a complex mosaics of distinct geological landforms. These landscape mosaics have been produced through the temporally episodic and spatially discontinuous aggradation of alluvial surfaces and the destruction of other parts of the landscape by erosion. These geomorphic processes produce abrupt juxtapositions of soils of different ages and degrees of profile development. Vegetation patterns correspond closely to this geomorphic mosaic. Larrea tridentata predominates on most Holocene--aged surfaces and all parts of highly dissected, early Pleistocene surfaces. This shrub is generally excluded from Pleistocene--aged surfaces containing soils with strongly developed argillic(clay--rich) horizons. The highest species diversity is encountered on some of the most unstable, erosional slopes of early Pleistocene surfaces. Comparisons among the three study areas indicated of igneous lithology(highly weatherable intrusives vs. weathering--resistant extrusives) in controlling geomorphic processes, and ultimately, vegetation patterns. The areal extent of late Holocene alluvial aggradation and patterns of erosion and dissection of older Pleistocene deposits are stongly influenced by by the weatherability of different lithologies and provide a strong control over the spatial scale of ecological patterns. Processes limiting the distributions and abundances of plants are directly linked to landscape characteristics in many ways. Landform age and stability affect the structure of populations of long-lived Larrea tridentata. Individuals of this shrub species can exhibit clone--like growth and increase considerably in in size(diameter) over time spans of many centuries to millennia. The growth and persistence of these long--lived clones in some parts of the landscape apparently contributes to exclusion of other species. However, development of large clones and dominance by L.tridentata are impossible or greatly inhibited in several landscape settings including:-
1) extremely young alluvial deposits that have existed for too short a time for large clones to have developed
2) hillslopes subject to considerable erosional disturbance, and
3) extremely thin soils underlain by impenetrable petrocalcic horizons(caliche), which magnify drought conditions and apparently contribute to episodic mortality in L.tridentata
Soil horizon development as determined by landform age controls the vertical movement and distribution of soil water, in turn affecting the distribution of various plant lifeforms. Clay--rich argillic horizons that have required tens to hundreds of thousands of years to form greatly limit the downward infiltration, vertical distribution, and temporal availability of soil water. Despite surficial stability for extremely long periods of time, sites with strongly developed argillic horizons lack L.tridentata and are instead by drought--deciduous or succulent plants that are capable of highly seasonal activity in soils that exhibit high seasonal water variability. Syntheses involving the study of various ecological(e.g. plant physiological, demographic, and interspecific interactions) with a larger landscape perspective provide a rich framework for further studies of aridland systems
Field observations of Larrea divaricata Cav. throughout its United States range were used to determine
1) the age distribution of stand members
2) the spatial distribution of stand members
3) the magnitude of certain soil changes across abrupt community boundaries.
Significant non-central tendencies in age distribution of most stands indicated that germination and survival are rare events, contributing to one- or several-age stands. Shrubs were distributed at random, in clumps, or at regular intervals depending on the environment. Soil pH and salinity changes across ecotones were neither predictable nor usually great enough to affect germination and early growth of Larrea.
Questions: We asked several linked questions about phenology and precipitation relationships at local, landscape, and regional spatial scales within individual seasons, between seasons, and between year temporal scales. (1) How do winter and summer phenological patterns vary in response to total seasonal rainfall? (2) How are phenological rates affected by the previous season rainfall? (3) How does phenological variability differ at landscape and regional spatial scales and at season and inter-annual temporal scales?
Location: Southern Arizona, USA.
Methods: We compared satellite-derived phenological variation between 38 distinct 625-km2 landscapes distributed in the northern Sonoran Desert region from 2000 to 2007. Regression analyses were used to identify relationships between landscape phenology dynamics in response to precipitation variability across multiple spatial and temporal scales.
Results: While both summer and winter seasons show increases of peak greenness and peak growth with more precipitation, the timing of peak growth was advanced with more precipitation in winter, while the timing of peak greenness was advanced with more precipitation in summer. Surprisingly, summer maximum growth was negatively affected by winter precipitation. The spatial variations between summer and winter phenology were similar in magnitude and response. Larger-scale spatial and temporal variation showed strong differences in precipitation patterns; however the magnitudes of phenological spatial variability in these two seasons were similar.
Conclusions: Vegetation patterns were clearly coupled to precipitation variability, with distinct responses at alternative spatial and temporal scales. Disaggregating vegetation into phenological variation, spanning value, timing, and integrated components revealed substantial complexity in precipitation-phenological relationships.
Spatial and temporal rainfall variability over watersheds directly impacts the hydrologic response over virtually all watershed scales. Changes in the precipitation regime over decades due to some combination of inherent local variability and climate change may contribute to changes in vegetation, water supply, and, over longer timescales, landscape evolution. Daily, seasonal, and annual precipitation volumes and intensities from the dense network of rain gauges on the Agricultural Research Service, U. S. Department of Agriculture Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona are evaluated for multidecadal trends in amount and intensity over a range of watershed scales (1.5 ha to 149 km2) using observations from 1956 to 2006. Rainfall and runoff volume and rate variability are compared over the same spatial scales over a 40 year period (1966-2006). The major findings of this study are that spatial variability of cumulative precipitation decreases exponentially with time, and, on average, became spatially uniform after 20 years of precipitation accumulation. The spatial variability of high-intensity, runoff-producing precipitation also decreased exponentially, but the variability was still well above the measurement error after 51 years. There were no significant temporal trends in basin scale precipitation. A long-term decrease in runoff from 1966 to 1998 from ephemeral tributaries like the WGEW may be a critical factor in decreasing summer flows in the larger San Pedro due to changes in higher-intensity, runoff-producing rainfall.
Thesis (M.S.)--University of Maryland at College Park, 1990. Includes bibliographical references (leaf 105).
Leaf-level CO2 assimilation (A(area)) can largely be predicted from stomatal conductance (g(s)), leaf morphology (SLA) and nitrogen (N) content (N(area)) in species across biomes and functional groups. The effects of simulated global change scenarios, increased summer monsoon rain (+H2O), N deposition (+N) and the combination (+H2O +N), were hypothesized to affect leaf trait-photosynthesis relationships differently in the short- and long-term for the desert shrub Larrea tridentata. During the spring, +H2O and +H2O +N plants had lower A(area) and g(s), but similar shoot water potential (Psi(shoot)) compared with control and +N plants; differences in A(area) were attributed to lower leaf N(area) and g(s). During the summer, +H2O and +H2O +N plants displayed higher A(area) than control and +N plants, which was attributed to higher Psi(shoot), g(s) and SLA. Throughout the year, A(area) was strongly correlated with g(s) but weakly correlated with leaf N(area) and SLA. We concluded that increased summer monsoon had a stronger effect on the performance of Larrea than increased N deposition. In the short term, the +H2O and +H2O +N treatments were associated with increasing A(area) in summer, but also with low leaf N(area) and lower A(area) in the long term the following spring.