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Scheme of water uptake by trees in the riparian forest. (a) Soil water content and water potential versus depth, (b) 18 O profile in the soil (dotted line shows υ 18 O ‰ in the trunk), (c) density of roots versus soil depth and (d) sap fluxes in trunk and root over the 24-h cycle
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Water flow in the soil–root–stem system was studied in a flooded riparian hardwood forest in the upper Rhine floodplain. The study was undertaken to identify the vertical distribution of water uptake by trees in a system where the groundwater is at a depth of less than 1 m. The three dominant ligneous species (Quercus robur, Fraxinus excelsior and...
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... comparing the isotopic composition of water in the soil and the oak, it was concluded that the water was absorbed from between 40 and 60 cm deep, which is in good agreement with the distribution of the oak roots and the soil water potential. Thus, hardwood trees such as oak and ash take up water in the unsaturated zone, unlike softwoods ( Salix and Populus ; Busch et al ., 1992), which also used water from saturated zones. In this study the white poplar exhibited the same root extension as the oak. The root structure, and as a consequence the distribution of water uptake, could be an adaptative strategy of a riparian forest growing on more elevated stands than softwoods where the groundwater level is closer to the soil surface. The analysis carried out following measurement of root structure, soil water, water potential, isotopic analysis and sap flux, allowed a scheme for the water movement in the alluvial forest ecosystem to be proposed ( Figure 8): the root system works as a pump able to raise groundwater to the unsaturated zone by capillarity from the soil towards the superficial layers with a high root density. This work was supported by the ‘Programme National de Recherche en Hydrologie’ a research programme on hydrology. The authors wish to thank A. V. Auzet and M. Trautmann for ...
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... There are further (2010 onwards) sap studies of deciduous oak which do not manipulate CO2 but which offer helpful data for comparison, for example within Europe (Aszalós et al., 2017;Hassler et al., 2018;Perkins et al., 2018;Schoppach et al., 2021;Süßel and Brüggemann, 2021;Wiedemann et al., 2016) and North America 105 (Fontes and Cavender-Bares, 2019). Older studies of oak transpiration, using other techniques such as highpressure flow meters, have been carried out in Europe (Rust and Roloff, 2002;Sánchez-Pérez et al., 2008). Robert et al. (2017) have also reviewed the characteristics of these old growth species from multiple studies which help us to place our results in context. ...
... When the soil moisture 455 stored in the shallow depths (0 to 40 cm) of soil is depleted (cf. MacKenzie et al., 2021, Fig. 4), we deduce that the oaks rely on hydraulic recharge (from depths of soil greater than 1 m at this site) to rehydrate the fine roots, especially in dry years such as 2018 (Sánchez-Pérez et al., 2008). We speculate that in this instance, with ring porous oaks, there may be a wider sapwood area towards the bark as the season progresses and, since the radial growth is variable between trees, this may lead to variation in comparative peaking in high growth periods, 460 especially for the larger trees which are growing faster (Dragoni et al., 2009). ...
Predicting how increased atmospheric carbon dioxide levels will affect water usage by whole mature trees remains a challenge. The present study focuses on diurnal (i.e. daylight) water usage of old growth oaks within an experimental treatment season from April to October inclusive. Over five years, from 2017 to 2022, we collected 12,259 days of individual tree data (770,667 diurnal sap flux measurements across all treatment months) from eighteen oaks (Quercus robur L.) within a large-scale manipulative experiment at the Birmingham Institute of Forest Research (BIFoR) Free-Air CO2 Enrichment (FACE) temperate forest in central England, UK. Sap flux data were measured using the compensation heat pulse (HPC) method and used to calculate diurnal tree water usage per day (TWU) across the leaf-on seasons. Six trees were monitored in each of three treatments: FACE infrastructure arrays of elevated (+150 mmol mol-1) CO2 (eCO2); FACE infrastructure control ambient CO2 (aCO2) arrays; and control Ghost (no-treatment-no-infrastructure) arrays. For each tree, sap flux demonstrated a circumferential imbalance across two orientations of the stem. Median and peak (95 %ile) diurnal sap flux increased in the spring from first leaf to achieve peak daily values in summer months (July, August) for all trees in the study. TWU increased similarly, declining more slowly towards full leaf senescence (Oct/ Nov). Water usage varied between individual oaks in July of each year. TWU was linearly proportional to tree bark radius, Rb, at the point of probeset insertion ca. 1.1–1.3 m above ground level (ca. 3.1 litres d-1 mm-1 radius; 274 mm ≤ radius ≤ 465 mm). We also found that bark radius is a very good proxy for canopy area, Ac. Ac was linearly proportional to Rb (ca. 616.5 m2 mm-1 radius), which implies a mean July water usage of almost 5 litres m-2 of projected canopy area in the BIFoR FACE forest. In comparing seasonal responses, TWU was seen to vary by treatment season precipitation amounts and in response to cloudy days, also seen from the diurnal sap flux data. We normalised TWU by individual tree bark radius Rb, which we call TWUn. TWUn treatment comparisons differed year on year. Trees treated with eCO2 compared to the aCO2 controls exhibited different median TWUn results both within and between treatment years, but with no consistency in this difference. Infrastructure control trees exhibited higher TWUn than Ghost, no-infrastructure, trees, especially for the larger trees. The greater TWUn may be due to one or more of several factors: the installation or operation of FACE infrastructure; or to array-specific differences in soil moisture, slope, soil respiration; or sub-dominant tree species presence. The results indicate the importance of infrastructure controls in forest FACE experiments. This first set of plant water usage results encourages the conclusion that old growth oak forests cope well with eCO2 conditions in the FACE(sic) of climate change. From our tree-centred viewpoint, the results reported improve our understanding of future-forest water dynamics of old growth forest and could contribute to the development of more realistic dynamic vegetation models.
... Cumulative stress due to consecutive hotter droughts (heatwaves coinciding with low precipitation) could affect tree growth in temperate floodplain forests (Schnabel et al., 2022). Our findings agree with those of previous research demonstrating the high radial growth sensitivity of ash species to drought , which confirms that this species is vulnerable to the drying of upper unsaturated soil horizons (Sánchez-Pérez et al., 2008;Singer et al., 2013Singer et al., , 2014. The correlations between SPEI and ash growth found in Ticino also agree with the findings of a previous study conducted in a nearby stand . ...
... Changes in the period of wood formation in response to more lasting summer droughts should be further investigated in ash and other riparian tree species. Such plastic behavior and differences among coexisting species in terms of hydraulic and photosynthetic traits or water-uptake soil depths could also mitigate drought stress in diverse floodplain forests through complementary effects (Sánchez-Pérez et al., 2008). Additionally, microenvironmental conditions can modulate tree responses to drought in floodplain forests. ...
Floodplain forests are sensitive to climate warming and increased drought, as showed by recent oak (Quercus robur) dieback and mortality episodes. However, a comprehensive comparison of coexisting tree species under different climate settings or biomes are lacking. Herein, we compared growth rates, growth responses to climate and drought severity, and modeled climate mediated growth of oak and three coexisting tree species (ash, Fraxinus angustifolia; alder, Alnus glutinosa; elm, Ulmus minor). Two floodplain forests subjected to cooler (temperate climate, Ticino) and warmer (Mediterranean climate, Bosco Pantano) conditions in northern and southern Italy, respectively, were analyzed. Ash seemed to be the most sensitive to drought, particularly at the Mediterranean site where oak and elm growth were also negatively affected by water shortages. Alder appeared to be the least sensitive species in terms of growth variability to drought under both temperate and Mediterranean climate conditions. Furthermore, the growth model revealed the influence of soil moisture in spring and summer on the constrained growth of ash and oak and illustrated how oak growth could be severely reduced during drastic hotter droughts. Alder seemed to be the most drought-resistant species under both environmental conditions. These results could represent the first attempts in documenting the ecological consequences of drought in terms of projected climate trends in less investigated Mediterranean floodplain forests. Furthermore, these results highlight how climate and tree-ring data combined with growth models could be useful tools to detect early warning signals of growth decline and impending dieback in floodplain forests in response to dry spells.
... These results support the idea that the majority of the roots are at the mid-soil layers taking most of the existing water at this middle range depth, when possible (e.g. Sánchez-Pérez et al., 2008), even being advantageous for more oxygenated conditions (Hahm et al., 2020). Thus, whenever there is enough available water, trees do not need to take it from deeper layers, where the investment in roots is costly, and the probability of oxygen limitation is higher. ...
One of the suggested mechanisms behind the success of non‐native plants in recipient ecosystems is competition avoidance with natives by means of different resource‐use strategies, such as deeper water uptake under dry conditions.
We aimed at evaluating water source partitioning between native and non‐native tree species coexisting in central Spain floodplains; determining the dependency on drought stress of such water sources use; and assessing if the reliance on deeper water sources relates with physiological and growth performance.
We assessed water uptake depth, leaf functional traits related to physiological performance and growth of native ( Populus alba ) and non‐native trees ( Ailanthus altissima , Robinia pseudoacacia ) coexisting in riparian forests under different drought conditions (drier, intermediate and wetter). We analysed δ ² H and δ ¹⁸ O isotopes in xylem water and in soil water from top, mid and deep soil depths and determined the contribution of each water source to overall plant xylem water. Leaf traits related with resource use and secondary growth were assessed for each species.
We found stronger differences between sites than between species, with all species taking more deep water in the driest site (~45% of the xylem water) than in the wettest (~15%). However, under drier conditions, species differences were significant for top‐soil water use, with R. pseudocacia withdrawing more superficial water (~22%) than A. altissima (~8%). These results indicate stronger water partitioning under drier conditions. Non‐native species showed a physiological strategy characterized by greater leaf N, water content, and enriched δ ¹³ C and δ ¹⁵ N values independently of the deep‐water uptake. However, a positive relationship between deep soil water use and such strategy was found for P. alba.
We highlight that those native and non‐native species differences were more evident regarding physiological performance at leaf level than for deep‐water uptake or growth. Furthermore, our results suggest that differences in water sources used by coexisting species may increase under drier conditions.
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... In the South Moravian Region, oak creates a more extensive root system than ash (Tatarinov et al., 2008;Jaworski, 2011). Studies of root system architecture revealed much higher total root density and higher root density in deeper soil layers in oak, compared to ash (Sánchez-Pérez et al., 2008). Therefore, ash may develop a lower absorptive root area than oak growing on the same site. ...
... While lower SWC limited Q in ash and hornbeam, oak responded in the opposite way, increasing transpiration (Fig. 7). This suggests that oak could uptake water from the deeper well-supplied layers during studied years (Sánchez-Pérez et al., 2008). ...
... (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)to oak(Sánchez-Pérez et al., 2008;Nadal-Sala et al., 2017). On the other hand,Gebauer et al. (2008) andHölscher et al. (2005) observed higher sap flux density and larger sapwood area in hornbeam, compared to ash. ...
Recent changes in the floodplain forests of Central Europe, caused mainly by changes in hydrological management and the increased frequency of droughts due to climate change, have led to severe degradation of floodplain ecosystems. Our main objective was to determine the sensitivity of trees to drought by observing the response of the tree phenology, stem radial growth, and physiology (sap flow) of three predominant tree species, namely English oak, narrow-leaved ash, and common hornbeam, to the environmental variables (climate). Stem radial growth began before bud break in ring-porous oak and ash, whereas in diffuse-porous hornbeam, growth onset occurred after leaf formation. The early onset with intense growth during favorable months (April–May) observed in ring-porous species was a major prerequisite for the successful growth of oak and ash at this site. Tree water deficit (TWD), an indicator of stem hydration, was triggered by decreasing soil moisture in all species, and was most prominent in ash, followed by oak. Intriguingly, sap flow was decoupled from TWD in all species and was driven primarily by evaporative demand from the atmosphere. Oak was the least conservative in regulating sap flow under atmospheric drought followed by hornbeam, whereas ash was most restricted and reduced its transpiration during dry periods. In contrast, ash was characterized by the highest radial growth and growth-based water-use efficiency. The lower water storage capacity of oak and ash is likely compensated by deep rooting and drought avoidance strategies, respectively. Tree species that tend to use surface soil water could be severely limited by more extractive species such as hornbeam. Despite the contrasting leaf and wood phenology, stomatal control, and rooting depth among the studied floodplain tree species, they exhibited analogous sap flow and water storage dynamics responses to drier conditions that enabled them to co-exist in the South Moravian Region. Nevertheless, our results suggest that the severe droughts and human-induced alterations in groundwater pose serious threats to floodplain forests in Central Europe, with certain tree species being unable to adapt to these altered conditions.
... Tree species vary greatly in their susceptibility to drought due to physiological and morphological differences. Among other features such as fine-root distribution and their dieback in response to drought (Brunner et al., 2015;Sánchez-Pérez et al., 2008), two key factors that might drive tree species reactions to drought are stomatal control and resistance to cavitation (Choat et al., 2012;Martínez-Vilalta & Garcia-Forner, 2017;McDowell et al., 2008). Stomatal closure in response to water deficits enables plants to avoid critically low water potentials through transpiration losses and thus hydraulic failure but species differ largely in their type of stomatal control (Martínez-Vilalta & Garcia-Forner, 2017;McDowell et al., 2008): ...
... Other traits may have influenced the responses observed but establishing species-specific differences remains challenging. For instance, ash was reported to have fine-and coarse-root biomass concentrated to shallower soil layers than oak in another riparian hardwood forests (Sánchez-Pérez et al., 2008). However, other studies reported rather deep rooting in ash and an intruding ability to plastically shift its water uptake to deeper soil layers (Brinkmann et al., 2019;Meißner et al., 2012). ...
... Finally, here reported drought effects may be influenced through the naturally high tree species richness of floodplain forests (Ward et al., 1999), as diverse tree communities with dissimilar hydraulic traits may outperform species poor communities through complementarity in water use (Sánchez-Pérez et al., 2008;Schnabel et al., 2019). ...
Droughts increasingly threaten the world's forests and their potential to mitigate climate change. In 2018–2019, Central European forests were hit by two consecutive hotter drought years, an unprecedented phenomenon that is likely to occur more frequently with climate change. Here, we examine tree growth and physiological stress responses (increase in carbon isotope composition; Δδ13C) to this consecutive drought based on tree rings of dominant tree species in a Central European floodplain forest. Tree growth was not reduced for most species in 2018, indicating that water supply in floodplain forests can partly buffer meteorological water deficits. Drought stress responses in 2018 were comparable to former single drought years but the hotter drought in 2018 induced drought legacies in tree growth while former droughts did not. We observed strong decreases in tree growth and increases in Δδ13C across all tree species in 2019, which are likely driven by the cumulative stress both consecutive hotter droughts exerted. Our results show that consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with comparably high levels of water supply.
... Cumulative stress due to consecutive hotter droughts (heatwaves coinciding with low precipitation) could affect tree growth in temperate floodplain forests (Schnabel et al., 2022). Our findings agree with those of previous research demonstrating the high radial growth sensitivity of ash species to drought , which confirms that this species is vulnerable to the drying of upper unsaturated soil horizons (Sánchez-Pérez et al., 2008;Singer et al., 2013Singer et al., , 2014. The correlations between SPEI and ash growth found in Ticino also agree with the findings of a previous study conducted in a nearby stand . ...
... Changes in the period of wood formation in response to more lasting summer droughts should be further investigated in ash and other riparian tree species. Such plastic behavior and differences among coexisting species in terms of hydraulic and photosynthetic traits or water-uptake soil depths could also mitigate drought stress in diverse floodplain forests through complementary effects (Sánchez-Pérez et al., 2008). Additionally, microenvironmental conditions can modulate tree responses to drought in floodplain forests. ...
... Isotopic ratios of oxygen (δ 18 O) extracted from xylem samples, annual growth rings, and potential endmember waters have been particularly helpful in identifying and disentangling contributions from different source waters, where deeper water is usually associated with lighter isotopic values and soil water that has undergone evaporative losses becomes isotopically heavier [23][24][25]47]. They have enabled improved understanding of the evolution of seasonally available water sources at critical periods of growth, and to identify the dominant water source usage over an entire season of growth [48,49]. Recent developments in the study of δ 18 O from tree ring cellulose have enabled the analysis of water source variability at sub-annual resolution in the same reference frame as the potential contributing endmember sources for co-occurring riparian tree species that typically use different water sources [48,50]. ...
... Each site contains common, and co-occurring Mediterranean tree species, Fraxinus excelsior and Populus nigra/alba (figure 2(a)). Fraxinus and Populus have been shown to exhibit contrasting rooting depths, where the former is restricted to soils above the gravel layer (∼ less than 1 m deep) and the latter has most of its root mass within gravels deeper than 1 m [31,49]. The investigation utilizes an 11-year dataset of sub-annual tree-ring cellulose (δ 18 O cell ) (n = 792) from individual, co-located Fraxinus-Populus pairs from both near channel (NC) and interior floodplain (IF) locations, within each study plot along the Rhône hydroclimatic gradient (figure 2(a)). ...
As global climate change continues to impact regional water cycles, we may expect further shifts in water availability to forests that create challenges for certain species and biomes. Lowland deciduous riparian forests are particularly vulnerable because tree species cannot migrate out of the stream corridor, and they rely on root zone water availability that is controlled by variations in both local climate conditions (e.g. precipitation, evaporation, and infiltration) and non-local hydroclimatic forcing (e.g. streamflow, snowmelt, recharge). To determine how the seasonal water source usage of riparian trees is controlled by local versus non-local variability in hydroclimatic regime, we reconstructed the seasonal oxygen isotope (δ ¹⁸ O) signature of water used by two riparian tree species with contrasting rooting depths, comprising ∼800 δ ¹⁸ O tree-ring cellulose measurements from 12 tree-level decadal time-series at sub-annual resolution (six samples per year), along a strong hydroclimatic gradient within the Rhône River basin, SE France. These results were evaluated alongside δ ¹⁸ O measurements made from potential endmember water sources and independent hydroclimatic metrics. Thus we characterize the seasonal evolution of both potential water availability at distinct rooting depths and tree water source use and investigate the generalized riparian tree response to seasonal variations in local versus non-local hydroclimatic forcing over a decade. We show: (a) distinct seasonal water use between species, based on differential access to groundwater; (b) substantial source switching in both species based on evolving water availability; and (c) that riparian trees are more dependent on locally controlled soil moisture with distance downstream, creating increased vulnerability to locally increasing temperatures. We also find that deeply rooted trees in lowland riparian floodplains are potentially vulnerable to climate change because of their high dependence on water supply from mountains. This effect is more pronounced downstream, where seasonal water table decline may lead to loss of water required for deeply rooted trees.
... In Japan, little data are available on the depth to which trees take up soil water in the unsaturated soil layer under open-air field conditions. Most of the limited data have been obtained from field tracer tests using D 2 O with different tree species in boreal forests 10,11 , in dry riverbed forests in France 12 , in a subtropical mesic savannah 13 , and in an Amazonian tropical forest 14 . Although these forests grow under different climatic conditions from the Japanese humid monsoon climate, most trees consume soil water from the surface to a 50-cm depth. ...
Although 10 years have passed since Japan’s Fukushima nuclear accident, the future radiation risk from 137Cs contamination of wood via root uptake is a serious concern. We estimated the depth at which the roots of evergreen coniferous sugi (Cryptomeria japonica) and broadleaf deciduous konara (Quercus serrata) trees actively take up soil water by using positive δD values from the artificial D2O tracer and seasonal changes in the δ18O values of soil water as a natural environmental tracer. We compared the tracer concentration changes in xylem sap with those in the soil water and ascertained that both tree species primarily took up water from a depth of 20 cm, though with mixing of water from other depths. Using sap hydrodynamics in tree stems, we found that water circulation was significantly slower in heartwood than in sapwood. Heartwood water was not supplied by direct root uptake of soil water. The measured diffusion coefficients for D2O, K+, Cs+, and I− in xylem stems were greater in sapwood than in heartwood, and their magnitude was inversely correlated with their molecular weights. The distribution of D2O and 137Cs concentrations along the radial stem could be explained by simulations using the simple advective diffusion model.
... Deeply rooted phreatophytic may species prefer phreatic water sources derived from shallow aquifers (Busch et al., 1992) and capillary rise, yet they may also exhibit opportunistic behavior, switching to shallow soil moisture (or using combination of both) under periods of water deficit (Singer et al., 2013(Singer et al., , 2014Snyder & Williams, 2000;Sun et al., 2016). Shallowly rooted species are unable to access deeper phreatic water, so they rely on vadose moisture that is sensitive to balance between precipitation inputs and near-surface evaporation, although it is possible that capillary rise during a brief period of elevated water table can supply phreatic water to the unsaturated zone (Sánchez-Pérez et al., 2008). Additionally, plant roots can redistribute water across a soil-water potential gradient (hydraulic redistribution), both vertically and laterally (Brooks et al., 2002;Caldwell et al., 1998;Richards & Caldwell, 1987), allowing water from different depths and distances to be utilized by neighboring plants (Dawson, 1993), thereby complicating source-water identification. ...
... Additionally, plant roots can redistribute water across a soil-water potential gradient (hydraulic redistribution), both vertically and laterally (Brooks et al., 2002;Caldwell et al., 1998;Richards & Caldwell, 1987), allowing water from different depths and distances to be utilized by neighboring plants (Dawson, 1993), thereby complicating source-water identification. Studies which provide detailed ecohydrological information are typically based on comparisons between the contemporaneous measurements of tree xylem waters with those of local water sources (e.g., Plamboeck et al., 1999;Sánchez-Pérez et al., 2008). While direct analysis of xylem δ 18 O circumvents the leaf fractionation and exchange mechanisms which mask the source water δ 18 O information stored in δ 18 O cell (McCarroll & Loader, 2004), xylem isotope studies are limited by the temporal domain of field work (typically 2-3 years), restricting the development of broader conclusions about ecohydrological interactions (Pettit & Froend, 2018). ...
... Identifying tree source water(s) requires information on how the potential local endmembers vary in their δ 18 O signature on both annual and sub-annual timescales, which is particularly complex in situations where there may exist more than one potential water source available for tree growth (Sargeant & Singer, 2016;Singer et al., 2014). Furthermore, fluctuations in source water mixtures modify the isotopic signature of root-available water, thus challenging interpretations of water sources consistently available to trees (Busch et al., 1992;Dawson & Ehleringer, 1991;Sánchez-Pérez et al., 2008;Snyder & Williams, 2000). Despite these challenges, the identification of δ 18 O sw is critically important, especially considering that changes in soil water content may outpace the ability for new root growth to track such shifts (Plamboeck et al., 1999). ...
Hydrological regimes are being perturbed under climate change due to the regional expression of the water cycle across the globe, leading to alterations in the spatial and temporal distribution of water near the Earth's surface. Water is a critical resource for plant ecosystems, and hydrological limitations on vegetative health are particularly complex. To anticipate how subsurface water availability may evolve in the future and affect the dynamics of plant water source usage, as well as the health and functioning of vegetation in various biomes, we need a robust, quantitative framework for linking water availability to past plant water use, which is constrained by historical data. Here, we outline the Identification of Source‐water Oxygen isotopes in trees Toolkit (ISO‐Tool), designed to retrospectively investigate the dynamics of tree water uptake. ISO‐Tool utilizes tree‐ring isotopes (δ¹⁸O) combined with a biomechanistic fractionation model to retrodict the δ¹⁸O of water utilized during any period of growth. Through comparisons with measured δ¹⁸O in local water sources, climatic, and hydrological variables, ISO‐Tool can reconstruct and inform on past ecohydrological interactions. We provide an overview of the modeling components and data requirements necessary to constrain the retrodictions of source‐water δ¹⁸O. We demonstrate the utility and efficacy of ISO‐Tool for three riparian field sites characterized by differences in climatic, geomorphic, and hydrologic complexity. We also state that ISO‐Tool can be applied to a range of vegetated environments where distinct isotopic endmembers exist. We present a set of tool groups, which can be applied adaptively, ensuring that scientific progress in understanding retrospective ecohydrology can be made, even under varying degrees of data availability.
... Considering that European ash's population is currently growing at the southernmost border of its European distribution in the La font del Regàs stand, and its inability to obtain water from other sources rather than vadose soil layer (Poblador, 2018), even in comparable humid environments such as the Rhoyne river floodplain (e.g. Sánchez-Pérez et al., 2008;Singer et al., 2014;Sargeant and Singer, 2016), our results suggest that this tree species endurance might be threatened in La font del Regàs stand under worst-case climate change scenarios. Furthermore, additional research should be focused in Mediterranean riparian tree species' responses to climate change. ...
... On the other hand, European ash's water uptake has been described to be closely related to SWC within first soil layers (e.g. Sánchez-Pérez et al., 2008;Singer et al., 2014). Moreover, Poblador (2018) reports close to zero phreatic water uptake for European ash trees in La font del Regàs stand. ...
Mediterranean riparian forests are comparably humid environments that provide shelter for several broadleaved deciduous tree species at their southernmost distribution margin. The stability of these communities, however, is threatened by climate change as well as invasive tree species, such as black locust (Robinia pseudoacacia L.). So far, black locust's European distribution appears to be mostly limited by low temperatures, but global warming might enhance its growth in colder areas. Moreover, R. pseudoacacia can better access water from the phreatic level than some native non-phreatophytic tree species such as European ash (Fraxinus excelsior L.). In this study, we compare the performance of European ash, a native deciduous tree species at its southernmost distribution border, with the invasive black locust, under a range of climate change projections, in a stand located at N.E. Spain. We first use Bayesian inference to calibrate the GOTILWA + vegetation model against sap flow data for both tree species. We then project each tree species' performance under several climate change scenarios. Our results indicate that increasing temperatures will enlarge black locust's vegetative period, leading to substantially increased annual productivity if the phreatic water table keeps reachable. For European ash, we project a slight increase in productivity, but with higher uncertainty. Our findings suggest that black locust will profit more from global warming than the native European ash, which is concerning because of the already detrimental impact of black locust for the local ecosystems. We conclude that climate change has the potential to stimulate black locust growth on Mediterranean riparian forests. Forest management should therefore include mechanisms to avoid black locust establishment, such as avoid clear-cutting and maintaining closed riparian forest canopies.
