Relative contribution of groundwater to plant transpiration estimated with stable isotopes

Nature-based solution enhances resilience to flooding and catalyzes multi-benefits in coastal cities in the Global South

Coastal cities are facing a rise in groundwater levels induced by sea level rise, further triggering saturation excess flooding where groundwater levels reach the topographic surface or reduce the storage capacity of the soil, thus stressing the existing infrastructure. Lowering groundwater levels is a priority for sustaining the long-term livelihood of coastal cities. In the absence of studies assessing the possibility of using tree-planting as a measure of alleviating saturation excess flooding in the context of rising groundwater levels, the multi-benefit nature of tree-planting programs as sustainable Nature-based solutions (NBSs) in coastal cities in the Global South is discussed. In environments where groundwater is shallow, trees uptake groundwater or reduce groundwater recharge, thereby contributing to lower groundwater levels and increasing the unsaturated zone thickness, further reducing the risk of saturation excess flooding. Tree-planting programs represent long-term solutions sustained by environmental factors that are complementary to conventional engineering solutions. The multi-benefit nature of such NBSs and the expected positive environmental, economic, and social outcomes make them particularly promising. Wide social acceptance was identified as crucial for the long-term success of any tree-planting program, as the social factor plays a major role in addressing most weaknesses and threats of the solution. In the case of Nouakchott City (Mauritania), where a rise in groundwater levels has led to permanent saturation excess flooding, a tree-planting program has the potential to lower the groundwater levels, thereby reducing flooding during the rainy season.

The determining factors of hydrogen isotope offsets between plants and their source waters

A fundamental assumption when using hydrogen and oxygen stable isotopes to understand ecohydrological processes is that no isotope fractionation occurs during plant water uptake/transport/redistribution. A growing body of evidence has indicated that hydrogen isotope fractionation occurs in certain environments or for certain plant species. However, whether the plant water source hydrogen isotope offset (δ2 H offset) is a common phenomenon and how it varies among different climates and plant functional types remains unclear. Here, we demonstrated the presence of positive, negative, and zero offsets based on extensive observations of 12 plant species of 635 paired stable isotopic compositions along a strong climate gradient within an inland river basin. Both temperature and relative humidity affected δ2 H offsets. In cool and moist environments, temperature mainly affected δ2 H offsets negatively due to its role in physiological activity. In warm and dry environments, relative humidity mainly affected δ2 H offsets, likely by impacting plant leaf stomatal conductance. These δ2 H offsets also showed substantial linkages with leaf water 18 O enrichment, an indicator of transpiration and evaporative demand. Further studies focusing on the ecophysiological and biochemical understanding of plant δ2 H dynamics under specific environments are essential for understanding regional ecohydrological processes and for conducting paleoclimate reconstructions.

The suitability of isotopic methods in identifying water sources of a shallow-rooted herbaceous plant in a desert steppe

Isotopic methodologies have gained prominence in investigating the composition of plant water sources; however, concerns regarding their suitability and reliability in diverse environments have emerged in recent years. This study presents a comparative analysis of root, soil, and liquid water (precipitation, dew, and groundwater) samples obtained from a desert steppe using isotope ratio infrared spectrometry (IRIS) and isotope ratio mass spectrometry (IRMS). The objective was to evaluate the applicability of these techniques in discerning the water sources of Stipa breviflora, a shallow-rooted herbaceous plant species. Additionally, we explored the root water uptake characteristics and water use strategy of S. breviflora. Our findings indicate that the IRIS method had more enriched values of D compared to the IRMS method across all samples, while no discernible pattern was observed for 18O. Notably, the differences observed among all samples exceeded the instruments' accuracies. Moreover, an unexpected occurrence was noted, whereby both D and 18O values in the root water were more enriched than in any of the considered water sources, rendering identification of the plant water sources unattainable. By conducting a re-analysis of more refined soil layer samples, we discovered that S. breviflora exhibits the ability to absorb and utilize water sources in close proximity to the soil surface. It further suggested that the shallow-rooted herbaceous plants in desert steppes can exploit small rainfalls, frequently overlooked in their ecological importance. Considering the distinctive soil and plant characteristics of desert steppes, we recommend adopting IRMS methods in conjunction with refined surface soil sampling for isotopic analysis aiming to identify water sources of shallow-rooted herbaceous plants. This study provides novel insights into assessing the suitability of isotopic techniques for analyzing plant water sources, while enhancing our understanding of water use strategies and environmental adaptation mechanisms employed by shallow-rooted herbaceous plants within xerophytic grassland ecosystems.

Effects and significance of groundwater for vegetation: A systematic review

Groundwater represents a critical water source for plants, especially during drought, with continuous groundwater availability widely associated with the presence of ecological refugia and the preservation of biodiversity during periods of adverse conditions. Here, we present a systematic quantitative literature review of global groundwater and ecosystem interactions to synthesise current knowledge and identify key knowledge gaps and research priorities through a management lens. Despite increasing research on groundwater dependent vegetation since the late 1990s, significant geographical and ecological biases are evident with papers focused on arid regions or areas with significant anthropogenic changes. Of the 140 papers reviewed, desert and steepe arid landscapes accounted for 50.7 % and desert and xeric shrublands were represented in 37.9 % of papers. A third of papers (34.4 %) quantified groundwater uptake by ecosystems and groundwater contributions to transpiration, with studies examining the influence of groundwater on vegetation productivity, distribution, and composition also well represented. In contrast, groundwater influences on other ecosystem functions are relatively poorly explored. The research biases introduce uncertainty in the transferability of findings between locations and ecosystems limiting the generality of our current understanding. This synthesis contributes to consolidating a solid knowledge base of the hydrological and ecological interrelationships for managers, planners, and other decision-makers that is relevant to the landscapes and environments they manage, so can more effectively deliver ecological and conservation outcomes.

Effects of nitrogen application on winter wheat growth, water use, and yield under different shallow groundwater depths

Shallow groundwater plays a vital role in physiology morphological attributes, water use, and yield production of winter wheat, but little is known of its interaction with nitrogen (N) application. We aimed to explore the effects of N fertilization rate and shallow groundwater table depth (WTD) on winter wheat growth attributes, yield, and water use. Experiments were carried out in micro-lysimeters at WTD of 0.6, 0.9, 1.2, and 1.5 m with 0, 150, 240, and 300 kg/ha N application levels for the winter wheat (Triticum aestivum L.). The results showed that there was an optimum groundwater table depth (Op-wtd), in which the growth attributes, groundwater consumption (GC), yield, and water use efficiency (WUE) under each N application rate were maximum, and the Op-wtd decreased with the increase in N application. The Op-wtd corresponding to the higher velocity of groundwater consumption (Gv) appeared at the late jointing stage, which was significantly higher than other WTD treatments under the same N fertilization. WTD significantly affected the Gv during the seeding to the regreening stage and maturity stage; the interaction of N application, WTD, and N application was significant from the jointing to the filling stage. The GC, leaf area index (LAI), and yield increased with an increase of N application at 0.6-0.9-m depth-for example, the yield and the WUE of the NF300 treatment with 0.6-m depth were significantly higher than those of the NF150-NF240 treatment at 20.51%, and 14.81%, respectively. At 1.2-1.5-m depth, the N application amount exceeding 150-240 kg/ha was not conducive to wheat growth, groundwater use, grain yield, and WUE. The yield and the WUE of 150-kg/ha treatment were 15.02% and 10.67% higher than those of 240-300-kg/ha treatment at 1.2-m depth significantly. The optimum N application rate corresponding to yield indicated a tendency to decrease with the WTD increase. Considering the winter wheat growth attributes, GC, yield, and WUE, application of 150-240 kg/ha N was recommended in our experiment.

Root Water Uptake Patterns for Nitraria during the Growth Period Differing in Time Interval from a Precipitation Event in Arid Regions

Vegetation root water uptake is one of the most central water transport processes along the soil-vegetation-atmosphere interface particularly in (semi-)arid ecosystems. The identification and quantification of root activities and water uptake patterns of arid vegetation remain challenging. This paper aims at the quantitative examination of water uptake behaviors of Nitraria, a prevalent desert species in arid environments, during the growth phase via a multivariate linear mixed model based on water stable isotopes, with a main focus on the time interval from a precipitation pulse. The observations indicate that the precipitation events exert periodic significant pulse-effects on vegetation water uptake through direct absorption (contribution of almost 75%) and activation of deep root activity at a certain depth. While in most occasions without rainfall, Nitraria relies on its extremely extensive shallow roots in surface-near lateral zone (contribution of about 60%) to extract massive soil as well as the hydraulic lifting mechanism to survive drought. Achievements would be beneficial to enhancing the understanding of entangled water transport processes and eco-hydrological feedbacks along soil-vegetation interface in arid ecosystems and contribute to a scientific allocation to water resources with the consideration of ecological protection.

Effects of Groundwater Mineralization and Groundwater Depth on Eco-Physiological Characteristics of Robinia pseudoacacia L. in the Yellow River Delta, China

Groundwater plays a significant role in influencing the growth and distribution of Robinia pseudoacacia L. plantations, with the largest planting area in the Yellow River Delta, by affecting the soil water–salt environment. This study aimed to clarify the mechanism of groundwater’s influence on the growth of R. pseudoacacia under different levels of groundwater mineralization (GWM) and groundwater depth (GWD). We simulated GWM of 0, 2 and 4 g L−1, and GWD of 0.8, 1.3 and 1.8 m. As GWM increased, soil relative water content (SRWC) and soil salt (dissolved salt) content (SSC) increased; sapling biomass (SB), stem mass (SM), leaf mass (LM), photosynthesis characteristics (maximum net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (E) and water use efficiency (WUE)) decreased; root mass (RM), root mass ratio (RMR) and root–shoot ratio (RSR) first increased then decreased; stem mass ratio (SMR) first decreased then increased; and leaf mass ratio (LMR) increased. As GWD increased, SRWC decreased, but SSC first increased then decreased; SB, RM, RMR, RSR, and photosynthesis characteristics increased; SM and LM first increased then decreased; and SMR and LMR decreased. SRWC and SSC were negatively correlated with SB and photosynthesis characteristics. SRWC was negatively correlated with RMR and RSR, whereas it was positively correlated with LMR. SSC was negatively correlated with SMR, whereas it was positively correlated with LMR. The first principal component, including SB, RM, and photosynthesis characteristics, was related to sapling growth. The second principal component, including RMR, SMR, and RSR, was mainly related to biomass allocation. In conclusion, GWM and GWD affected the soil water and salt content, which were key factors influencing the photosynthesis and growth of R. pseudoacacia. Adjustments in biomass allocation and photosynthesis were the main adaptive strategies of R. pseudoacacia to salt, drought, and flooding stress.

Quantitative Stable Isotope Tracing of Water Uptake in Cynanchum sibiricum during Its Growth Period in Arid Areas of Northwest China

Vegetation root water uptake plays a significant role in water transport processes and has multiple effects along the soil–vegetation–atmosphere interface, particularly in semi-arid and arid ecosystems. The objective of this study was to quantitatively examine the water uptake in Cynanchum sibiricum, a common desert rattan plant in arid regions, during its growth period, as well as to assess the relevance of the contributions of various water sources via a multivariate linear mixed model based on water-stable isotopes and eliminating the significant short-term impact of precipitation pulses on root water uptake. The results indicated that when the influence of a precipitation event was minimized, 0–10 cm soil water was the dominant water source for Cynanchum sibiricum in its growth phase, and that the species’ relative uptake of rainfall was positively proportional to that of 10–70 cm soil water while negatively proportional to that of 0–10 cm soil water. We concluded that Cynanchum sibiricum relies on its active roots extending transversely to extract water from shallow soil to survive under extreme drought conditions and would be inseparable from the symbiosis formed with other deep-rooted vegetation. Additional research into disentangling the water transport processes and studying the ecohydrological feedback along the soil–vegetation interface in arid ecosystems would be beneficial and could contribute to the evidence-based management of water resources for ecological protection.

Evidence for distinct isotopic compositions of sap and tissue water in tree stems: consequences for plant water source identification

The long-standing hypothesis that the isotopic composition of plant stem water reflects that of source water is being challenged by studies reporting bulk water from woody stems with an isotopic composition that cannot be attributed to any potential water source. The mechanism behind such source-stem water isotopic offsets is still poorly understood. Using a novel technique to extract selectively sap water from xylem conduits, we show that, in cut stems and potted plants, the isotopic composition of sap water reflects that of irrigation water, demonstrating unambiguously that no isotopic fractionation occurs during root water uptake or sap water extraction. By contrast, water in nonconductive xylem tissues is always depleted in deuterium compared with sap water, irrespective of wood anatomy. Previous studies have shown that isotopic heterogeneity also exists in soils at the pore scale in which water adsorbed onto soil particles is more depleted in deuterium than unbound water. Data collected at a riparian forest indicated that sap water matches best unbound soil water from depth below -70 cm, while bulk stem and soil water differ markedly. We conclude that source-stem isotopic offsets can be explained by micrometre-scale heterogeneity in the isotope ratios of water within woody stems and soil micro-pores.

Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers

The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial. We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use. Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting. These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Formation and Development of Taproots in Deciduous Tree Species

Trees are generally long-lived and are therefore exposed to numerous episodes of external stimuli and adverse environmental conditions. In certain trees e.g., oaks, taproots evolved to increase the tree's ability to acquire water from deeper soil layers. Despite the significant role of taproots, little is known about the growth regulation through internal factors (genes, phytohormones, and micro-RNAs), regulating taproot formation and growth, or the effect of external factors, e.g., drought. The interaction of internal and external stimuli, involving complex signaling pathways, regulates taproot growth during tip formation and the regulation of cell division in the root apical meristem (RAM). Assuming that the RAM is the primary regulatory center responsible for taproot growth, factors affecting the RAM function provide fundamental information on the mechanisms affecting taproot development.

Tree-ring oxygen isotopes record a decrease in Amazon dry season rainfall over the past 40 years

Extant climate observations suggest the dry season over large parts of the Amazon Basin has become longer and drier over recent decades. However, such possible intensification of the Amazon dry season and its underlying causes are still a matter of debate. Here we used oxygen isotope ratios in tree rings (δ¹⁸O_TR) from six floodplain trees from the western Amazon to assess changes in past climate. Our analysis shows that δ¹⁸O_TR of these trees is negatively related to inter-annual variability of precipitation during the dry season over large parts of the Amazon Basin, consistent with a Rayleigh rainout model. Furthermore δ¹⁸O_TR increases by approximately 2‰ over the last four decades (~ 1970-2014) providing evidence of an Amazon drying trend independent from satellite and in situ rainfall observations. Using a Rayleigh rainout framework, we estimate basin-wide dry season rainfall to have decreased by up to 30%. The δ¹⁸O_TR record further suggests such drying trend may not be unprecedented over the past 80 years. Analysis of δ¹⁸O_TR with sea surface temperatures indicates a strong role of a warming Tropical North Atlantic Ocean in driving this long-term increase in δ¹⁸O_TR and decrease in dry season rainfall.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00382-021-06046-7.

Spatiotemporal origin of soil water taken up by vegetation

Vegetation modulates Earth's water, energy and carbon cycles. How its functions might change in the future largely depends on how it copes with droughts^1-4. There is evidence that, in places and times of drought, vegetation shifts water uptake to deeper soil^5-7 and rock^8,9 moisture as well as groundwater^10-12. Here we differentiate and assess plant use of four types of water sources: precipitation in the current month (source 1), past precipitation stored in deeper unsaturated soils and/or rocks (source 2), past precipitation stored in groundwater (source 3, locally recharged) and groundwater from precipitation fallen on uplands via river-groundwater convergence toward lowlands (source 4, remotely recharged). We examine global and seasonal patterns and drivers in plant uptake of the four sources using inverse modelling and isotope-based estimates. We find that (1), globally and annually, 70% of plant transpiration relies on source 1, 18% relies on source 2, only 1% relies on source 3 and 10% relies on source 4; (2) regionally and seasonally, source 1 is only 19% in semi-arid, 32% in Mediterranean and 17% in winter-dry tropics in the driest months; and (3) at landscape scales, source 2, taken up by deep roots in the deep vadose zone, is critical in uplands in dry months, but source 4 is up to 47% in valleys where riparian forests and desert oases are found. Because the four sources originate from different places and times, move at different spatiotemporal scales and respond with different sensitivity to climate and anthropogenic forces, understanding the space and time origins of plant water sources can inform ecosystem management and Earth system models on the critical hydrological pathways linking precipitation to vegetation.

Stable Isotopes in Tree Rings of Pinus heldreichii Can Indicate Climate Variability over the Eastern Mediterranean Region

A long-term context is important for understanding past climatic variability. Although tree-ring widths (TRWs) are widely used as a proxy for reconstructing past climate, the use of annually-resolved values of δ13C and δ18O tree-ring stable isotopes (TRSIs) is increasing and may provide further valuable information. Here, we present a 487-year-long TRW chronology and 240-year-long TRSI chronology for Bosnian pine (Pinus heldreichii H. Christ) and compare them to each other. We demonstrate that both δ13C and δ18O values are better proxies for temperature, precipitation, and drought than TRW. The correlations between these climate parameters and TRSIs are strongest for the combined summer (JJA) period. The results of temporal and spatial field correlation indicate that TRSI chronologies are stable, reliable proxies for JJA precipitation reconstruction over the whole Balkan Peninsula and surrounding eastern Mediterranean region. However, the stability of the temperature signal of the both δ13C and δ18O chronologies declines after the 1950s. Our work supports the emerging evidence that TRSI data track climate variability more accurately than a conventional TRW approach and can be subsequently used for the reconstruction of past climate.

From glacial refugia to hydrological microrefugia: Factors and processes driving the persistence of the climate relict tree Zelkova sicula

With only two tiny populations, the climate relict Zelkova sicula (Sicily, Italy) is one of the rarest trees in the world. It also represents the most marginal member of genus Zelkova that was widespread in the broadleaved forests thriving in warm-temperate climates throughout Eurasia until the Last Glacial Age. Occurring at the westernmost range of the genus under typical Mediterranean climate, the micro-topographic settings have always appeared crucial for the survival of this relict. However, the factors and processes actually involved in its persistence in the current refugia, as well as the response of similar relict trees in arid environments, are poorly understood worldwide. In the aim to elucidate these aspects, in the two sites hosting Z. sicula analyses of topographical attributes were combined with investigations on soil moisture dynamics. Additionally, plants' growth and spatial distribution patterns were analyzed to detect fine-scale differences between populations and assess the possible ecological amplitude of the species. Results revealed that convergent topographies are basic determinants of microrefugia in arid environments. Within the investigated sites, underground moisture never decreases below 25%, buffering seasonal rainfall fluctuations. Therefore, hydrological microrefugia play a key role in decoupling from regional climate, supporting the target species in coping with an unsuitable climatic envelope. Additionally, the inter-population variability of biometric attributes showed that individual growth is site-dependent and the species retains a relative ecological plasticity, whereas the strongly clumped spatial patterns confirmed the common clonal growth. On one hand, deeply incised landforms have acted as effective hydrologic microrefugia, on the other clonality coupled with triploidy supposedly improved the resistance of Z. sicula to harsh environments, though entailing inability to reproduce sexually. Most likely, sterility and environmental/physical barriers that have existed for millennia have prevented this relict from leaving the last suitable microrefugia, resulting in the two current rear edge populations.

Tracing plant–environment interactions from organismal to planetary scales using stable isotopes: a mini review

Natural isotope variation forms a mosaic of isotopically distinct pools across the biosphere and flows between pools integrate plant ecology with global biogeochemical cycling. Carbon, nitrogen, and water isotopic ratios (among others) can be measured in plant tissues, at root and foliar interfaces, and in adjacent atmospheric, water, and soil environments. Natural abundance isotopes provide ecological insight to complement and enhance biogeochemical research, such as understanding the physiological conditions during photosynthetic assimilation (e.g. water stress) or the contribution of unusual plant water or nutrient sources (e.g. fog, foliar deposition). While foundational concepts and methods have endured through four decades of research, technological improvements that enable measurement at fine spatiotemporal scales, of multiple isotopes, and of isotopomers, are advancing the field of stable isotope ecology. For example, isotope studies now benefit from the maturation of field-portable infrared spectroscopy, which allows the exploration of plant–environment sensitivity at physiological timescales. Isotope ecology is also benefiting from, and contributing to, new understanding of the plant–soil–atmosphere system, such as improving the representation of soil carbon pools and turnover in land surface models. At larger Earth-system scales, a maturing global coverage of isotope data and new data from site networks offer exciting synthesis opportunities to merge the insights of single-or multi-isotope analysis with ecosystem and remote sensing data in a data-driven modeling framework, to create geospatial isotope products essential for studies of global environmental change.

Shallow urban aquifers under hyper-recharge equatorial conditions and strong anthropogenic constrains. Implications in terms of groundwater resources potential and integrated water resources management strategies

Humid equatorial regions are recognized as the least documented in term of hydrogeological functioning of aquifers despite the fact that they house a lot of developing countries and that groundwater is often the main water resource. Regarding this aspect, a study was conducted in sub-Saharan Africa, focusing on the Mio-Pliocene aquifer in Douala megacity (Cameroon) which is the rainiest city in West-Africa (about 4000 mm/year) with one of the greatest demographic growth rate of the African continent. Firstly, groundwater recharge rate has been calculated through water balance and Water Table Fluctuation methods. Results show that the aquifer is characterized by a high recharge of 600-760 mm/year. Then infiltration process and groundwater flow conditions have been examined by combining hydrogeological and isotopic methods. Rainwater infiltrated is recycled in the vadose zone through plants roots transpiration and groundwater flows with a Darcy velocity of 5 m/day. From the recharge area to the estuary, the mineralization increases controlled by anthropogenic activities and water-rocks interactions which are amplified by the residence time and accelerated by the hot and humid climate of Douala. The paper ends with the determination of natural background levels (NBLs) and threshold values (TV) of chemical components in groundwater to assess the contamination for different flow paths. This multi-proxy study and the establishment of NBLs and TV can be beneficial to improve groundwater resources management. Moreover, the conceptual model provided in this study could be used as a reference for porous aquifers submitted to high rainfall amount.

Water uptake depth is coordinated with leaf water potential, water-use efficiency and drought vulnerability in karst vegetation

Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake depth, leaf-level water-use efficiency (WUEi) and water potential in drought-prone plant communities is not well understood. We conducted a 135-d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co-occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season. Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller-diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought-induced canopy defoliation and/or mortality. By contrast, larger-diameter species with deeper water uptake, higher leaf-level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation. Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities.

Detecting Tree Species Effects on Forest Canopy Temperatures with Thermal Remote Sensing: The Role of Spatial Resolution

Canopy temperatures are important for understanding tree physiology, ecology, and their cooling potential, which provides a valuable ecosystem service, especially in urban environments. Linkages between tree species composition in forest stands and air temperatures remain challenging to quantify, as the establishment and maintenance of onsite sensor networks is time-consuming and costly. Remotely-sensed land surface temperature (LST) observations can potentially acquire spatially distributed crown temperature data more efficiently. We analyzed how tree species modify canopy air temperature at an urban floodplain forest (Leipzig, Germany) site equipped with a detailed onsite sensor network, and explored whether mono-temporal thermal remote sensing observations (August, 2016) at different spatial scales could be used to model air temperatures at the tree crown level. Based on the sensor-network data, we found interspecific differences in summer air temperature to vary temporally and spatially, with mean differences between coldest and warmest tree species of 1 °C, and reaching maxima of up to 4 °C for the upper and lower canopy region. The detectability of species-specific differences in canopy surface temperature was found to be similarly feasible when comparing high-resolution airborne LST data to the airborne LST data aggregated to 30 m pixel size. To realize a spatial resolution of 30 m with regularly acquired data, we found the downscaling of Landsat 8 thermal data to be a valid alternative to airborne data, although detected between-species differences in surface temperature were less expressed. For the modeling of canopy air temperatures, all LST data up to the 30 m level were similarly appropriate. We thus conclude that satellite-derived LST products could be recommended for operational use to detect and monitor tree species effects on temperature regulation at the crown scale.

Comparing Approaches for Reconstructing Groundwater Levels in the Mountainous Regions of Interior British Columbia, Canada, Using Tree Ring Widths

Observed groundwater level records are relatively short (<100 years), limiting long-term studies of groundwater variability that could provide valuable insight into climate change effects. This study uses tree ring data from the International Tree Ring Database (ITRDB) and groundwater level data from 22 provincial observation wells to evaluate different approaches for reconstructing groundwater levels from tree ring widths in the mountainous southern interior of British Columbia, Canada. The twenty-eight reconstruction models consider the selection of observation wells (e.g., regional average groundwater level vs. wells classified by recharge mechanism) and the search area for potential tree ring records (climate footprint vs. North American Ecoregions). Results show that if the climate footprint is used, reconstructions are statistically valid if the wells are grouped according to recharge mechanism, with streamflow-driven and high-elevation recharge-driven wells (both snowmelt-dominated) producing valid models. Of all the ecoregions considered, only the Coast Mountain Ecoregion models are statistically valid for both the regional average groundwater level and high-elevation recharge-driven systems. No model is statistically valid for low-elevation recharge-driven systems (rainfall-dominated). The longest models extend the groundwater level record to the year 1500, with the highest confidence in the later portions of the reconstructions going back to the year 1800.

An explanation for the isotopic offset between soil and stem water in a temperate tree species

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured δ² H and δ¹⁸ O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. Regardless of substrate, soil and stem water δ² H were similar only near permanent wilting point. Under moister conditions, stem water δ² H was 11 ± 3‰ more negative than soil water δ² H, coherent with field studies. Under drier conditions, stem water δ² H became progressively more enriched than soil water δ² H. Although stem water δ¹⁸ O broadly reflected that of soil water, soil-stem δ² H and δ¹⁸ O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies. Soil-stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil-plant continuum.

Variation in the access to deep soil water pools explains tree-to-tree differences in drought-triggered dieback of Mediterranean oaks

Individual differences in the access to deep soil water pools may explain the differential damage among coexisting, conspecific trees as a consequence of drought-induced dieback. We addressed this issue by comparing the responses to a severe drought of three Mediterranean oak species with different drought tolerance, Quercus pubescens L. and Quercus frainetto Ten., mainly thriving at xeric and mesic sites, respectively, and Quercus cerris L., which dominates at intermediate sites. For each species, we compared coexisting declining (D) and non-declining (ND) trees. The stable isotope composition (δ2H, δ18O) of xylem and soil water was used to infer a differential use of soil water sources. We also measured tree size and radial growth to quantify the long-term divergence of wood production between D and ND trees and non-structural carbohydrates (NSCs) in sapwood to evaluate if D trees presented lower NSC values. The ND trees had access to deeper soil water than D trees except in Q. frainetto, as indicated by significantly more depleted xylem water values. However, a strong δ2H offset between soil and xylem water isotopes observed in peak summer could suggest that both tree types were not physiologically active under extreme drought conditions. Alternative processes causing deuterium fractionation, however, could not be ruled out. Tree height and recent (last 15-25 years) growth rates in all species studied were lower in D than in ND trees by 22 and 44%, respectively. Lastly, there was not a consistent pattern of NSC sapwood concentration; in Q. pubescens, it was higher in ND trees while in Q. frainetto, the D trees were the ones exhibiting the higher NSC concentration. We conclude that the vulnerability to drought among conspecific Mediterranean oaks depends on the differential access to deep soil water pools, which may be related to differences in rooting depth, tree size and growth rate.

17 O-excess as a detector for co-extracted organics in vapor analyses of plant isotope signatures

RATIONALE

The stable isotope compositions of hydrogen and oxygen in water (δ² H and δ¹⁸ O values) have been widely used to investigate plant water sources, but traditional isotopic measurements of plant waters are expensive and labor intensive. Recent work with direct vapor equilibration (DVE) on laser spectroscopy has shown potential to side step limitations imposed by traditional methods. Here, we evaluate DVE analysis of plants with a focus on spectral contamination introduced by organic compounds. We present ¹⁷ O-excess as a way of quantifying organic compound interference in DVE.

METHODS

We performed isotopic analysis using the δ² H, δ¹⁸ O and δ¹⁷ O values of water on an Off-Axis Integrated Cavity Output Spectroscopy (IWA-45EP OA-ICOS) instrument in vapor mode. We used a set of methanol (MeOH) and ethanol (EtOH) solutions to assess errors in isotope measurements. We evaluated how organic compounds affect the ¹⁷ O-excess. DVE was used to measure the isotopic signatures in natural plant material from Pinus banksiana, Picea mariana, and Larix laricina, and soil from boreal forest for comparison with solutions.

RESULTS

The ¹⁷ O-excess was sensitive to the presence of organic compounds in water. ¹⁷ O-excess changed proportionally to the concentration of MeOH per volume of water, resulting in positive values, while EtOH solutions resulted in smaller changes in the ¹⁷ O-excess. Soil samples did not show any spectral contamination. Plant samples were spectrally contaminated on the narrow-band and were enriched in ¹ H and ¹⁶ O compared with source water. L. laricina was the only species that did not show any evidence of spectral contamination. Xylem samples that were spectrally contaminated had positive ¹⁷ O-excess values.

CONCLUSIONS

¹⁷ O-excess can be a useful tool to identify spectral contamination and improve DVE plant and soil analysis in the laboratory and in situ. The ¹⁷ O-excess flagged the presence of MeOH and EtOH. Adding measurement of δ¹⁷ O values to traditional measurement of δ² H and δ¹⁸ O values may shed new light on plant water analysis for source mixing dynamics using DVE.

A pool-weighted perspective on the two-water-worlds hypothesis

The 'two-water-worlds' hypothesis is based on stable isotope differences in stream, soil and xylem waters in dual isotope space. It postulates no connectivity between bound and mobile soil waters, and preferential plant water uptake of bound soil water sources. We tested the pool-weighted impact of isotopically distinct water pools for hydrological cycling, the influence of species-specific water use and the degree of ecohydrological separation. We combined stable isotope analysis (δ¹⁸ O and δ² H) of ecosystem water pools of precipitation, groundwater, soil and xylem water of two distinct species (Quercus suber, Cistus ladanifer) with observations of soil water contents and sap flow. Shallow soil water was evaporatively enriched during dry-down periods, but enrichment faded strongly with depth and upon precipitation events. Despite clearly distinct water sources and water-use strategies, both species displayed a highly opportunistic pattern of root water uptake. Here we offer an alternative explanation, showing that the isotopic differences between soil and plant water vs groundwater can be fully explained by spatio-temporal dynamics. Pool weighting the contribution of evaporatively enriched soil water reveals only minor annual impacts of these sources to ecosystem water cycling (c. 11% of bulk soil water and c. 14% of transpired water).

Woody Plant Encroachment Impacts on Groundwater Recharge: A Review

Woody plant encroachment has profound impacts on the sustainable management of water resources in water-limited ecosystems. However, our understanding of the effects of this global phenomenon on groundwater recharge at local and regional scales is limited. Here, we reviewed studies related to (i) recharge estimation methods; (ii) mechanisms by which woody plants impact groundwater recharge; (iii) impacts of woody plant on recharge across different soil and geology; (iv) hydrological repercussions of woody plant removal; and (v) research gaps and needs for groundwater studies. We identified six different methods: water balance, water table, isotopes, chloride mass balance, electrical geophysical imaging, and modeling were used to study the impact of woody encroachment on groundwater. Woody plant encroachment could alter soil infiltration rates, soil water storage, transpiration, interception, and subsurface pathways to affect groundwater recharge. The impact is highly variable, with the extent and the magnitude varying across the soil, substrate, plant cover, and topographic locations. Our review revealed mixed effects of woody plant removal on groundwater recharge. Studies of litter interception, root water uptake, soil moisture dynamics, and deep percolation along with the progression of woody plant encroachment are still limited, warranting further experimental studies focusing on groundwater recharge. Overall, information about woody plant encroachment impacts on groundwater resources across a range of scales is essential for long-range planning of water resources.

Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi-arid coastal ecosystem

Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi-arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (a) how is woody vegetation ecophysiological performance affected by water table depth during the dry season? and (b) does the vegetation response to increasing depth to groundwater differ among water-use functional types? We examined a suite of physiological parameters and water-uptake depths of the dominant, functionally distinct woody vegetation along a water-table depth gradient in a Mediterranean semi-arid coastal ecosystem that is currently experiencing anthropogenic groundwater extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water-source use exist, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi-arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi-arid coastal ecosystems to groundwater drawdown.

Separation of Scales in Transpiration Effects on Low Flows: A Spatial Analysis in the Hydrological Open Air Laboratory

The objective of this study was to understand whether spatial differences in runoff generation mechanisms affect the magnitudes of diurnal streamflow fluctuations during low flow periods and which part of the catchment induces the diurnal streamflow signal. The spatiotemporal variability of the streamflow fluctuations observed at 12 locations in the 66-ha Hydrological Open Air Laboratory experimental catchment in Austria was explained by differences in the vegetation cover and runoff generation mechanisms. Almost a quarter of the volume associated with diurnal streamflow fluctuations at the catchment outlet was explained by transpiration from vegetation along the tributaries; more than three quarters was due to transpiration by the riparian forest along the main stream. The lag times between radiative forcing and evapotranspiration estimated by a solar radiation-driven model increased from 3 to 11 hr from spring to autumn. The recession time scales increased from 21 days in spring to 54 days in autumn. Observations and model simulations suggest that a separation of scales in transpiration effects on low flows exists both in time and space; that is, the diurnal streamflow fluctuations are induced by transpiration from the riparian vegetation, while most of the catchment evapotranspiration, such as evapotranspiration from the crop fields further away from the stream, do not influence the diurnal signal in streamflow.