The role of deep vadose zone water in tree transpiration during drought periods in karst settings - Insights from isotopic tracing and leaf water potential

Introduction of broadleaf tree species can promote the resource use efficiency and gross primary productivity of pure forests

Long-term pure forest (PF) management and successive planting has result resulted in "low-efficiency artificial forests" in large areas. However, controversy persists over the promoting effect of introduction of broadleaf tree species on production efficiency of PF. This study hypothesised that introduced broadleaf tree species can significantly promote both water-nutrient use efficiency and gross primary productivity (GPP)of PF. Tree ring chronologies, water source, water use efficiency and GPP were analysed in coniferous Cunninghamia lanceolata and broadleaved Phoebe zhennan growing over the past three decades. The introduction of P. zhennan into C. lanceolata plantations resulted in inter-specific competition for water, probably because of the similarity of the main water source of these two tree species. However, C. lanceolata absorbed more water with a higher nutrient level from the 40-60-cm soil layer in mixed forests (MF). Although the co-existing tree species limited the basal area increment and growth rates of C. lanceolata in MF plots, the acquisition of dissolved nutrients from the fertile topsoil layer were enhanced; this increased the water use efficiency and GPP of MF plots. To achieve better ecological benefits and GPP, MFs should be constructed in southern China.

Coordination between degree of isohydricity and depth of root water uptake in temperate tree species

In an increasingly dry environment, it is crucial to understand how tree species use soil water and cope with drought. However, there is still a knowledge gap regarding the relationships between species-specific stomatal behaviour, spatial root distribution, and root water uptake (RWU) dynamics. Our study aimed to investigate above- and below-ground aspects of water use during soil drying periods in four temperate tree species that differ in stomatal behaviour: two isohydric tracheid-bearing conifers, Scots pine and Norway spruce, and two more anisohydric deciduous species, the diffuse-porous European beech, and the ring-porous Downy oak. From 2015 to 2020, soil-tree-atmosphere-continuum parameters were measured for each species in monospecific forests where trees had no access to groundwater. The hourly time series included data on air temperature, vapor pressure deficit, soil water potential, soil hydraulic conductivity, and RWU to a depth of 2 m. Analysis of drought responses included data on stem radius, leaf water potential, estimated osmotically active compounds, and drought damage. Our study reveals an inherent coordination between stomatal regulation, fine root distribution and water uptake. Compared to conifers, the more anisohydric water use of oak and beech was associated with less strict stomatal closure, greater investment in deep roots, four times higher maximum RWU, a shift of RWU to deeper soil layers as the topsoil dried, and a more pronounced soil drying below 1 m depth. Soil hydraulic conductivity started to limit RWU when values fell below 10-3 to 10-5 cm/d, depending on the soil. As drought progressed, oak and beech may also have benefited from their leaf osmoregulatory capacity, but at the cost of xylem embolism with around 50 % loss of hydraulic conductivity when soil water potential dropped below -1.25 MPa. Consideration of species-specific water use is crucial for forest management and vegetation modelling to improve forest resilience to drought.

Sustainable Development versus Extractivist Deforestation in Tropical, Subtropical, and Boreal Forest Ecosystems: Repercussions and Controversies about the Mother Tree and the Mycorrhizal Network Hypothesis

This research reviews the phenomenon of extractive deforestation as a possible trigger for cascade reactions that could affect part of the forest ecosystem and its biodiversity (surface, aerial, and underground) in tropical, subtropical, and boreal forests. The controversy and disparities in criteria generated in the international scientific community around the hypothesis of a possible link between "mother trees" and mycorrhizal networks in coopetition for nutrients, nitrogen, and carbon are analyzed. The objective is to promote awareness to generate more scientific knowledge about the eventual impacts of forest extraction. Public policies are emphasized as crucial mediators for balanced sustainable development. Currently, the effects of extractive deforestation on forest ecosystems are poorly understood, which requires caution and forest protection. Continued research to increase our knowledge in molecular biology is advocated to understand the adaptation of biological organisms to the new conditions of the ecosystem both in the face of extractive deforestation and reforestation. The environmental impacts of extractive deforestation, such as the loss of biodiversity, soil degradation, altered water cycles, and the contribution of climate change, remain largely unknown. Long-term and high-quality research is essential to ensure forest sustainability and the preservation of biodiversity for future generations.

Water sources for street trees in mesic urban environments

Street trees support climate resiliency through a variety of pathways, such as offsetting urban heat and attenuating storm water runoff. While urban trees in arid and semiarid ecosystems have been shown to take up water from irrigation, it is unknown where street trees in mesic cities obtain their water. In this study, we use natural abundance stable isotopes to estimate the proportional sources of water taken up by Acer platanoides street trees in Boston, Massachusetts, United States, including precipitation, irrigation, groundwater, and wastewater. We use Bayesian multisource mixing models to estimate water sources by comparing the natural abundance isotopic ratios of hydrogen and oxygen across potential water sources with water extracted from tree stem samples. We find that during the summer of 2021, characterized by anomalously high rainfall, street trees predominantly utilized water from precipitation. Precipitation accounted for 72.3 % of water extracted from trees sampled in August and 65.6 % from trees sampled in September. Of the precipitation taken up by street trees, most water was traced back to large storm events in July, with July rainfall alone accounting for up to 84.0 % of water found within street trees. We find strong relationships between canopy cover fractions and the proportion of precipitation lost to evapotranspiration across the study domain, supporting the conclusion that tree planting initiatives result in storm water mitigation benefits due to utilization of water from precipitation by urban vegetation. However, while the mature trees studied here currently support their water demand from precipitation, the dependency of street trees on precipitation in mesic cities may lead to increased water stress in a changing climate characterized by a higher frequency and severity of drought.

Quercus ilex L. dieback is genetically determined: Evidence provided by dendrochronology, δ13C and SSR genotyping

Quercus ilex L. dieback has been reported in several Mediterranean forests, revealing different degree of crown damages even in close sites, as observed in two Q. ilex forest stands in southern Tuscany (IT). In this work, we applied a novel approach combining dendrochronological, tree-ring δ13C and genetic analysis to test the hypothesis that different damage levels observed in a declining (D) and non-declining (ND) Q. ilex stands are connected to population features linked to distinct response to drought. Furthermore, we investigated the impact of two major drought events (2012 and 2017), that occurred in the last fifteen years in central Italy, on Q. ilex growth and intrinsic water use efficiency (WUEi). Overall, Q. ilex showed slightly different ring-width patterns between the two stands, suggesting a lower responsiveness to seasonal climatic variations for trees at D stand, while Q. ilex at ND stand showed changes in the relationship between climatic parameters and growth across time. The strong divergence in δ13C signals between the two stands suggested a more conservative use of water for Q. ilex at ND compared to D stand that may be genetically driven. Q. ilex at ND resulted more resilient to drought compared to trees at D, probably thanks to its safer water strategy. Genotyping analysis based on simple-sequence repeat (SSR) markers revealed the presence of different Q. ilex populations at D and ND stands. Our study shows intraspecific variations in drought response among trees grown in close. In addition, it highlights the potential of combining tree-ring δ13C data with SSR genotyping for the selection of seed-bearing genotypes aimed to preserve Mediterranean holm oak ecosystem and improve its forest management.

Increased uptake of deep soil water promotes drought resistance in mixed forests

The intensity and frequency of droughts are projected to rise in recent years and adversely affect forests. Thus, information on plant water use and acclimation during and after droughts is crucial. This study used the stable isotope and thermal dissipation probes to detect the water-use adaptation of mixed forests to drought using a precipitation gradient control experiment in the field. The results showed that Platycladus orientalis and Quercus variabilis mainly absorbed stable water from deep soil layers during the drought (32.05% and 28.2%, respectively). The synergetic nocturnal sap flow in both species replenished the water loss, but P. orientalis experienced a greater decline in transpiration acclimation to drought. The transpiration of Q. variabilis remained high since it was mainly induced by radiation. After short-term exposure to drought, P. orientalis majorly obtained shallow soil water, confirming its sensitivity to shallow water. Contrarily, Q. variabilis mainly absorbed stable water from deep soil layers regardless of the soil water content. Therefore, these findings suggest that Q. variabilis cannot physiologically adjust to extreme drought events, possibly limiting their future distributions and altering the composition of boreal forests.

Seasonal variations in water uptake and transpiration for plants in a karst critical zone in China

Despite substantial drought conditions in the karst critical zone (KCZ), the KCZ landscapes are often covered with forest woody plants. However, it is not well understood how these plants balance water supply and demand to survive in such a water-limited environment. This study investigated the water uptake and transpiration relationships of four coexisting woody species in a subtropical karst forest ecosystem using measurements of microclimate, soil moisture, stable isotopes (δ¹⁸O, δ²H, and δ¹³C), intrinsic water-use efficiency (WUE_i), sap flow, and rooting depth. The focus was on identifying differences within- and between-species across soil- and rock-dominated habitats (SDH and RDH) during the rainy growing season (September 2017) and dry season (February 2018). Species across both habitats tended to have higher transpiration with lower WUE_i during the rainy season and lower transpiration with higher WUE_i during the dry season. Compared to those in the SDH, species in the RDH showed lower transpiration with higher WUE_i in both seasons. The dominant water sources were soil water and rainwater for supporting rainy-season transpiration in the SDH and RDH, respectively, and groundwater was the main water source for supporting dry-season transpiration in both habitats. A clear ecohydrological niche differentiation was also revealed among species. Across both habitats, shallower-rooted species with higher soil-water uptake, compared to deeper-rooted species with higher groundwater uptake, showed higher transpiration and lower WUE_i during the rainy season and vice versa during the dry season. This study provides integrated insights into how forest woody plants in the KCZ regulate transpiration and WUE_i in response to drought stress through interactions with seasonal water sources in the environment.

Effects of high Ca and Mg stress on plants water use efficiency in a Karst ecosystem

Background

Karst ecosystems are widely distributed in the world, with one of the largest continuous Karst landforms in Southwest China. Karst regions are characterized by water shortage, high soil calcium (Ca) and magnesium (Mg) content, and soil nutrient leaching, resulting in drought stress and growth limitation of plants.

Methods

This study compared nitrogen (N), phosphorus (P), potassium (K), Ca, and Mg of herbaceous and woody plants in a small Karst ecosystem in Southwest China. The indexes of water use efficiency (WUE) were calculated to identify the drought stress of plants in this Karst ecosystem. Meanwhile, the relationship between Ca and Mg accumulation and WUE was evaluated in herbaceous and woody plants.

Results

Herbaceous plants showed a higher content of leaf N (13.4 to 40.1 g·kg^-1), leaf P (2.2 to 4.8 g·kg^-1) and leaf K (14.6 to 35.5 g·kg^-1) than woody plants (N: 10.4 g to 22.4 g·kg^-1; P: 0.4 to 2.3 g·kg^-1; K: 5.7 to 15.5 g·kg^-1). Herbaceous plants showed a significantly positive correlation between WUE and K:Ca ratio (R = 0.79), while WUE has a strongly positive correlation with K:Mg ratio in woody plants (R = 0.63).

Conclusion

Herbaceous plants suffered from nitrogen (N) limitation, and woody plants were constrained by P or N+P content. Herbaceous plants had higher leaf N, P, and K than woody plants, while Ca and Mg showed no significant differences, probably resulting from the Karst environment of high Ca and Mg contents. Under high Karst Ca and Mg stress, herbaceous and woody plants responded differently to Ca and Mg stress, respectively. WUE of herbaceous plants is more sensitive to Ca stress, while that of woody plants is more sensitive to Mg stress. These findings establish a link between plant nutrients and hydraulic processes in a unique Karst ecosystem, further facilitating studies of the nutrient-water cycling system in the ecosystem.

Soil water stress overrides the benefit of water-use efficiency from rising CO2 and temperature in a cold semi-arid poplar plantation

The counteractive effect of atmospheric CO₂ (c_a ) enrichment and drought stress on tree growth results in great uncertainty in the growth patterns of forest plantations in cold semi-arid regions. We analysed tree ring chronologies and carbon isotopes in Populus simonii plantations in cold semi-arid areas in northern China over the past four decades. We hypothesized that the hydraulic stress from drought would override the stimulating effect of increasing c_a and temperature (T) on stem growth (basal area increment [BAI]). We found the stimulating effect of rising c_a and T on the growth, indicated by continuous increase of intrinsic water-use efficiency in all stands and a positive correlation between T and BAI. However, these effects failed to alleviate the negative impacts of drought on tree growth. Concurrent acceleration of BAI reversed during the intensive drought episodes. Water stress resulted from inaccessibility of roots to deep soil water rather than from lack of precipitation, suggested by the decoupling of BAI from precipitation and vapour pressure deficit. Local soil water limitation might also cause greater stomatal regulation in declining trees, indicated by lower intercellular CO₂ concentration. Thus, site-specific soil moisture conditions growth sensitivity to global warming resulting in site-specific decline episodes in drought-prone areas.

Widespread woody plant use of water stored in bedrock

In the past several decades, field studies have shown that woody plants can access substantial volumes of water from the pores and fractures of bedrock^1-3. If, like soil moisture, bedrock water storage serves as an important source of plant-available water, then conceptual paradigms regarding water and carbon cycling may need to be revised to incorporate bedrock properties and processes^4-6. Here we present a lower-bound estimate of the contribution of bedrock water storage to transpiration across the continental United States using distributed, publicly available datasets. Temporal and spatial patterns of bedrock water use across the continental United States indicate that woody plants extensively access bedrock water for transpiration. Plants across diverse climates and biomes access bedrock water routinely and not just during extreme drought conditions. On an annual basis in California, the volumes of bedrock water transpiration exceed the volumes of water stored in human-made reservoirs, and woody vegetation that accesses bedrock water accounts for over 50% of the aboveground carbon stocks in the state. Our findings indicate that plants commonly access rock moisture, as opposed to groundwater, from bedrock and that, like soil moisture, rock moisture is a critical component of terrestrial water and carbon cycling.

Plant adaptability in karst regions

Karst ecosystems are formed by dissolution of soluble rocks, usually with conspicuous landscape features, such as sharp peaks, steep slopes and deep valleys. The plants in karst regions develop special adaptability. Here, we reviewed the research progresses on plant adaptability in karst regions, including drought, high temperature and light, high-calcium stresses responses and the strategies of water utilization for plants, soil nutrients impact, human interference and geographical traits on karst plants. Drought, high temperature and light change their physiological and morphological structures to adapt to karst environments. High-calcium and soil nutrients can transfer surplus nutrients to special parts of plants to avoid damage of high nutrient concentration. Therefore, karst plants can make better use of limited water. Human interference also affects geographical distribution of karst plants and their growing environment. All of these aspects may be analyzed to provide guidance and suggestions for related research on plant adaptability mechanisms.

Vertical decoupling of soil nutrients and water under climate warming reduces plant cumulative nutrient uptake, water-use efficiency and productivity

Warming-induced desiccation of the fertile topsoil layer could lead to decreased nutrient diffusion, mobility, mineralization and uptake by roots. Increased vertical decoupling between nutrients in topsoil and water availability in subsoil/bedrock layers under warming could thereby reduce cumulative nutrient uptake over the growing season. We used a Mediterranean semiarid shrubland as model system to assess the impacts of warming-induced topsoil desiccation on plant water- and nutrient-use patterns. A 6 yr manipulative field experiment examined the effects of warming (2.5°C), rainfall reduction (30%) and their combination on soil resource utilization by Helianthemum squamatum shrubs. A drier fertile topsoil ('growth pool') under warming led to greater proportional utilization of water from deeper, wetter, but less fertile subsoil/bedrock layers ('maintenance pool') by plants. This was linked to decreased cumulative nutrient uptake, increased nonstomatal (nutritional) limitation of photosynthesis and reduced water-use efficiency, above-ground biomass growth and drought survival. Whereas a shift to greater utilization of water stored in deep subsoil/bedrock may buffer the negative impact of warming-induced topsoil desiccation on transpiration, this plastic response cannot compensate for the associated reduction in cumulative nutrient uptake and carbon assimilation, which may compromise the capacity of plants to adjust to a warmer and drier climate.

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.

From the comfort zone to crown dieback: Sequence of physiological stress thresholds in mature European beech trees across progressive drought

Drought responses of mature trees are still poorly understood making it difficult to predict species distributions under a warmer climate. Using mature European beech (Fagus sylvatica L.), a widespread and economically important tree species in Europe, we aimed at developing an empirical stress-level scheme to describe its physiological response to drought. We analysed effects of decreasing soil and leaf water potential on soil water uptake, stem radius, native embolism, early defoliation and crown dieback with comprehensive measurements from overall nine hydrologically distinct beech stands across Switzerland, including records from the exceptional 2018 drought and the 2019/2020 post-drought period. Based on the observed responses to decreasing water potential we derived the following five stress levels: I (predawn leaf water potential >-0.4 MPa): no detectable hydraulic limitations; II (-0.4 to -1.3): persistent stem shrinkage begins and growth ceases; III (-1.3 to -2.1): onset of native embolism and defoliation; IV (-2.1 to -2.8): onset of crown dieback; V (<-2.8): transpiration ceases and crown dieback is >20%. Our scheme provides, for the first time, quantitative thresholds regarding the physiological downregulation of mature European beech trees under drought and therefore synthesises relevant and fundamental information for process-based species distribution models. Moreover, our study revealed that European beech is drought vulnerable, because it still transpires considerably at high levels of embolism and because defoliation occurs rather as a result of embolism than preventing embolism. During the 2018 drought, an exposure to the stress levels III-V of only one month was long enough to trigger substantial crown dieback in beech trees on shallow soils. On deep soils with a high water holding capacity, in contrast, water reserves in deep soil layers prevented drought stress in beech trees. This emphasises the importance to include local data on soil water availability when predicting the future distribution of European beech.

Long- and Short-Term Inorganic Nitrogen Runoff from a Karst Catchment in Austria

Excess nitrogen (N) deposition and gaseous N emissions from industrial, domestic, and agricultural sources have led to increased nitrate leaching, the loss of biological diversity, and has affected carbon (C) sequestration in forest ecosystems. Nitrate leaching affects the purity of karst water resources, which contribute around 50% to Austria’s drinking water supply. Here we present an evaluation of the drivers of dissolved inorganic N (DIN) concentrations and fluxes from a karst catchment in the Austrian Alps (LTER Zöbelboden) from 27 years of records. In addition, a hydrological model was used together with climatic scenario data to predict expected future runoff dynamics. The study area was exposed to increasing N deposition during the 20th century (up to 30 to 35 kg N ha−1 y−1), which are still at levels of 25.5 ± 3.6 and 19.9 ± 4.2 kg N ha−1 y−1 in the spruce and the mixed deciduous forests, respectively. Albeit N deposition was close to or exceeded critical loads for several decades, 70–83% of the inorganic N retained in the catchment from 2000 to 2018, and NO3- concentrations in the runoff stayed <10 mg L−1 unless high-flow events occurred or forest stand-replacing disturbances. We identified tree growth as the main sink for inorganic N, which might together with lower runoff, increase retention of only weakly decreasing N deposition in the future. However, since recurring forest stand-replacement is predicted in the future as a result of a combination of climatically driven disturbance agents, pulses of elevated nitrate concentrations in the catchment runoff will likely add to groundwater pollution.

Tree xylem water isotope analysis by Isotope Ratio Mass Spectrometry and laser spectrometry: A dataset to explore tree response to drought

Water isotopes from plant xylem and surrounding environment are increasingly used in eco-hydrological studies. Carrière et al. [1] analyzed a dataset of water isotopes in (i) the xylem of three different tree species, (ii) the surrounding soil and drainage water and (iii) the underlying karst groundwater, to understand tree water uptake during drought in two different Mediterranean forests on karst setting. The xylem and soil water were extracted by cryogenic distillation. The full dataset was obtained with Isotope Ratio Mass Spectrometry (IRMS) and Isotope Ratio Infrared Spectrometer (IRIS), and included 219 measurements of δ²H and δ¹⁸O. Prompted by unexpected isotopic data characterized by a very negative deuterium excess, a subsample of 46 xylem samples and 9 soil water samples were double checked with both analytical techniques. IRMS and IRIS analyses yielded similar data. Therefore, the results reveal that laser spectrometry allows an accurate estimation of xylem and soil water isotopes. The dataset highlights a strong ²H depletion in xylem water for all species. Deuterium does not seem adequate to interpret ecological processes in this dataset given the strong fractionation.

Karst recharge-discharge semi distributed model to assess spatial variability of flows

Aquifer recharge assessment is a key factor for sustainable groundwater resource management. Although main factors of the spatial and temporal variability of recharge are known, taking them into account in a distributed or semi-distributed model is still a challenging task. This difficulty is increased in karst environments. Indeed, recharge of karst aquifers also depends on the organization of the karst network, which is both highly heterogeneous and difficult to characterize. We developed a reservoir model to simulate the spatial and temporal variability of recharge on karst watersheds. Special attention was paid to the link between model parameters and measurable or qualitative environmental factors of recharge. The spatial variability of soil reservoir capacity was estimated by multifactorial modelling (neural network). Intrinsic vulnerability indices were used to constrain the partitioning between slow and fast flows within the karst aquifer. Comparison of simulated and measured discharge at the outlet was used to calibrate and assess recharge model. The karst hydrosystem of the Fontaine de Vaucluse is renowned for its significant heterogeneity and anisotropy, which has so far limited the application of 2D or 3D modelling. The model developed was successfully applied to this system. Our results showed that the annual recharge is very heterogeneous on the test site. Spatialization of recharge improves discharge modelling as evidenced by increased KGE (from 0.8 to 0.9) and more realistic flows during drought periods. It is therefore essential to spatialize recharge in karst hydrogeological modelling to improve predictive capacity and better understand functioning of the whole hydrosystem.