Observational evidence for groundwater influence on crop yields in the United States.

As climate change shifts crop exposure to dry and wet extremes, a better understanding of factors governing crop response is needed. Recent studies identified shallow groundwater-groundwater within or near the crop rooting zone-as influential, yet existing evidence is largely based on theoretical crop model simulations, indirect or static groundwater data, or small-scale field studies. Here, we use observational satellite yield data and dynamic water table simulations from 1999 to 2018 to provide field-scale evidence for shallow groundwater effects on maize yields across the United States Corn Belt. We identify three lines of evidence supporting groundwater influence: 1) crop model simulations better match observed yields after improvements in groundwater representation; 2) machine learning analysis of observed yields and modeled groundwater levels reveals a subsidy zone between 1.1 and 2.5 m depths, with yield penalties at shallower depths and no effect at deeper depths; and 3) locations with groundwater typically in the subsidy zone display higher yield stability across time. We estimate an average 3.4% yield increase when groundwater levels are at optimum depth, and this effect roughly doubles in dry conditions. Groundwater yield subsidies occur ~35% of years on average across locations, with 75% of the region benefitting in at least 10% of years. Overall, we estimate that groundwater-yield interactions had a net monetary contribution of approximately $10 billion from 1999 to 2018. This study provides empirical evidence for region-wide groundwater yield impacts and further underlines the need for better quantification of groundwater levels and their dynamic responses to short- and long-term weather conditions.

Elevating water table reduces net ecosystem carbon losses from global drained wetlands.

Drained wetlands are thought to be carbon (C) source hotspots, and rewetting is advocated to restore C storage in drained wetlands for climate change mitigation. However, current assessments of wetland C balance mainly focus on vertical fluxes between the land and atmosphere, frequently neglecting lateral carbon fluxes and land-use effects. Here, we conduct a global synthesis of 893 annual net ecosystem C balance (NECB) measures that include net ecosystem exchange of CO2, along with C input via manure fertilization, and C removal through biomass harvest or hydrological exports of dissolved organic and inorganic carbon, across wetlands of different status and land uses. We find that elevating water table substantially reduces net ecosystem C losses, with the annual NECB decreasing from 2579 (95% interval: 1976 to 3214) kg C ha-1 year-1 in drained wetlands to -422 (-658 to -176) kg C ha-1 year-1 in natural wetlands, and to -934 (-1532 to -399) kg C ha-1 year-1 in rewetted wetlands globally. Climate, land-use history, and time since water table changes introduce variabilities, with drainage for (sub)tropical agriculture or forestry uses showing high annual C losses, while the net C losses from drained wetlands can continue to affect soil C pools for several decades. Rewetting all types of drained wetlands is needed, particularly for those formerly agriculture-used (sub)tropical wetlands where net ecosystem C losses can be largely reduced. Our findings suggest that elevating water table is an important initiative to reduce C losses in degraded wetlands, which could contribute to policy decisions for managing wetlands to enhance their C sequestration.

Short-Term Groundwater Level Fluctuations Drive Subsurface Redox Variability.

As global change processes modify the extent and functions of terrestrial-aquatic interfaces, the variability of critical and dynamic transitional zones between wetlands and uplands increases. However, it is still unclear how fluctuating water levels at these dynamic boundaries alter groundwater biogeochemical cycling. Here, we used high-temporal resolution data along gradients from wetlands to uplands and during fluctuating water levels at freshwater coastal areas to capture spatiotemporal patterns of groundwater redox potential (Eh). We observed that topography influences groundwater Eh that is higher in uplands than in wetlands; however, the high variability within TAI zones challenged the establishment of distinct redox zonation. Declining water levels generally decreased Eh, but most locations exhibited significant Eh variability, which is associated with rare instances of short-term water level fluctuations, introducing oxygen. The Eh-oxygen relationship showed distinct hysteresis patterns, reflecting redox poising capacity at higher Eh, maintaining more oxidizing states longer than the dissolved oxygen presence. Surprisingly, we observed more frequent oxidizing states in transitional areas and wetlands than in uplands. We infer that occasional oxygen entering specific wetland-upland boundaries acts as critical biogeochemical control points. High-resolution data can capture such rare yet significant biogeochemical instances, supporting redox-informed models and advancing the predictability of climate change feedback.

Non-flooding conditions caused by water table drawdown alter microbial network complexity and decrease multifunctionality in alpine wetland soils.

Several soil functions of alpine wetland depend on microbial communities, including carbon storage and nutrient cycling, and soil microbes are highly sensitive to hydrological conditions. Wetland degradation is often accompanied by a decline in water table. With the water table drawdown, the effects of microbial network complexity on various soil functions remain insufficiently understood. In this research, we quantified soil multifunctionality of flooded and non-flooded sites in the Lalu Wetland on the Tibetan Plateau. We employed high-throughput sequencing to investigate the microbial community responses to water table depth changes, as well as the relationships between microbial network properties and soil multifunctionality. Our findings revealed a substantial reduction in soil multifunctionality at both surface and subsurface soil layers (0-20 cm and 20-40 cm) in non-flooded sites compared to flooded sites. The α-diversity of bacteria in the surface soil of non-flooded sites was significantly lower than that in flooded sites. Microbial network properties (including the number of nodes, number of edges, average degree, density, and modularity of co-occurrence networks) exhibited significant correlations with soil multifunctionality. This study underscores the adverse impact of non-flooded conditions resulting from water table drawdown on soil multifunctionality in alpine wetland soils, driven by alterations in microbial community structure. Additionally, we identified soil pH and moisture content as pivotal abiotic factors influencing soil multifunctionality, with microbial network complexity emerging as a valuable predictor of multifunctionality.

Convergence of carbon sink magnitude and water table depth in global wetlands.

Wetlands are strategic areas for carbon uptake, but accurate assessments of their sequestration ability are limited by the uncertainty and variability in their carbon balances. Based on 2385 observations of annual net ecosystem production from global wetlands, we show that the mean net carbon sinks of inland wetlands, peatlands and coastal wetlands are 0.57, 0.29 and 1.88 tons of carbon per hectare per year, respectively, with a mean value of 0.57 tons of carbon per hectare per year weighted by the distribution area of different wetland types. Carbon sinks are mainly in Asia and North America. Within and across wetland types, we find that water table depth (WTD) exerts greater control than climate- and ecosystem-related variables, and an increase in WTD results in a stronger carbon sink. Our results highlight an urgent need to sustain wetland hydrology under global change; otherwise, wetlands are at high risk of becoming carbon sources to the atmosphere.

Ericoid shrub encroachment shifts aboveground-belowground linkages in three peatlands across Europe and Western Siberia.

In northern peatlands, reduction of Sphagnum dominance in favour of vascular vegetation is likely to influence biogeochemical processes. Such vegetation changes occur as the water table lowers and temperatures rise. To test which of these factors has a significant influence on peatland vegetation, we conducted a 3-year manipulative field experiment in Linje mire (northern Poland). We manipulated the peatland water table level (wet, intermediate and dry; on average the depth of the water table was 17.4, 21.2 and 25.3 cm respectively), and we used open-top chambers (OTCs) to create warmer conditions (on average increase of 1.2°C in OTC plots compared to control plots). Peat drying through water table lowering at this local scale had a larger effect than OTC warming treatment per see on Sphagnum mosses and vascular plants. In particular, ericoid shrubs increased with a lower water table level, while Sphagnum decreased. Microclimatic measurements at the plot scale indicated that both water-level and temperature, represented by heating degree days (HDDs), can have significant effects on the vegetation. In a large-scale complementary vegetation gradient survey replicated in three peatlands positioned along a transitional oceanic-continental and temperate-boreal (subarctic) gradient (France-Poland-Western Siberia), an increase in ericoid shrubs was marked by an increase in phenols in peat pore water, resulting from higher phenol concentrations in vascular plant biomass. Our results suggest a shift in functioning from a mineral-N-driven to a fungi-mediated organic-N nutrient acquisition with shrub encroachment. Both ericoid shrub encroachment and higher mean annual temperature in the three sites triggered greater vascular plant biomass and consequently the dominance of decomposers (especially fungi), which led to a feeding community dominated by nematodes. This contributed to lower enzymatic multifunctionality. Our findings illustrate mechanisms by which plants influence ecosystem responses to climate change, through their effect on microbial trophic interactions.

Comparing the variations and influencing factors of CH4 emissions from paddies and wetlands under CO2 enrichment: A data synthesis in the last three decades.

Understanding and quantifying the impact of elevated tropospheric carbon dioxide concentration (e [CO2]) on methane (CH4) globally is important for effectively assessing and mitigating climate warming. Paddies and wetlands are the two important sources of CH4 emissions. Yet, a quantitative synthetic investigation of the effects of e [CO2] on CH4 emissions from paddies and wetlands on a global scale has not been conducted. Here, we conducted a meta-analysis of 488 observation cases from 40 studies to assess the long-term effects of e [CO2] (ambient [CO2]+ 53-400 µmol mol-1) on CH4 emissions and to identify the relevant key drivers. On aggregate, e [CO2] increased CH4 emissions by 25.7% (p < 0.05) from paddies but did not affect CH4 emissions from wetlands (-3.29%; p > 0.05). The e [CO2] effects on paddy CH4 emissions were positively related to that on belowground biomass and soil-dissolved CH4 content. However, these factors under e [CO2] resulted in no significant change in CH4 emissions in wetlands. Particularly, the e [CO2]-induced abundance of methanogens increased in paddies but decreased in wetlands. In addition, tillering number of rice and water table levels affected e [CO2]-induced CH4 emissions in paddies and wetlands, respectively. On a global scale, CH4 emissions changed from an increase (+0.13 and + 0.86 Pg CO2-eq yr-1) under short-term e [CO2] into a decrease and no changes (-0.22 and + 0.03 Pg CO2-eq yr-1) under long-term e [CO2] in paddies and wetlands, respectively. This suggested that e [CO2]-induced CH4 emissions from paddies and wetlands changed over time. Our results not only shed light on the different stimulative responses of CH4 emissions to e [CO2] from paddy and wetland ecosystems but also suggest that estimates of e [CO2]-induced CH4 emissions from global paddies and wetlands need to account for long-term changes in various regions.

Effects of stand age, tree species, and climate on water table fluctuations and estimated evapotranspiration in managed peatland forests.

Lowland conifer forests dominated by black spruce (Picea mariana) and tamarack (Larix laricina) typically occur in peatlands in the boreal North American forest with near-surface water tables throughout the year. These forests are ecologically and economically important resources that may be impacted by climate change. However, information characterizing effects of forest disturbance, such as even-aged harvest on water table dynamics is needed to evaluate which forest tree species cover types are most hydrologically susceptible to even-aged harvest and changes in precipitation. We used a chronosequence approach to evaluate water table fluctuations and evapotranspiration across four stand age classes (<10, 15-30, 40-80, and >100-years old) and three distinct forest cover types (productive black spruce, stagnant black spruce, and tamarack) for a period of three years in Minnesota, USA. In general, there is limited evidence for elevated water tables in the younger age classes; the <10-year age class had no significant difference in mean weekly water table depth compared to the older age classes across all cover types. Estimated actual daily evapotranspiration (ET) generally agreed with the water table observations, with the exception of the tamarack cover type where ET was significantly lower in the <10-year age class. Productive black spruce sites that are 40-80-years old had higher evapotranspiration, and lower water table, possibly reflecting increased transpiration associated with the stem exclusion stage of stand development. Tamarack in the 40-80-year age class had higher water tables but no difference in ET compared to all other age classes, indicating that other external factors are driving higher water tables in that age class. To evaluate susceptibility to changing climate, we also assessed the sensitivity and response of water table dynamics to pronounced differences in growing season precipitation that occurred across study years. In general, tamarack forests are more sensitive to changes in precipitation compared to the two black spruce forest cover types. These findings can inform expected responses of site hydrology for a range of precipitation scenarios that may occur under future climate and be used by forest managers to evaluate hydrologic impacts of forest management activities across lowland conifer forest cover types.

Stories of a water-table: anomalous depressions, aquitard breaches and seasonal implications, Shelby County, Tennessee, USA.

An extensive water level survey of the water-table aquifer (i.e., shallow aquifer) within Shelby County, Tennessee, was conducted in the dry (fall 2020) and wet (spring 2021) seasons. Water-table surfaces were generated using cokriging to observe seasonal differences to identify anomalous water-table depressions, indicative of an underlying aquitard breach. Seasonal differences were attributed to non-coincident control and timing between the surveys and when optimum dry (fall) and wet (spring) conditions existed, as observed through comparisons with continuous historical water levels from 12 shallow monitoring wells. Additionally, data from fall 2020 were compared to previous studies in 2005 and 2015 to determine decadal changes in levels and shape of the water-table surface which were mostly attributed to changes in data control and potential climate variations. A prediction error map was generated from the 2020 dataset to identify areas of the county with high-prediction error (>7.0 m) to offer guidance on where future well control would be optimal.

Proxy modeling approach to evaluate groundwater recharge potentiality zones in the data scarce area of upper Blue Nile Basin, Ethiopia.

Prioritization of groundwater recharge potentiality evaluation is critical for sustainable water resources management. Since recharge is a main source for enhancing groundwater availability. Water scarcity is extremely severe in the upper Blue Nile Basin (i.e., Gunabay watershed). Therefore, this study emphasizes groundwater recharge delineating and mapping 3920.25 km2 in the data-limited area of the upper Blue Basin using proxy modeling (i.e., WetSpass-M model and geodetector model) and tools. The driving/influencing factors are rainfall, temperature, wind speed, evapotranspiration, elevation, slope, land cover, soil, groundwater depth, drainage density, geomorphology, and geology that control the movement of groundwater recharge. However, the first nine factors were used as inputs in the WetSpass-M model to evaluate groundwater recharge. To validate the groundwater recharge availability, water table fluctuation was established based on recorded groundwater levels. Furthermore, the major influencing factors and their interaction have been quantified using geodetector model. Spatiotemporal recharge distribution (in mm) is classified as very low (0-6), low (6-30), moderate (30-51), high (51-83), and very high (83-508) comprising 21%, 20%, 20%, 20%, and 19% of the total area, respectively. Very high groundwater recharge zone has been found in the northwest part of the area. The geodetector results showed that soil (0.841) and temperature (0.287) had larger individual contributions, but the interaction between soil and temperature (0.962) was more significant. It indicates that the interaction between climate and soil has the largest influence on groundwater recharge variability. Generally, the overall approach of this study can be applied to water sectors, policymakers, and decision-makers to overcome future water scarcity.

Hydraulic tradeoffs underlie local variation in tropical forest functional diversity and sensitivity to drought.

Tropical forests are important to the regulation of climate and the maintenance of biodiversity on Earth. However, these ecosystems are threatened by climate change, as temperatures rise and droughts' frequency and duration increase. Xylem anatomical traits are an essential component in understanding and predicting forest responses to changes in water availability. We calculated the community-weighted means and variances of xylem anatomical traits of hydraulic and structural importance (plot-level trait values weighted by species abundance) to assess their linkages to local adaptation and community assembly in response to varying soil water conditions in an environmentally diverse Brazilian Atlantic Forest habitat. Scaling approaches revealed community-level tradeoffs in xylem traits not observed at the species level. Towards drier sites, xylem structural reinforcement and integration balanced against hydraulic efficiency and capacitance xylem traits, leading to changes in plant community diversity. We show how general community assembly rules are reflected in persistent fiber-parenchyma and xylem hydraulic tradeoffs. Trait variation across a moisture gradient is larger between species than within species and is realized mainly through changes in species composition and abundance, suggesting habitat specialization. Modeling efforts to predict tropical forest diversity and drought sensitivity may benefit from adding hydraulic architecture traits into the analysis.

Plate Load Tests on an Unsaturated Sand-Kaolin Mixture with Varying Water Table.

Clayey sand is widely distributed and commonly encountered in geotechnical engineering practice. To understand its bearing capacity behavior under unsaturated conditions, plate load tests are performed on sand-kaolin mixture samples with varying water tables. The distributions of suction and volumetric water content with depth are measured by vibrating wire piezometers and soil moisture sensors, respectively. It is shown by the test results that the bearing capacity increases when the water table in the soil sample drops. The influence of suction on the bearing capacity is found to be dependent on the height of the water table and the hydraulic loading history of the soil sample. The plate load test results are interpreted using bearing capacity equations. Good agreement is obtained between measured and calculated bearing capacities. This study provides a simple method to estimate the bearing capacity of in situ unsaturated soil foundations.

Variations in Groundwater Level and Microtopography Influence Desert Plant Communities in Shallow Aquifer Areas.

An improved understanding of the relationships among vegetation, groundwater level, and microtopography is crucial for making well-informed management decisions in areas with shallow groundwater resources. We measured plant species abundance/composition and richness in relation to depth to groundwater (DTW) and microtopography in Owens Valley, California, particularly in areas where DTW ranged from 0 to 4 m. Sampling occurred along 67 vegetation transects across three community types. Relationships between DTW and community composition were evaluated using non-metric multidimensional scaling (NMS), while non-parametric multiplicative regression was used to relate DTW and microtopography to species abundance. The dominant gradient in species composition (NMS Axis 1) explained ~51% of variation in our distance matrix and was most strongly associated (r = 0.55) with DTW. The graminoids Juncus arcticus, Leymus triticoides, and Distichlis spicata had strong affinities toward areas with the shallowest DTW (<1.5 m). One salt-adapted species Sporobolus airoides and one shrub Ericameria nauseosa dominated areas with intermediate DTW (1.5-2.0 m), whereas the shrubs Atriplex torreyi, Sarcobatus vermiculatus, and Artemisia tridentata were dominant in areas with deeper DTW (>2.0 m). Variation in microtopography affected species abundance and increased species richness for vegetation communities at either extreme of the water table gradient, shallow, and deep DTW but not the intermediate DTW. Findings indicate that desert plant communities from shallow aquifers have adapted to different DTW and microtopography conditions and that considering those adaptations may be important to manage groundwater and vegetation resources in these areas.

Peatland microbial community responses to plant functional group and drought are depth-dependent.

Peatlands store one-third of Earth's soil carbon, the stability of which is uncertain due to climate change-driven shifts in hydrology and vegetation, and consequent impacts on microbial communities that mediate decomposition. Peatland carbon cycling varies over steep physicochemical gradients characterizing vertical peat profiles. However, it is unclear how drought-mediated changes in plant functional groups (PFGs) and water table (WT) levels affect microbial communities at different depths. We combined a multiyear mesocosm experiment with community sequencing across a 70-cm depth gradient, to test the hypotheses that vascular PFGs (Ericaceae vs. sedges) and WT (high vs. low) structure peatland microbial communities in depth-dependent ways. Several key results emerged. (i) Both fungal and prokaryote (bacteria and archaea) community structure shifted with WT and PFG manipulation, but fungi were much more sensitive to PFG whereas prokaryotes were much more sensitive to WT. (ii) PFG effects were largely driven by Ericaceae, although sedge effects were evident in specific cases (e.g., methanotrophs). (iii) Treatment effects varied with depth: the influence of PFG was strongest in shallow peat (0-10, 10-20 cm), whereas WT effects were strongest at the surface and middle depths (0-10, 30-40 cm), and all treatment effects waned in the deepest peat (60-70 cm). Our results underline the depth-dependent and taxon-specific ways that plant communities and hydrologic variability shape peatland microbial communities, pointing to the importance of understanding how these factors integrate across soil profiles when examining peatland responses to climate change.

Wetland Heterogeneity Determines Methane Emissions: A Pan-Arctic Synthesis.

Methane (CH4) emissions from pan-Arctic wetlands provide a potential positive feedback to global warming. However, the differences in CH4 emissions across wetland types in these regions have not been well understood. We synthesized approximately 9000 static chamber CH4 measurements during the growing season from 83 sites across pan-Arctic regions. We highlighted spatial variations of CH4 emissions corresponding to environmental heterogeneity across wetland types. CH4 emission is the highest in fens, followed by marshes, bogs, and the lowest in swamps. This gradient is controlled by the water table, soil temperature, and dominant plant functional types and their interactions. The water table position for maximum CH4 emission is below, close to, and above the ground surface in bogs, marshes/fens, and swamps, respectively. The temperature sensitivity (Q10) of CH4 emissions varied among different wetland types, ranging from the lowest in swamps to the highest in fens. The interactive impact of temperature and the water table positions on CH4 emissions are regulated with dominant plant functional types. CH4 emissions from wetlands dominated by vascular plants rely more on species composition than that dominated by non-vascular plants. Wetlands with greater abundance of graminoids (e.g., fens) have higher CH4 emissions than tree-dominated wetlands (e.g., swamps). This synthesis emphasizes the role of wetland heterogeneity in determining the strength of CH4 emissions.

Development of potential map for groundwater abstraction in the northwest region of Bangladesh using RS-GIS-based weighted overlay analysis and water-table-fluctuation technique.

The increasing trend of population growth along with the rapid groundwater-based agricultural expansion and decreasing trend of mean annual rainfall in the Northwest region of Bangladesh has been exacerbating the declination of groundwater for further expansion. Therefore, the present study attempts to demarcate the potential groundwater abstraction zones from the assessment of potential recharge and available recharge. Potential recharge was obtained with commonly used geospatial-based weighted linear combination (WLC) technique. Here, WLC analysis was based on eight factors related to physiographic (e.g. drainage density, lineament density, slope), geomorphologic (e.g. geomorphology, lithology, soil), land use and land cover (LULC) and hydrology (i.e. rainfall). Available net recharge was assessed for the period 1993-2017 by employing the water table fluctuation method. Finally, the resultant map on potential abstraction was characterized into five different classes, viz. 'very low', 'low', 'moderate', 'high' and 'very high'. The derived map reveals that 'very high' potential zone is distributed along the Teesta river floodplain, especially the northeastern part. In contrast, the Barind Tract (i.e. the southwestern and the southcentral parts) area shows 'very low' groundwater prospect. Such fused interpretations are expected to contribute to the planning of integrated management of water resources.

Hydrologic regulation of plant rooting depth.

Plant rooting depth affects ecosystem resilience to environmental stress such as drought. Deep roots connect deep soil/groundwater to the atmosphere, thus influencing the hydrologic cycle and climate. Deep roots enhance bedrock weathering, thus regulating the long-term carbon cycle. However, we know little about how deep roots go and why. Here, we present a global synthesis of 2,200 root observations of >1,000 species along biotic (life form, genus) and abiotic (precipitation, soil, drainage) gradients. Results reveal strong sensitivities of rooting depth to local soil water profiles determined by precipitation infiltration depth from the top (reflecting climate and soil), and groundwater table depth from below (reflecting topography-driven land drainage). In well-drained uplands, rooting depth follows infiltration depth; in waterlogged lowlands, roots stay shallow, avoiding oxygen stress below the water table; in between, high productivity and drought can send roots many meters down to the groundwater capillary fringe. This framework explains the contrasting rooting depths observed under the same climate for the same species but at distinct topographic positions. We assess the global significance of these hydrologic mechanisms by estimating root water-uptake depths using an inverse model, based on observed productivity and atmosphere, at 30″ (∼1-km) global grids to capture the topography critical to soil hydrology. The resulting patterns of plant rooting depth bear a strong topographic and hydrologic signature at landscape to global scales. They underscore a fundamental plant-water feedback pathway that may be critical to understanding plant-mediated global change.

Embolism resistance drives the distribution of Amazonian rainforest tree species along hydro-topographic gradients.

Species distribution is strongly driven by local and global gradients in water availability but the underlying mechanisms are not clear. Vulnerability to xylem embolism (P50 ) is a key trait that indicates how species cope with drought and might explain plant distribution patterns across environmental gradients. Here we address its role on species sorting along a hydro-topographical gradient in a central Amazonian rainforest and examine its variance at the community scale. We measured P50 for 28 tree species, soil properties and estimated the hydrological niche of each species using an indicator of distance to the water table (HAND). We found a large hydraulic diversity, covering as much as 44% of the global angiosperm variation in P50 . We show that P50 : contributes to species segregation across a hydro-topographic gradient in the Amazon, and thus to species coexistence; is the result of repeated evolutionary adaptation within closely related taxa; is associated with species tolerance to P-poor soils, suggesting the evolution of a stress-tolerance syndrome to nutrients and drought; and is higher for trees in the valleys than uplands. The large observed hydraulic diversity and its association with topography has important implications for modelling and predicting forest and species resilience to climate change.

Smaller future floods imply less habitat for riparian plants along a boreal river.

Climate-change projections suggest large changes in riverine flow regime, which will likely alter riparian communities. In northern Europe, forecasts propose lower annual spring flood peaks and higher winter flows, resulting in narrower riparian zones. To estimate the impact of climate change on habitat extent of riparian plants, we developed a framework estimating the sensitivity and exposure of individual species to streamflow change, and surveyed five reaches along the free-flowing Vindel River in northern Sweden. We modeled the hydrologic niche of riparian plant species based on the probability of occurrence along gradients of flood frequency and duration and used predicted future water-level fluctuations (based on climate models and IPCC emission scenarios) to calculate changes in flow-related habitat availability of individual species. Despite projected increases in runoff, we predict most species to decrease in riparian elevational extent by on average 12-29% until the end of the century, depending on scenario. Species growing in the upper, spring-flood-controlled part of the riparian zone will likely lose most habitat, with the largest reductions in species with narrow ranges of inundation duration tolerance (decreases of up to 54%). In contrast, the elevational extent of most amphibious species is predicted to increase, but conditions creating isoëtid vegetation will become rarer or disappear: isoëtid vegetation is presently found in areas where ice formed in the fall settles on the riverbank during the winter as water levels subside. Higher winter flows will make these conditions rare. We argue that our framework is useful to project the effects of hydrologic change caused by climate change as well as other stressors such as flow regulation also in other regions. With few rivers remaining unaffected by dams and other human stressors, these results call for monitoring to detect species declines. Management to alleviate species losses might include mitigation of habitat degradation from land-use activities, more environmentally friendly flow schemes, and more intensive management options such as mowing riparian meadows no longer regularly maintained by recurrent floods.

Differential effects of soil waterlogging on herbaceous and woody plant communities in a Neotropical savanna.

The impacts of soil properties and fire regime on Neotropical savannas are well-known, but the importance of hydrological regime for plant species assembly has received less attention. Here, we assessed changes in diversity patterns of herbaceous and woody communities along a water table gradient in a fire-excluded Neotropical savanna. We found that increased waterlogging of soils was associated with declines in both herbaceous and woody species richness. Woody species richness decreased once the water table depth is less than 4 m and no woody species occurred once water table depth was less than 23 cm. Herbaceous communities remained species rich until the shallowest water table depth, where there is flooding at some point in the year, and even there, over a dozen species occurred. Woody species that occurred in areas with shallower water tables were a nested subset of those in areas with deeper water tables. In contrast, herbaceous communities showed turnover over the hydrological gradient, with distinct species specialized for different water table levels. However, we found that those specialists are restricted to few evolutionary lineages, evidenced by increased phylogenetic clustering over the water table gradient in herbaceous communities. We suggest that evolutionarily conserved hydrological niches define the herbaceous layer over the hydrological gradient, whereas only generalist woody species persist under high water tables. Our findings show that the effect of soil waterlogging differs between the herbaceous and woody layer of savannas, indicating that these communities will respond differently to shifts in the hydrological regime under future environmental change.