Figure - available from: Environmental Research Letters
This content is subject to copyright. Terms and conditions apply.
Modeled source water δ ¹⁸ O (δ ¹⁸ O msw) for the sub-annual δ ¹⁸ O cell samples obtained from each tree for the 2000–2010 period at study sites, Upstream (a)–(d), Midstream (e)–(h), and Downstream (j)–(l). Blue and red lines correspond to the sub-annual δ ¹⁸ O msw means for Fraxinus and Populus, respectively. Grey shading represents ±1 SD.
Source publication
As global climate change continues to impact regional water cycles, we may expect further shifts in water availability to forests that create challenges for certain species and biomes. Lowland deciduous riparian forests are particularly vulnerable because tree species cannot migrate out of the stream corridor, and they rely on root zone water avail...
Citations
... Different riparian species use varying water sources due to their rooting systems (Rood, Bigelow, and Hall 2011), resulting in unequal susceptibility to water stress (Sargeant and Singer 2016;Singer et al. 2014). Likewise, the magnitude of climate controls on water availability varies depending on the local climatic context (Palmer et al. 2008), especially along large rivers within basins characterised by significant hydroclimatic variability (Puckridge et al. 1998), resulting in different responses of riparian forests to climate forcing (Mayes et al. 2020;Sargeant and Singer 2021). In temperate climates with wet summers, the risk of vadose zone drying is limited, as evapotranspiration rarely exceeds precipitation. ...
... makes sense, given that riparian trees along the Rhône benefit from good vadose zone recharge from winter precipitation and high streamflow from snowmelt runoff, which maintains relatively high groundwater levels (Sargeant and Singer 2021). This limited effect of precipitation on greenness has been highlighted in several environments (Piedallu et al. 2019); however other studies have shown the opposite effect, especially in semi-arid climates (Chen et al. 2014;Grossiord et al. 2017;Vicente-Serrano et al. 2013), more similar to that observed for the late season. ...
In the context of rising global temperatures and their impact on weather patterns and water cycles, understanding the relationship between vegetation and hydroclimatic forcing is critical. Riparian forests are highly vulnerable to hydroclimatic variability, which can significantly affect water availability in the soil on which they primarily depend. Along large rivers, hydroclimatic forcings can vary, resulting in different vegetative responses depending on the local climatic context and site conditions. To explore this, we studied riparian forest greenness along a 512‐km river corridor with a 3° latitudinal gradient, analysing the relative contributions of climate (latitude, season, temperature, precipitation) and local hydrological conditions (groundwater). Here, we show that riparian forests along a latitudinal gradient respond differently to hydroclimatic controls, with vegetative dynamics that can be attenuated or accentuated by local site conditions. We combined Sentinel‐2 satellite Normalised Difference Vegetation Index (NDVI) data over seven years (2016–2022) with hydroclimatic data to examine riparian forest greenness responses to latitudinal, seasonal and interannual hydroclimatic variability. We found contrasting hydroclimatic controls across the latitudinal gradient, with the northernmost sites predominantly controlled by temperature, while those further south are limited by water availability. In addition, we identified temperature as the primary driver of NDVI throughout the growing season, either positively or negatively. Late season precipitation and high phreatic water availability positively influenced NDVI, emphasising the role of local conditions in governing riparian forest resilience. This study enhances understanding of climate controls on riparian tree greenness, which is critical for designing effective landscape‐scale riparian ecosystem management and restoration strategies.
... Riparian trees are particularly vulnerable and sensitive to fluctuations in water availability because they rely primarily on soil moisture and groundwater availability for their long-term survival Pettit and Froend, 2018;Sabathier et al., 2021;Sargeant and Singer, 2021;Singer et al., 2014;Warter et al., 2023). A common response of trees to drought is to reduce their stomatal conductance, which prevents excessive canopy transpiration and reduces water loss and the risk of cavitation, but at the cost of slower growth and development. ...
... Its catchment area covers 98,500 km 2 , with a mean annual discharge of 1700 m 3 /s at its mouth (Olivier et al., 2022). The river corridor is mainly north-south oriented (Fig. 1a) and characterized by an important hydroclimatic gradient, from wetter and cooler conditions in the north to drier and warmer conditions in the south (Sargeant and Singer, 2021;Sauquet et al., 2019). This hydroclimatic gradient strongly influences the growth and health of riparian trees, which are sensitive to both local and nonlocal climate forcing (Lochin et al., in review;Sargeant and Singer, 2021). ...
... The river corridor is mainly north-south oriented (Fig. 1a) and characterized by an important hydroclimatic gradient, from wetter and cooler conditions in the north to drier and warmer conditions in the south (Sargeant and Singer, 2021;Sauquet et al., 2019). This hydroclimatic gradient strongly influences the growth and health of riparian trees, which are sensitive to both local and nonlocal climate forcing (Lochin et al., in review;Sargeant and Singer, 2021). Despite these climatic differences, riparian forests are homogeneous in species and distribution along the hydroclimatic gradient (Olivier et al., 2022). ...
... This observation is consistent with other riparian studies that have found tree growth to be more directly influenced by local hydrology compared with climate (Antunes et al., 2018;Sabathier et al., 2021;Sargeant & Singer, 2021;Schook et al., 2020;Singer et al., 2014;Valor et al., 2020). However, it is possible that other influences, such as increased nutrient availability and release from competition following drought-induced mortality events, obscured climate-growth comparisons and could be partly responsible for the enhanced growth during the drought recovery period (Gessler F I G U R E 5 Scatterplot of correlation coefficients between annual values of Δ 13 C and ring-width indices as a function of groundwater (GW) trend for earlywood (panel a), and latewood (panel b) at sites along the lower Santa Clara River, California. ...
Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro‐isotopic analysis of radial growth and seasonal (semi‐annual) carbon isotope discrimination (Δ¹³C) to investigate the response of riparian cottonwood stands to the unprecedented California‐wide drought from 2012 to 2019, along the largest remaining free‐flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ¹³C) during peak drought years. However, patterns of radial growth and Δ¹³C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ¹³C for individual trees, and higher inter‐correlation of Δ¹³C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year⁻¹ beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded ~3 m. These findings indicate that drought‐induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures.
... Riparian plant species are distributed in relation to past and present river flows, particularly where flows replenish alluvial soil water and groundwater (Diehl et al., 2018;Lytle & Poff, 2004;Sargeant & Singer, 2021;Stromberg & Merritt, 2016). Across the globe, human management of rivers has changed streamflow patterns (Nilsson et al., 2005). ...
Riparian trees and their annual growth rings can be used to reconstruct drought histories related to streamflow. Because the death of individual trees reduces competition for survivors, however, tree‐ring chronologies based only on surviving trees may underestimate drought impacts. This problem can be addressed by calculating productivity at the stand scale to account for tree mortality and establishment. In the semi‐arid Great Basin in the western United States, we calculated riparian wood production from 1946 to 2016 along a stream where most flow has been removed by a diversion pipeline since 1961. The water table was found to be generally below the root zone of cottonwoods (Populus angustifolia and P. angustifolia × trichocarpa) in the pipeline‐dewatered reach but within it in reference reaches. To reconstruct forest productivity through time, we separately combined measurements of tree‐ring basal area increment with either changing forest area from aerial photos or a census of cross‐dated living and dead cottonwoods. Both approaches revealed productivity declines in the dewatered reach relative to adjacent reference reaches, and the decline accelerated in the 2000s. Tree‐ring narrowing resulted in divergence between the dewatered reach and one reference reach within 5 years after diversion. However, the dewatered reach did not diverge from the other reference reach until 40 years later, when an unprecedented early 2000s atmospheric drought coupled with diversion to cause extensive cottonwood mortality. We conclude that dendrochronological investigations of forest response to environmental stress should incorporate stand dynamics and that the full impacts of flow diversion can be delayed for decades.
The adverse impacts of Australia’s Millennium Drought on both surface and groundwater hydrological systems are extensively documented. During the Millennium Drought, the Murray Basin experienced a severe rainfall deficit. Our study revisited groundwater table trends in 451 wells within the Murray Basin during the drought from 1997 to 2009. These trends varied, 70% showed significant downward shifts, 19% were insignificant, and 11% even displayed upward trends. The results from K-means clustering analysis indicate a markedly slow recuperation of groundwater levels post-drought. We used multiple regression models to link interannual groundwater dynamics with climate variables, revealing climate as the primary driver of declining groundwater levels. This connection is influenced by land cover and thickness of the vadose zone, resulting in hysteresis effects and spatial variations. In cases with a thick vadose zone and minimal evapotranspiration, the influence of the Millennium Drought on the groundwater system is reduced. The increasing trends may also be related to lateral recharge from mountainous areas, human activities in adjacent irrigation districts, and east-west geostress. Our findings reveal the complex interactions between climate, land characteristics, and groundwater behavior during and after the Millennium Drought, holding significant implications for understanding hydrological processes under extreme drought conditions and for the sustainable management of water resources.
