Abstract
As global climate change continues to impact regional water cycles, we may expect further shifts in water availability to forests that create challenges for certain species and biomes. Lowland deciduous riparian forests are particularly vulnerable because tree species cannot migrate out of the stream corridor, and they rely on root zone water availability that is controlled by variations in both local climate conditions (e.g. precipitation, evaporation, and infiltration) and non-local hydroclimatic forcing (e.g. streamflow, snowmelt, recharge). To determine how the seasonal water source usage of riparian trees is controlled by local versus non-local variability in hydroclimatic regime, we reconstructed the seasonal oxygen isotope (δ 18 O) signature of water used by two riparian tree species with contrasting rooting depths, comprising ∼800 δ 18 O tree-ring cellulose measurements from 12 tree-level decadal time-series at sub-annual resolution (six samples per year), along a strong hydroclimatic gradient within the Rhône River basin, SE France. These results were evaluated alongside δ 18 O measurements made from potential endmember water sources and independent hydroclimatic metrics. Thus we characterize the seasonal evolution of both potential water availability at distinct rooting depths and tree water source use and investigate the generalized riparian tree response to seasonal variations in local versus non-local hydroclimatic forcing over a decade. We show: (a) distinct seasonal water use between species, based on differential access to groundwater; (b) substantial source switching in both species based on evolving water availability; and (c) that riparian trees are more dependent on locally controlled soil moisture with distance downstream, creating increased vulnerability to locally increasing temperatures. We also find that deeply rooted trees in lowland riparian floodplains are potentially vulnerable to climate change because of their high dependence on water supply from mountains. This effect is more pronounced downstream, where seasonal water table decline may lead to loss of water required for deeply rooted trees.
Highlights
Despite a growing consensus that forests across the globe are becoming increasingly vulnerable to water stress [1,2,3,4,5], insufficient information exists on the availability of specific water sources and the usage of this water by vegetation throughout the growing season (GS) across hydroclimatic gradients.Knowledge of water availability to plants and their use of this water in favorable hydrologic years versus drought periods will improve understanding of the proportional balance of groundwater versus infiltrated precipitation
Lowland deciduous riparian forests are vulnerable because tree species cannot migrate out of the stream corridor, and they rely on root zone water availability that is controlled by variations in both local climate conditions and non-local hydroclimatic forcing
We characterize the seasonal evolution of both potential water availability at distinct rooting depths and tree water source use and investigate the generalized riparian tree response to seasonal variations in local versus non-local hydroclimatic forcing over a decade
Summary
Knowledge of water availability to plants and their use of this water in favorable hydrologic years versus drought periods will improve understanding of the proportional balance of groundwater versus infiltrated precipitation. It will inform us about the evolution of key climatically controlled water sources for plant species rooted at different depths, and the corresponding vegetation responses. C I Sargeant and M B Singer to hydroclimatic forcing, whether locally or nonlocally expressed This knowledge gap is compounded by a lack of historical (i.e. decadal-multidecadal) baseline data, from which spatiotemporal trends of water source usage can be evaluated to predict and mitigate deleterious forest responses to shifting hydroclimate regimes under global climate change [1, 6, 7]. A new predictive understanding of the variation in water source availability and usage in response to local and nonlocal hydroclimatic forcing could support management interventions that prevent or mitigate largescale forest diebacks, such as those recently attributed to increasing temperatures, precipitation variability, and root-zone moisture deficits [1, 7, 13,14,15,16,17]
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