Low Subsurface Water Storage Capacity Relative to Annual Rainfall Decouples Mediterranean Plant Productivity and Water Use From Rainfall Variability

Hahm, W. Jesse; Dralle, David; Rempe, D. M.; Bryk, A. B.; Thompson, Sally; Dawson, Todd E.; Dietrich, W. E.

doi:10.1029/2019gl083294

Cited by 75 publications

(114 citation statements)

References 48 publications

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“…Several studies suggest that the responses of vegetation growing in seasonally dry climates to meteorological drought depend on the interactions between climate, subsurface hydrology, and landscape features. Hahm, Dralle, et al (), for example, demonstrate that in a subset of locations across California, limited subsurface water storage capacity, rather than variations in seasonal precipitation, appears to constrain plant productivity, such that variability in annual vegetation growth is independent of variations in annual precipitation. In Australian savannas, plant drought mortality varied by parent rock lithology (with mortality ranging from greatest to least across igneous metamorphic to sedimentary rock types to alluvium, Fensham & Holman, ), presumably reflecting differences in vadose zone porosity and permeability profiles.…”

Section: Introductionmentioning

confidence: 99%

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought

McLaughlin

Blakey

Weitz

et al. 2020

Global Change Biology

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Drought extent and severity have increased and are predicted to continue to increase in many parts of the world. Understanding tree vulnerability to drought at both individual and species levels is key to ongoing forest management and preparation for future transitions in community composition. The influence of subsurface hydrologic processes is particularly important in water-limited ecosystems, and is an under-studied aspect of tree drought vulnerability. With California's 2013-2016 extraordinary drought as a natural experiment, we studied four co-occurring woodland tree species, blue oak (Quercus douglasii), valley oak (Quercus lobata), gray pine (Pinus sabiniana), and California juniper (Juniperus californica), examining drought vulnerability as a function of climate, lithology and hydrology using regional aerial dieback surveys and site-scale field surveys. We found that in addition to climatic drought severity (i.e., rainfall), subsurface processes explained variation in drought vulnerability within and across species at both scales. Regionally for blue oak, severity of dieback was related to the bedrock lithology, with higher mortality on igneous and metamorphic substrates, and to regional reductions in groundwater. At the site scale, access to deep subsurface water, evidenced by stem water stable isotope composition, was related to canopy condition across all species. Along hillslope gradients, channel locations supported similar environments in terms of water stress across a wide climatic gradient, indicating that subsurface hydrology mediates species' experience of drought, and that areas associated with persistent access to subsurface hydrologic resources may provide important refugia at species' xeric range edges.Despite this persistent overall influence of the subsurface environment, individual species showed markedly different response patterns. We argue that hydrologic niche segregation can be a useful lens through which to interpret these differences in vulnerability to climatic drought and climate change. K E Y W O R D S climate change, drought, hydrologic niche, hydrology, lithology, oak woodlands, Quercus douglasii, subsurface, water S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: McLaughlin BC, Blakey R, Weitz AP, et al. Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought. Glob Change Biol. 2020;26:3091-3107.

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Section: Introductionmentioning

confidence: 99%

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought

McLaughlin

Blakey

Weitz

et al. 2020

Global Change Biology

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“…As the climate warms, increase in plant water use in Mediterranean climates due to higher‐energy availability in spring can be offset by decreases due to soil limitation in summer (Pangle et al, ; Tague & Peng, ), a phenomena consistent with global trends (Angert et al, ; Buermann et al, ; Wolf et al, ). This temporal alignment of seasonal water and energy availability (or its change) also influences partitioning of subsurface water resources between groundwater recharge, transpiration, and streamflow (Dralle et al, ; Hahm et al, ). Additionally, in Mediterranean climates that are snow‐dominated, warming reduces the fraction of precipitation that falls as snow and promotes earlier snowmelt (which is slower and less likely to saturate soils; Musselman et al, ): all changes that tend to increase evapotranspiration at the expense of streamflow (Barnhart et al, ).…”

Section: Introductionmentioning

confidence: 99%

Quantifying Asynchronicity of Precipitation and Potential Evapotranspiration in Mediterranean Climates

Feng

Thompson

Woods

et al. 2019

Geophysical Research Letters

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Recent climate change has contributed to shifts in the seasonal interplay between precipitation and potential evapotranspiration, which have in turn increased droughts and reduced freshwater availability in Mediterranean climate regions. To overcome limitations in existing indices for comparing these seasonal hydroclimatic drivers at the global scale, we introduce an information theory-based, nonparametric asynchronicity index that captures both the temporal alignment and relative magnitudes of precipitation and potential evapotranspiration. We use this asynchronicity index to first identify Mediterranean climates around the world. We then apply the asynchronicity index over two Mediterranean climate regions and show that their boundaries have shifted between 1960 and 2018, resulting in a regional expansion in the U.S. Pacific Northwest and a contraction in southwestern Australia. These results highlight the need for globally consistent measures of seasonal climatic water supply and demand for diagnosing potential changes in water resources and ecosystem responses within Mediterranean climate regions. Plain Language Summary Climate change is likely to change the boundaries of what aretypically considered Mediterranean climate regions, which have dry summers and mild, wet winters. To analyze how these regions' water availability may be influenced by climate change, we introduce a metric for quantifying the mismatch in the timing and amount of precipitation relative to atmospheric water demand. This new metric is able to identify the boundaries of Mediterranean climate regions more effectively than other existing indices and can be used to analyze how these boundaries shift over time.

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“…For example, Hahm et al (2019b) recently revealed through intensive hillslopescale monitoring in the Northern California Coast Ranges how the magnitude of plantavailable water storage is capped by subsurface water storage capacity, which is in turn related to the depth and extent of weathering in the CZ. Hahm et al (2019a) also demonstrated, via remote-sensing of plant greenness and water-balance tracking, that subsurface water storage capacity can decouple summer water availability, and thus plant productivity, from year-toyear rainfall variability. However, methods and datasets that explicitly map CZ structure are generally lacking at larger spatial scales, or have limited applicability.…”

Section: Introductionmentioning

confidence: 92%

“…In the Results, we demonstrate that knowledge of CV [S 0 ] = CV [ET dry ] can be used to directly estimate S max . This method for estimating storage is more widely applicable than the flux-tracking methodology presented in Hahm et al (2019a), because it does not rely on closure of the water budget which, crucially, requires unimpaired stream gauging. Moreover, the storage capacities that could be inferred from flux tracking are total catchment dynamic water storages, rather than root-zone plantavailable water storage capacities, which is estimated here.…”

Section: Model Inversion For Estimating S Maxmentioning

confidence: 99%

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Plants as sensors: vegetation response to rainfall predicts subsurface water storage capacity in Mediterranean climates

Dralle¹,

Hahm²,

Rempe³

et al. 2019

Preprint

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Methods are lacking to characterize critical zone (CZ) structure at spatial scales relevant to earth system and dynamic global vegetation models. This knowledge gap results in poor quantification of CZ plant-available water storage capacity, hindering realistic prediction of the response of plants and streamflow to anticipated changes in the hydrological cycle. Here, we exploit the phase offset between water and energy delivery in rain-dominated Mediterranean climates to use plants as sensors to infer belowground water storage capacity. We hypothesize that if the magnitude of stored plant-available subsurface water is the primary control on dry season plant water use, then (remotely sensed) measures of transpiration may be used to infer rooting zone storage capacity. We encapsulate this idea within an ecohydrological modeling framework that describes how the stochastic properties of rainfall interact with storage capacity on intra-annual timescales to control annual variations in plant-available water storage, and thus dry season plant water use. The model reveals that where storage capacity is high relative to mean annual rainfall, plant-available water storage is not replenished in all years, and so storage and thus plant water use are sensitive to annual total rainfall. Where storage capacity is low, storage is typically replenished but can be depleted rapidly between storm events, resulting in plant insensitivity to annual total rainfall but sensitivity to spring rainfall patterns. Both high and low storage capacity result in relatively highly variable stored water for summer, and thus predicted highly variable summer transpiration; in contrast, variability is minimized at intermediate storage capacity. The model captures these diverse responses of stored water (and consequently summer vegetation water use) -- as mediated by water storage capacity -- to precipitation dynamics. Consequently, we show that a simple model inversion can be used to estimate rooting zone water storage capacity. We validate model inversion predictions using direct observations of plant-available water storage capacity in soils and weathered bedrock at two intensively monitored sites in the Northern California Coast Ranges. The model accurately predicts the magnitude of the combined dry season soil and rock moisture loss that supports transpiration, in contrast to existing soils maps, which underestimate plant-available water storage by up to a factor of three. Strongly contrasting weathering profiles and hence porosity structures at the study sites demonstrate the method is robust across diverse modes of storage and runoff generation.

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Low Subsurface Water Storage Capacity Relative to Annual Rainfall Decouples Mediterranean Plant Productivity and Water Use From Rainfall Variability

Cited by 75 publications

References 48 publications

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought

Quantifying Asynchronicity of Precipitation and Potential Evapotranspiration in Mediterranean Climates

Plants as sensors: vegetation response to rainfall predicts subsurface water storage capacity in Mediterranean climates

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