As droughts become more frequent and more severe under anthropogenic climate change, water stress due to diminished subsurface supplies may threaten the health and function of semi-arid riparian woodlands, which are assumed to be largely groundwater dependent. To better support the management of riparian woodlands under changing climatic conditions, it is essential to understand the sensitivity of riparian woodlands to depth to groundwater (DTG) across space and time. In this study, we examined six stands of riparian woodland along 28 km of the Santa Clara River in southern California. Combining remote sensing data of fractional land cover, based on spectral mixture analysis, with historical groundwater data, we assessed changes in riparian woodland health in response to DTG during the unprecedented 2012–2019 California drought. We observed a coherent ‘brown wave’ of tree mortality, characterized by decreases in healthy vegetation cover and increases in dead/woody vegetation cover, which progressed downstream through the Santa Clara River corridor between 2012 and 2016. We also found consistent, significant relationships between DTG and healthy vegetation cover, and separately between DTG and dead/woody vegetation cover, indicating that woodland health deteriorated in a predictable fashion as the water table declined at different sites and different times. Based on these findings, we conclude that the brown wave of vegetation dieback was likely caused by local changes in DTG associated with the propagation of precipitation deficits into a depleted shallow alluvial aquifer. These factors suggest that semi-arid riparian woodlands are strongly dependent on shallow groundwater availability, which is in turn sensitive to climate forcing.

Recovery trajectories derived from remote sensing data are widely used to monitor ecosystem recovery after disturbance events, but these trajectories are often retrieved without a precise understanding of the land cover within a scene. As a result, the sources of variability in post-disturbance recovery trajectories are poorly understood. In this study, we monitored the recovery of chaparral and conifer species following the 2007 Zaca Fire, which burned 97,270 ha in Santa Barbara County, California. We combined field survey data with two time series remote sensing products: the relative delta normalized burn ratio (RdNBR) and green vegetation (GV) fractions derived from spectral mixture analysis. Recovery trajectories were retrieved for stands dominated by six different chaparral species. We also retrieved recovery trajectories for stands of mixed conifer forest. We found that the two remote sensing products were equally effective at mapping vegetation cover across the burn scar. The GV fractions (r(78) = 0.552, p < 0.001) and normalized burn ratio (r(78) = 0.555, p < 0.001) had nearly identical correlations with ground reference data of green vegetation cover. Recovery of the chaparral species was substantially affected by the 2011-2017 California drought. GV fractions for the chaparral species generally declined between 2011 and 2016. Physiological responses to fire and drought were important sources of variability between the species. The conifer stands did not exhibit a drought signal that was directly correlated with annual precipitation, but the drought likely delayed the return to pre-fire conditions. As of 2018, 545 of the 756 conifer stands had not recovered to their pre-fire GV fractions. Spatial and temporal variation in species composition were important sources of spectral variability in the chaparral and conifer stands. The chaparral stands in particular had highly heterogeneous species composition. Dominant species accounted for between 30% and 53% of the land cover in the surveyed chaparral patches, so non-dominant land cover types strongly influenced remote sensing signals. Our study reveals that prolonged drought can delay or alter the post-fire recovery of Mediterranean ecosystems. It is also the first study to critically examine how fine-scale variability in land cover affects time series remote sensing analyses.

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&lt;p&gt;Isotopic tracing of water sources for plants is an increasingly common method that supports insight into climatic controls on water availability to plants and their use of this available water, especially in water-limited environments where isotopic endmembers are distinct. Recent advances in this field of research have enabled characterization of annual and seasonal water use by plants, whose water sources vary in contribution along a continuum between groundwater (isotopically light) to infiltrated precipitation (isotopically heavy). Xylem samples are commonly used to characterize real-time uptake of water from roots, and they can be contextualized with respect to endmember water sources via sampling of root zone water, providing these endmembers are isotopically distinct. The time integration of seasonally varying water source usage results in the annually recorded isotopic signal recorded in tree ring cellulose for temperate trees and shrubs, which reflects the dominant water source used in the season of growth. This has enabled dendro-isotopic methods that are commonly used to reconstruct past climates (isotopically light = colder/wetter; isotopically heavy = warmer/drier). However, questions have arisen about the utility of these annually integrated dendro-isotopic signatures, given the strong seasonal variations of water use that are particularly pronounced in dryland ecosystems, including notable water source switching by plants. &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;In our recent work, we have been pushing isotopic methods in new directions to better understand what plants can tell us about how climate affected hydrology across dryland regions, and about the associated plant responses. Drylands pose interesting research challenges, since water is typically the key limiting factor on dryland plant growth, and it is fundamental to the health, functioning, composition, distribution, and evolution of vegetation communities. In drylands, water availability to plants may vary dramatically across space and time, creating challenges for simple analyses of annual water use signatures. To aid the understanding of climatically-controlled ecohydrology in drylands, we have developed a new tool (ISO-Tool) based on established biochemical fractionation theory, which allows for back-calculation of water sources used for growth from tree-ring isotopes. This tool generates critical knowledge for evaluating dendro-isotopic signatures within the same reference frame as sampled endmember water sources, and it can be used for both annual and seasonal analyses of plant water use. We have also been working on a set of interdisciplinary metrics we call water stress indicators (WSIs), which support corroboration of information on climatic forcing, water availability, plant water uptake, and ecological health of terrestrial vegetation. &amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;Using these new methods, we have been able to identify important hydroclimatic gradients in water usage for the same species that reflect the local expression of climate into plant-available water. We have also begun to understand the whole continuum from climate forcing to root-zone water availability to tree growth to canopy health. We believe this broader continuum perspective is critical for tackling key ecohydrological questions especially in drylands, where we expect large variability in water availability across space and time. &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;