Future vulnerability mapping based on response to extreme climate events: Dieback thresholds in an endemic California oak

Abstract

Aim: This study presents a bioclimate modelling approach, using responses to extreme climate events, rather than historical distributional associations, to project future species vulnerability and refugia. We aim to illustrate the compounding effects of groundwater loss and climate on species vulnerability.Location: California, USA.Methods: As a case study, we used the 2012-2015 California drought and resulting extensive dieback of blue oak (Quercus douglasii). We used aerial dieback surveys, downscaled climate data and subsurface water change data to develop boosted regression tree models identifying key thresholds associated with dieback throughout the blue oak distribution. We (1) combined observed dieback-climatic threshold relationships with climate futures to anticipate future areas of vulnerability and (2) used satellite-derived measurements of subsurface water loss in drought/dieback modelling to capture the mediating effect of groundwater on species response to climatic drought.Results: A model including climate, climate anomalies and subsurface water change explained 46% of the variability in dieback. Precipitation in 2015 and subsurface water change accounted for 62.6% of the modelled probability of dieback. We found an interaction between precipitation and subsurface water in which dieback probability increased with low precipitation and subsurface water loss. The relationship between precipitation and dieback was nonlinear, with 99% of dieback occurring in areas that received <363 mm precipitation. Based on a MIROC_rcp85 future climate scenario, relative to historical conditions, 13% of the blue oak distribution is predicted to experience more frequent years below this precipitation threshold by midcentury and 81% by end of century. Main conclusions:As ongoing climate change and extreme events impact ecological processes, the identification of thresholds associated with observed dieback may be combined with climate futures to help identify vulnerable populations and refugia and prioritize climate change-related conservation efforts. | 1187BROWN et al.

Which trees die during drought? The key role of insect host‐tree selection

“…First, as mentioned in Section 4.3, an evergreen gymnosperm not analysed here, S. giganteum , had extremely low mortality (<1%) during the drought (Stephenson et al, ), even though mortality was elevated in A. concolor and P. lambertiana growing mixed with S. giganteum . Second, two other angiosperms in the genus Quercus ( Q. douglasii [deciduous] and Q. wislizeni [evergreen]), which mostly occur at elevations lower than we sampled, had at least some size classes with estimated drought mortality >25% (A. J. Das & N. L. Stephenson, unpublished data; also see Brown, McLaughlin, Blakey, & Morueta‐Holme, ). (However, these low‐elevation Quercus may have experienced more severe drought stress than Q. kelloggii .)…”

No local adaptation in leaf or stem xylem vulnerability to embolism, but consistent vulnerability segmentation in a North American oak

“…Given that Q. douglasii occurs in a Mediterranean‐type climate region characterized by very little rainfall in the summer months that has been present since the mid‐ to late Miocene (Raven & Axelrod, ; Major, ), where even the wetter sites regularly experience intra‐annual water stress (Osuna et al ., , and data shown in Table ), we suggest that populations are likely to be exposed to reliable contemporary environmental signals. In addition, interannual variation in rainfall within sites occasionally produces severe drought events known to cause adult mortality (Brown et al ., ), suggesting that selection pressure for drought tolerance traits is likely to be strong.…”

“…Given that Q. douglasii occurs in a Mediterranean-type climate region characterized by very little rainfall in the summer months that has been present since the mid-to late Miocene (Raven & Axelrod, 1978;Major, 1988), where even the wetter sites regularly experience intra-annual water stress (Osuna et al, 2015, and data shown in Table 1), we suggest that populations are likely to be exposed to reliable contemporary environmental signals. In addition, interannual variation in rainfall within sites occasionally produces severe drought events known to cause adult mortality (Brown et al, 2018), suggesting that selection pressure for drought tolerance traits is likely to be strong. Although uniform selection pressure could be acting in geographically disparate populations (possibly because populations are adapting to local extremes rather than the average; Gutschick & BassiriRad, 2003), we sampled across sites covering a substantial range in aridity, equal to the range over which other species display local adaptation (Wortemann et al, 2011;Lamy et al, 2014;Lobo et al, 2018) and over which we see variation in vulnerability between congeneric oak species (Skelton et al, 2018, see also Larter et al, 2017.…”

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

“…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, 1999), presumably reflecting differences in vadose zone porosity and permeability profiles. Some studies have considered how landscape positioning along site-scale topographic, climatic and soil moisture gradients influence water stress and mortality outcomes (Anderegg, Anderegg, Abatzoglou, Hausladen, & Berry, 2013;Gitlin et al, 2006;Young et al, 2017), but few address processes in the subsurface (Brown, McLaughlin, Blakey, & Morueta-Holme, 2018;Fensham & Holman, 1999).…”

“…In this study, we use the unprecedented 2013-2016 California drought (which included the hottest, driest 3-year period since instrumental records commenced in [since records commenced in 1895, Mann & Gleick, 2015]) as a case study to explore how subsurface processes and variation in species' hydrologic niches (which influence their regional and site-scale distributions) led to different drought responses of species and individuals. In a previous study (Brown et al, 2018), our group explored regional patterns of blue oak dieback and climate with a focus on response thresholds, predicting future areas of the species' persistence using climate futures and the influence of regional groundwater loss. In this paper, we expand on that work with a cross-species and multiscale analysis of the variations in species' drought response and vulnerability and the importance of the subsurface environment more broadly.…”