Identifying Habitat Holdouts for High Elevation Tree Species Under Climate Change

Charles J Maxwell,Robert M Scheller

doi:10.3389/ffgc.2019.00094

Charles J Maxwell, Robert M Scheller

Open Access

https://doi.org/10.3389/ffgc.2019.00094

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Abstract

High elevation tree species are at great risk of decline under climate change—particularly in ranges below tree line where upslope movement is not possible —as warmer temperatures reduce snowpack and increase evaporative demand. Forecasting future locations of persistence is key to the conservation of those species. In this study, we had two major objectives: 1) to determine the potential decline in the extent of three montane conifers in California, USA, and 2) to assess how model resolution affected our estimates of decline and whether this could inform identifying potential holdouts. To do so, we forecast forest dynamics, disturbances, and future distributions of three montane conifer species under a changing climate in the Klamath Mountains using the LANDIS-II forest simulation model. Simulations were run under two grain sizes, 0.81 ha and 7.29 ha cells, and four GCMs representing three relative concentration pathways. The area occupied by the three montane conifers declined by the end of the 21st century, with only a few areas where the species were able to persist. Higher levels of climate forcing resulted in greater declines. Moreover, higher temperatures reduced tree regeneration although adult populations persisted despite the climate disequilibrium. Model resolution but did not alter the overall trend of decline. These species were projected to remain in only a few limited areas by the end of the century, but because these species are widely dispersed on the larger landscape, managers must consider trade-offs between local and broader conservation efforts and consider the current and potential range of these conifers throughout the west.

Highlights

Climate change has caused, and will continue to cause, significant changes in tree species’ distributions (Lenoir et al, 2008)
We focused on the Marble Mountain Wilderness (MMW) and the surrounding high elevation area, covering ∼170,000 hectares in total (Figure 1)
A. magnifica saw the largest declines in terms of percentage from 2015 to 2105 as averaged across the four climate projections (−70%); there was a greater variation in decline with A. concolor when considering each climate projection separately (−68% under ACCESS rcp 8.5 vs. −57% under CNRM rcp 4.5) (Figure 3)

Summary

Introduction

Will continue to cause, significant changes in tree species’ distributions (Lenoir et al, 2008). Paleo-ecological studies have shown that geographical ranges of tree species across the globe have expanded and contracted since the last glacial maximum (see Svenning et al, 2015). These range shifts are, dependent on local context: topographic and climatic interactions result in microclimates that allow for the localized persistence of tree species that would otherwise be lost under the regional climate (Dobrowski, 2011; Keppel et al, 2012). For a microrefugium for any particular tree species to remain viable, it requires the persistence of favorable microclimate and maintenance of the disturbance regime for which the species is adapted (e.g., fire may be necessary to allow sufficient light for germination; Hannah et al, 2014). Climate change and other human factors are expected to alter these disturbance regimes, fire regimes for the western US (Westerling et al, 2006)

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