Identifying Western North American Tree Populations Vulnerable to Drought under Observed and Projected Climate Change

Abstract

Global climate change has affected forest health and productivity. A highly visible, direct climate impact is dieback caused by drought periods in moisture-limited forest ecosystems. Here, we have used a climate moisture index (CMI), which has been developed in order to map forest–grassland transitions, to investigate the shifts of the zero-CMI isopleths, in order to infer drought vulnerabilities. Our main objective was to identify populations of the 24 most common western North American forest tree species that are most exposed to drought conditions by using a western North American forest inventory database with 55,700 plot locations. We have found that climate change projections primarily increase the water deficits for tree populations that are already in vulnerable positions. In order to test the realism of this vulnerability assessment, we have compared the observed population dieback with changes in index values between the 1961–1990 reference period and a recent 1991–2020 average. The drought impacts that were predicted by negative CMI values largely conformed to the observed dieback in Pinus edulis, Populus tremuloides, and Pinus ponderosa. However, there was one notable counter-example. The observed dieback in the Canadian populations of Populus tremuloides were not associated with directional trends in the drought index values but were instead caused by a rare extreme drought event that was not apparently linked to directional climate change. Nevertheless, a macro-climatic drought index approach appeared to be generally suitable to identify and forecast the drought threats to the tree populations.

Evaluating climate change impacts on ecosystem resources through the lens of climate analogs

“…Areas with strong positive loadings indicated high overall stability whereas areas with strong negative loadings indicated low stability. PCA was conducted using the rasterPCA function in the RStoolbox package v0.3.1.9999 (Leutner et al, 2023) in R.…”

Selection of Representative General Circulation Models Under Climatic Uncertainty for Western North America

“…In this study, we consider WNA region, which is a vulnerable region due to its sensitivity to environmental pressures and low-to-moderate adaptability to climate change [1]. As shown in Fig.…”

“…Temperature reconstructions based on tree-ring data [28] indicate that spring temperatures in WNA during the previous 50 years have been greater than any comparable time during the last 900 years [29,30]. indices from the archive produced by the Expert Team on Climate Change Detection and Indices (ETCCDI) [1] and made available by the "Canadian Centre for Climate Modeling and Analysis" are then employed for the purpose of obtaining the annual extremes of the regular CMIP5 results. This information is collected and downloaded using the KNMI Climate Explorer; and (iii) Observational datasets: We employ reference temperature data from the European Centre for Medium-Range Weather Forecasts' global reanalyses, ERA5 [14], and reference precipitation data from the Global Precipitation Climatology Project (GPCP), version 2.3 [32], to evaluate the selected representative GCMs.…”

“…Evaluating possible climate changes on a local and regional scale is essential in understanding the potential impacts and hazards and enhancing a resilience plan. Analysis of the US Historical Climatology Network (USHCN) database between 1990 and 2020 for the Western North America (WNA) region reveals significant changes in temperature and precipitation [1]. The WNA has experienced severe droughts and gradual changes in hydrological regimes, leading to reduced population growth and increased mortality [2,3,4,5,6].…”