Tree biodiversity of warm drylands is likely to decline in a drier world

Abstract. Warm drylands represent 19% of land surfaces worldwide and host ca. 1,100 tree species. The risk of decline due to climate aridification of this neglected biodiversity has been overlooked despite its ecological and societal importance. To fill this gap, we assessed the risk of decline due to climate aridification of tree species in warm drylands based on spatialized occurrence data and climate models. We considered both species vulnerability and exposure, compared the risk of tree species decline across five bioregions and searched for phylogenetic correlates. Depending on the future climate model, from 44% to 88% of warm drylands’ tree species will undergo climate aridification with a high risk of decline even under the most optimistic conditions. On a regional scale, the rate of species that will undergo climate aridification in the future varies from 21% in the Old World North, to 90% in Australia, with a risk of decline confirming the high level of risk predicted at the global scale. Using generalized linear mixed models, we found that, species more exposed to climate aridification will be more at risk, but also that species vulnerability is a key driver of their risk of decline. Indeed, the warm drylands specialist species will be less at risk due to climate aridification than species being marginal in warm drylands. We also found that the risk of decline is widespread across the main clades of the phylogeny and involves several evolutionary distinct species. Estimating a high risk of decline for numerous tree species in all warm drylands, including emblematic dryland endemics, our work warns that future increase in aridity could result in an extensive erosion of tree biodiversity in these ecosystems. Table S3. The seven risk of decline, vulnerability and exposure metrics (RDA, ANO, ANB, Range size, Exp1, Exp2 and Exp3) for tree species submitted to climate aridification in warm drylands (Exp1 > 0) for each GCM and RCP, and distribution (bioregions,) and evolutionary distinctness for each of the whole 1,016 species list. [tableS3.xlsx]摘要。暖干旱区占全球陆地总面积的19%，孕育了约1100种树木。尽管这类被忽视的生物多样性具有重要的生态与社会价值，但因气候干旱化导致的物种衰退风险却长期被忽视。为填补这一研究空白，本研究基于空间化出现数据与气候模型，评估了暖干旱区内树木物种因气候干旱化面临的衰退风险。研究同时考量了物种脆弱性与暴露风险，对比了五个生物地理区域间树木物种的衰退风险，并探索了其系统发育相关性。根据不同的未来气候模型预测，即便在最乐观的情景下，仍有44%至88%的暖干旱区树木物种将经历气候干旱化，并面临极高的衰退风险。在区域尺度上，未来将经历气候干旱化的物种占比从旧大陆北部的21%到澳大利亚的90%不等，其衰退风险也印证了全球尺度下预测的高风险水平。通过广义线性混合模型（generalized linear mixed models），我们发现，暴露于气候干旱化程度更高的物种衰退风险更高，同时物种脆弱性也是影响其衰退风险的关键驱动因素。具体而言，暖干旱区特有专化物种因气候干旱化面临的衰退风险，要低于在暖干旱区处于边缘分布的物种。本研究还发现，衰退风险广泛分布于系统发育树的主要演化支中，且涉及多个演化独特性较高的物种。本研究评估发现，所有暖干旱区内的众多树木物种（包括标志性的干旱区特有物种）均面临较高的衰退风险，这警示我们：未来干旱程度的加剧可能会导致这些生态系统中树木生物多样性的大规模流失。 表S3。针对每个全球气候模式（GCM）和典型浓度路径（RCP）下，暖干旱区内（Exp1>0）受气候干旱化影响的树木物种的7项衰退风险、脆弱性与暴露指标（RDA、ANO、ANB、分布范围面积、Exp1、Exp2及Exp3），以及全部1016个物种列表中每个物种的分布区域（生物地理区）与演化独特性。[tableS3.xlsx]