Projected climate and canopy change lead to thermophilization and homogenization of forest floor vegetation in a hotspot of plant species richness, Berchtesgaden National Park, Bavaria, Germany

Mountain forests are plant diversity hotspots, but changing climate and increasing forest disturbances will likely lead to far-reaching plant community change. Projecting future change, however, is challenging for forest understory plants, which respond to forest structure and composition as well as climate. Here, we jointly assessed effects of both climate and forest change, including wind and bark beetle disturbances, using the process-based simulation model iLand in a protected landscape in the northern Alps (Berchtesgaden National Park, Germany), asking: (1) How do understory plant communities respond to 21st-century change in a topographically complex mountain landscape, representing a hotspot of plant species richness? (2) How important are climatic changes (i.e., direct climate effects) versus forest structure and composition changes (i.e., indirect climate effects and recovery from past land use) in driving understory responses at landscape scales? Stacked individual species distribution models fit with climate, forest, and soil predictors (248 species currently present in the landscape, derived from 150 field plots stratified by elevation and forest development, overall AUC = 0.86) were driven with projected climate (RCP4.5 and RCP8.5) and modeled forest variables to predict plant community change. Nearly all species persisted in the landscape in 2050, but on average 8% of the species pool was lost by the end of the century. By 2100, landscape mean species richness and understory cover declined (-13% and -8%, respectively), warm-adapted species increasingly dominated plant communities (i.e., thermophilization, +12%), and plot-level turnover was high (62%). Subalpine forests experienced the greatest richness declines (-16%), most thermophilization (+17%), and highest turnover (67%), resulting in plant community homogenization across elevation zones. Climate rather than forest change was the dominant driver of understory responses. The magnitude of unabated 21st-century change is likely to erode plant diversity in a species richness hotspot, calling for stronger conservation and climate mitigation efforts.

山地森林是植物多样性热点区域，但气候变化加剧与森林干扰频发或引发深远的植物群落变化。然而，针对同时响应森林结构、组成与气候的森林林下植物，预测其未来群落变化颇具挑战。本研究依托阿尔卑斯山脉北部德国贝希特斯加登国家公园（Berchtesgaden National Park）的保护景观，借助过程基模拟模型（process-based simulation model）iLand，联合评估了气候变化与森林变化（包括风扰与树皮甲虫干扰）的效应，并提出两个核心研究问题：（1）在地形复杂的山地植物物种多样性热点景观中，林下植物群落将如何响应21世纪的环境变化？（2）在景观尺度上，气候变化（即直接气候效应）与森林结构、组成变化（即间接气候效应与过去土地利用恢复过程）对林下植物群落响应的相对重要性如何？本研究基于气候、森林与土壤预测因子构建了堆叠式单物种分布模型（stacked individual species distribution models），涵盖该景观目前现存的248个物种，数据来自150块按海拔与森林发育阶段分层设置的野外样地，整体曲线下面积（Area Under Curve, AUC）为0.86，随后结合典型浓度路径（Representative Concentration Pathway, RCP）4.5与RCP8.5情景下的未来气候数据与模拟得到的森林变量，预测植物群落变化。结果显示，至2050年，几乎所有物种均能在该景观中存续，但至本世纪末，平均有8%的物种种群消失；至2100年，景观平均物种丰富度与林下盖度分别下降13%与8%，适应温暖环境的物种逐渐占据群落主导地位（即热适应化（thermophilization），增幅达12%），样地水平的物种周转率高达62%。亚高山森林的物种丰富度下降幅度最大（-16%），热适应化程度最高（+17%），物种周转率也最高（67%），进而引发不同海拔带的植物群落均质化。研究表明，气候变化是驱动林下植物群落响应的主导因素，而非森林变化。21世纪未受遏制的环境变化或将侵蚀这一物种多样性热点区域的植物多样性，因此亟需强化保护与气候减缓举措。