Climate warming, rather than nitrogen deposition, reduces plant diversity and increases community homogenization in a desert steppe

It is still unclear how increasing nitrogen (N) deposition, climate warming, and their interaction affect biotic impoverishment (decreases in α-diversity) and homogenization (decreases in β-diversity) of plant communities at taxonomic, functional, and phylogenetic levels. To address this, we conducted a long-term (17-year) field experiment in Inner Mongolia's temperate desert steppe to investigate the effects of warming and nitrogen deposition on plant taxonomic, functional, and phylogenetic α- and β-diversity. Over this period (2006–2022), warming (mean annual temperature + 1.4°C) significantly decreased species richness and functional α-diversity throughout the experiment, whereas its effects on phylogenetic α-diversity and community phylogenetic structure were significant only before 2015 and disappeared thereafter, likely due to an extreme drought that reset the community composition. In contrast, β-diversity steadily declined over the entire period. These dynamics led to a shift in the community phylogenetic structure from overdispersion towards randomness during the first 10 years. Nitrogen addition (10 g m2 yr-1) had no significant impact on diversity at the functional and phylogenetic levels but increased β-diversity at the taxonomic level. Our findings reveal that relatively rare species were more likely to go extinct, while species with higher leaf nitrogen concentrations were more prone to colonization, and these patterns were observed across all treatments. These findings suggest that warming can contribute to biotic impoverishment and homogenization by causing the extinction of species distantly related to the resident community.Species richness, functional α-diversity (FDα_q), phylogenetic α-diversity (PD) and net relatedness index (NRI) for experimental plots during experimental period.

目前尚不明确氮（N）沉降增加、气候变暖及其交互作用，如何在分类学、功能学与系统发育三个层面上，影响植物群落在该维度下的生物多样性丧失（α多样性下降）与群落同质化（β多样性下降）。为解答这一科学问题，我们在内蒙古温带荒漠草原开展了一项为期17年的野外定位试验，以探究增温和氮沉降对植物分类学、功能学及系统发育维度下的α与β多样性的影响。试验周期为2006年至2022年，其中增温处理使样地年平均气温升高1.4℃，在整个试验期间显著降低了物种丰富度与功能α多样性；但其对系统发育α多样性及群落系统发育结构的显著影响仅在2015年前存在，后续效应消失，这可能归因于一场极端干旱事件重置了群落组成。与之相反，β多样性在整个试验周期内持续下降。上述动态变化使群落系统发育结构在前10年从过度分散转向随机分布。氮添加处理（施氮量为10 g·m⁻²·yr⁻¹）对功能与系统发育维度的多样性无显著影响，但显著提升了分类学维度的β多样性。研究结果显示，相对稀有种更易发生灭绝，而叶片氮浓度更高的物种更易成功定植，且上述规律在所有处理组中均存在。本研究结果表明，气候变暖可通过导致与本地群落亲缘关系较远的物种灭绝，进而引发生物多样性丧失与群落同质化。本数据集包含试验期间各实验样地的物种丰富度、功能α多样性（FDα_q）、系统发育α多样性（PD）及净亲缘关系指数（NRI）。