Resource addition drives taxonomic divergence and phylogenetic convergence of plant communities

Anthropogenic environmental changes are known to affect the Earth's ecosystems. However, how these changes influence assembly trajectories of the impacted communities remains a largely open question. In this study, we investigated the effect of elevated nitrogen (N) deposition and increased precipitation on plant taxonomic and phylogenetic β‐diversity in a 9‐year field experiment in the temperate semi‐arid steppe of Inner Mongolia, China. We found that both N and water addition significantly increased taxonomic β‐diversity, whereas N, not water, addition significantly increased phylogenetic β‐diversity. After the differences in local species diversity were controlled using null models, the standard effect size of taxonomic β‐diversity still increased with both N and water addition, whereas water, not N, addition, significantly reduced the standard effect size of phylogenetic β‐diversity. The increased phylogenetic convergence observed in the water addition treatment was associated with colonizing species in each water addition plot being more closely related to species in other replicate plots of the same treatment. Species colonization in this treatment was found to be trait‐based, with leaf nitrogen concentration being the key functional trait. Synthesis. Our analyses demonstrate that anthropogenic environmental changes may affect the assembly trajectories of plant communities at both taxonomic and phylogenetic scales. Our results also suggest that while stochastic processes may cause communities to diverge in species composition, deterministic process could still drive communities to converge in phylogenetic community structure. Foreign Language 摘要 1. 众所周知, 人类活动引起的环境变化会影响地球生态系统。然而, 这些变化如何影响生物群落构建的 “轨迹” 仍然是一个悬而未决的问题。 2.在本研究中, 基于中国内蒙古温带半干旱草原的9年控制试验, 我们研究了氮（N）沉降增加和降水量增加对植物群落物种水平和系统发育水平β多样性的影响。 3. 我们发现添加N和水都能显著增加物种β多样性, 而只有添加N能显著增加系统发育β多样性。在利用零模型去除了不同样方间物种差异的影响后, 物种β多样性的标准效应值（standard effect size）依旧会随着N和水的添加而增大, 然而对于系统发育β多样性, 只有水的添加显著降低了它的标准效应值。在加水处理中, 不同样方之间的群落系统发育聚集程度随时间而增强。这种增强跟不同样方被亲缘关系相近的物种定殖有关。我们还发现加水处理中的物种定殖是基于性状的, 叶片氮含量是关键的功能性状。 4. 结论:我们的分析表明, 人类活动引起的环境变化可能影响植物群落在物种和系统发育水平上的构建 “轨迹”。虽然随机性过程可能导致群落在物种组成上出现分歧, 但确定性过程仍然可以促使群落在系统发育结构上趋于收敛。 Our analyses demonstrate that anthropogenic environmental changes may affect the assembly trajectories of plant communities at both taxonomic and phylogenetic scales. Our results also suggest that while stochastic processes may cause communities to diverge in species composition, deterministic process could still drive