Data from: Evidence of functional species sorting by rainfall and biotic interactions: A community monolith experimental approach

1. Understanding the mechanisms that underlie species assembly is a
central concern in community ecology. Abiotic and biotic filters are
probabilistic ‘sieves’ that allow species with certain functional traits
to become part of the community, or not. We manipulated natural plant
assemblieses in order to identify variations in the timings of biotic and
abiotic filters that determine community trait assemblies. 2. We extracted
soil portions when the investigated annual plant community was in its seed
phase (‘community monolith’), thereby maintaining the structure and
similar soil characteristics to the field conditions. Community monoliths
were subjected to experimental manipulation in terms of the rainfall
timing and amount, and perturbations of the biological soil crust (BSC;
intact vs. perturbed). We surveyed the experimental community assembly
over time based on the functional diversity by considering important
functional traits in different life stages. 3. We found that autumn
droughts acted as abiotic filters by favouring the germination and
establishment of species with greater investment in the root biomass.
Under severe droughts (66% water reduction), the experimental assemblies
were dominated by species with functional traits adapted to water shortage
conditions: high leaf dry matter content, low specific leaf area, small
individual size, low reproductive ratio, and high root:shoot ratio. We
identified two roles of BSCs in annual plant species assemblies: 1) as a
biotic filter that limited the establishment of species based on seed
size, and 2) as a buffer against water stress conditions by reducing soil
evapotranspiration. 4. Synthesis. We demonstrated the importance of the
timing and amount of rainfall for shaping annual plant communities, and
identified germination filters as the main process that determined
community assemblies. Our results suggest that the phenotypic integration
of functional traits facilitates resistance to drought during the life
cycle. The BSC–annual plant relationship shifted from negative by acting
as a germination filter to positive by acting as a buffer in later stages.
Climatic fluctuations and fine scale biotic determinants of spatial
heterogeneity emerged as sources of changes in the community assembly in
time and space to possibly promote species coexistence and trait
differences among the communities studied.

1. 解析物种组装的内在机制是群落生态学（community ecology）的核心议题。非生物与生物过滤是一类概率性的“筛子”，允许具备特定功能性状（functional traits）的物种纳入群落，或对其进行排除。为明确调控群落性状组装的生物与非生物过滤的时序差异，我们对天然植物群落开展了操控实验。
2. 当研究的一年生植物群落处于种子阶段时，我们采集了土壤块体，即“群落原位土块（community monolith）”，以此保留群落结构并维持与野外原位条件一致的土壤特性。我们对群落原位土块开展了两项操控实验：调控降雨的时序与总量，以及对生物土壤结皮（biological soil crust, BSC）进行扰动处理（完整结皮vs扰动结皮）。我们基于不同生活史阶段的关键功能性状，以功能多样性为指标，对实验群落的组装过程开展了长期动态观测。
3. 研究发现，秋季干旱通过偏向促进根系生物量投入更高的物种萌发与建植，发挥了非生物过滤的作用。在极端干旱（减水66%）处理下，实验群落由适应缺水环境的功能性状物种主导：高叶干物质含量（leaf dry matter content）、低比叶面积（specific leaf area）、较小的个体体量、较低的繁殖率以及较高的根冠比（root:shoot ratio）。我们明确了生物土壤结皮在一年生植物群落组装中的两项功能：其一，作为基于种子大小限制物种建植的生物过滤因子；其二，通过降低土壤蒸发蒸腾（evapotranspiration），作为抵御水分胁迫的缓冲屏障。
4. 综合分析。本研究证实了降雨时序与总量在塑造一年生植物群落结构中的关键作用，并明确了萌发过滤是调控群落组装的核心过程。研究结果表明，功能性状的表型整合（phenotypic integration）有助于提升植物整个生活史周期内的干旱抗性。生物土壤结皮与一年生植物的相互关系发生了动态转变：在萌发阶段，其作为过滤因子表现为负相关作用；而在后续发育阶段，其作为水分胁迫缓冲屏障则转变为正相关作用。气候波动以及调控空间异质性（spatial heterogeneity）的精细尺度生物因子，成为群落组装在时空维度上发生变化的驱动因素，或可促进本研究样地内的物种共存（species coexistence）以及不同群落间的性状分化。