Evidence that spatial scale and environment factors explains community assembly in grassland communities

Understanding community assembly of living organisms has been a prominent ecological question since the early days of the discipline. The impact of habitat filtering and limiting similarity on plant community structures is well-known, as both processes are influenced by individual responses to environmental changes. However, identifying and quantifying potential abiotic and biotic factors that ultimately influence community structures at a fine scale remains a challenge. Here, we employed different species pool null models to assess the importance of habitat filtering and limiting similarity at two spatial scales. We used 63 natural vegetation plots, each measuring 5m x 5m, with 3 nested subplots measuring 1m x 1m, from the 2021 field survey, to examine the alpha diversity of plots and subplots as well as Beta diversity. We also used linear mixed-effects models (LMEs) to assess how environmental factors affect the assembly process. Our findings indicate that habitat filtering was the dominant assembly process at both the plot and subplot levels, while limiting similarity was stronger at the subplot level. Plot-level limiting similarity was positively correlated with fine-scale partitioning, suggesting that trait divergence resulted from a combination of limiting similarity species and spatial partitioning. Our results also suggest that the assembly process varies more strongly along the mean annual temperature (MAT) gradient than the mean annual precipitation (MAP). Additionally, the community assembly process of different traits varied with these environmental factors, indicating the importance of multi-dimensional traits. This study provides a valuable example of non-random assembly rules from spatial scale and environmental factors in grassland communities in the loess hilly region. These results highlight the essential role of additional constraints with spatial scales and environmental factors for understanding the process of grassland community assembly.