Data from: Disentangling the relative importance of species occurrence, abundance and intraspecific variability in community assembly: a trait-based approach at the whole-plant level in Mediterranean forests

Understanding which factors and rules govern the process of assembly in communities constitutes one of the main challenges of plant community ecology. The presence of certain functional strategies along broad environmental gradients can help to understand the patterns observed in community assembly and the filtering mechanisms that take place. We used a trait-based approach, quantifying variations in aboveground (leaf and stem) and belowground (root) functional traits along environmental gradients in Mediterranean forest communities (south Spain). We proposed a new practical method to quantify the relative importance of species turnover (distinguishing between species occurrence and abundance) versus intraspecific variation, which allowed us to better understand the assemblage rules of these plant communities along environmental gradients. Our results showed that the functional structure of the studied plant communities was highly determined by soil environment. Results from our modelling approach based on maximum likelihood estimators showed a predominant influence of soil water storage on most of the community functional traits. We found that changes in community functional structure along environmental gradients were mainly promoted by species turnover rather than by intraspecific variability. Specifically, our new method of variance decomposition demonstrated that between-site trait variation was the result of changes in species occurrence rather than in the abundance of certain dominant species. In conclusion, this study showed that water availability promoted the predominance of specific trait values (both in above and belowground fractions) associated to a resource acquisition or conservation strategy. In addition, we provided evidence that changes on community functional structure along the environmental gradient were mainly promoted by a process of species replacement, which represent a crucial step towards a more general understanding of the relative importance of intraspecific versus interspecific trait variation in these woody Mediterranean communities.

厘清调控群落构建（community assembly）过程的核心因子与内在逻辑，是植物群落生态学（plant community ecology）面临的核心挑战之一。沿宽泛环境梯度（environmental gradients）分布的特定功能策略（functional strategies），有助于解析群落构建模式与其中发生的生态过滤机制（filtering mechanisms）。本研究采用基于功能性状的研究方法（trait-based approach），针对西班牙南部的地中海森林群落（Mediterranean forest communities），量化了地上（叶片与茎干）与地下（根系）功能性状（functional traits）沿环境梯度的变化特征。我们提出了一种全新的实用方法，用于量化物种周转（species turnover，区分物种出现频率与种群多度）与种内变异（intraspecific variation）的相对重要性，借此更深入地理解这些植物群落在环境梯度下的构建规则。研究结果表明，所调研植物群落的功能结构在很大程度上由土壤环境决定。基于最大似然估计器（maximum likelihood estimators）的建模分析结果显示，土壤蓄水量（soil water storage）对绝大多数群落功能性状具有主导性影响。我们发现，沿环境梯度的群落功能结构变化，主要由物种周转而非种内变异驱动。具体而言，我们提出的新型方差分解方法（variance decomposition）证实，样地间性状变异源于物种出现频率的变化，而非特定优势种（dominant species）种群多度的改变。综上，本研究表明水分可利用性推动了与资源获取或保守策略相关的特定性状值（涵盖地上与地下组分）占据主导地位。此外，我们的研究证实，沿环境梯度的群落功能结构变化主要由物种更替过程驱动——这一发现为进一步理解木本地中海群落（woody Mediterranean communities）中种内与种间性状变异（interspecific trait variation）的相对重要性迈出了关键一步。