Patterns of Understory Community Assembly and Plant Trait-Environment Relationships in Temperate SE European Forests

Janez Kermavnar,Lado Kutnar

doi:10.3390/d12030091

Abstract

We analyzed variation in the functional composition and diversity of understory plant communities across different forest vegetation types in Slovenia. The study area comprises 10 representative forest sites covering broad gradients of environmental conditions (altitude, geology, light availability, soil type and reaction, nutrient availability, soil moisture), stand structural features and community attributes. The mean and variation of the trait values were quantified by community-weighted means and functional dispersion for four key plant functional traits: plant height, seed mass, specific leaf area and leaf dry matter content. At each study site, forest vegetation was surveyed at two different spatial scales (4 and 100 m2) in order to infer scale-dependent assembly rules. Patterns of community assembly were tested with a null model approach. We found that both trait means and diversity values responded to conspicuous gradients in environmental conditions and species composition across the studied forests. Our results mainly support the idea of abiotic filtering: more stressful environmental conditions (e.g., high altitude, low soil pH and low nutrient content) were occupied by communities of low functional diversity (trait convergence), which suggests a selective effect for species with traits adapted to such harsh conditions. However, trait convergence was also detected in some more resource-rich forest sites (e.g., low altitude, high soil productivity), most likely due to the presence of competitive understory species with high abundance domination. This could, at least to some extent, indicate the filtering effect of competitive interactions. Overall, we observed weak and inconsistent patterns regarding the impact of spatial scale, suggesting that similar assembly mechanisms are operating at both investigated spatial scales. Our findings contribute to the baseline understanding of the role of both abiotic and biotic constraints in forest community assembly, as evidenced by the non-random patterns in the functional structure of distinct temperate forest understories.

Highlights

IntroductionAnalyzing patterns of functional composition (community trait means) and functional diversity (the degree of trait variation among coexisting species in a community) within and across plant communities has been recognized as a promising avenue in attempts to infer mechanisms of community assembly [1,2,3,4]
Analyzing patterns of functional composition and functional diversity within and across plant communities has been recognized as a promising avenue in attempts to infer mechanisms of community assembly [1,2,3,4]
Our sampled data set consisted of 267 different plant species in 60 large plots (Lplots) (10 × 10 m; total sampled area of 6000 m2 ), whereas in 100 Splots (2 × 2 m; total sampled area is 400 m2 ), 224 different species were recorded in the understory layer

Summary

Introduction

Analyzing patterns of functional composition (community trait means) and functional diversity (the degree of trait variation among coexisting species in a community) within and across plant communities has been recognized as a promising avenue in attempts to infer mechanisms of community assembly [1,2,3,4]. Functional diversity patterns serve as indicators of different assembly processes and have important implications for understanding global, regional and local plant species distributions [5,6]. Diversity 2020, 12, 91 community assembly processes [9] Such evaluations of trait-based (dis)similarities among coexisting plant species are used to objectively test the assumption that local plant communities do not result from the random gathering of species from a regional species pool [10]. Species need to display some degree of functional trait differentiation in order to minimize niche overlap and coexist with other species present in a local community [15,16,17]

Methods

Results

Discussion

Conclusion