Functional traits change but species diversity is not influenced by edge effects in an urban forest of Eastern China

Abstract

Forest edges have significant effects on plant species and trait compositions. However, most current studies on edge effects are based on natural forests, and studies on edge effects in urban forests are still limited. Here, we set up 50 survey plots in Wuchao Mountain National Forest Park, an urban forest of Hangzhou, China. We tested how species composition changed with distance from the forest edge. Specifically, we investigated dispersal-related traits for 86 species, and analyzed trait composition changes with distance from the forest edge. The NMDS analysis revealed a significant difference in species composition between forest edge and interior, but species richness and functional diversity remain unchanged. However, single traits showed higher divergence than expected. The percentage of fleshy-fruited species declined with distance from the edge, but plant abundance of mammal-dispersed and animal-dispersal seed mass was increased. The different trend in seed size of different dispersal mode and seed mass from edge to the interior within the community may be related to the loss of mammal-dispersed plant species at the forest edge. This suggests that edge effects have a high influence on functional traits in urban forests, but not on species diversity. Edges had significant effects on animal-dispersed species and trait composition of plant communities. Our study highlights the importance of maintaining forest integrity by protecting interior habitats in urban regions. We also recommend reducing the creation of new edges and increasing the buffer area for biodiversity conservation.

Introduction

More than half of the global forested areas in the world are influenced by edge effect (Haddad et al., 2015). Changes in microclimate, energy flows and biological interaction at the forest edges can result in the loss of species that are vulnerable to edge environment (Razafindratsima et al., 2017; Liu et al., 2019a; Hawes et al., 2020), which can influence species composition, reduce biodiversity, and alter ecosystem function (Anderson et al., 2003; Deal et al., 2014; Razafindratsima et al., 2017; Smith et al., 2018). In urban areas, forests are mainly small fragments divided by roads, or recreational use (Lehvavirta et al., 2014). With the decreasing of forest areas and increasing disturbance at the forest edges, edge effects can occur more critically in urban forests (Guerra et al., 2017; Razafindratsima et al., 2017).

On account of accelerating urbanization and increasing interference with human activities, urban forests may experience different causes of edge effects compared to natural forests (Latimer and Zuckerberg, 2016; Vakhlamova et al., 2016; Haddock et al., 2019; Meili et al., 2020). At habitat edge, increased artificial light at night may result in the reduction of available habitat areas and wildlife corridors, which can affect functional connectivity of local species (Haddock et al., 2019). There are a variety of human activities in urban forests, such as recreation, exercise, and climbing (Basu and Nagendra, 2020), thus a study found that edge effects in urban forest have a positive influence on the regeneration of some deciduous trees, but especially have direct effects of trampling on all species (Lehvavirta et al., 2014). The effect arising from edge habitat represents the interaction of urban environment and organism. Although forest specialists, such as animal-dispersed large-seed species, are generally replaced by ubiquitous species or pioneer tree species (small-seed tree species) as a result of lack of specialist seed dispersers and increasing non-animal-dispersed plants at the natural forest edge (Liu et al., 2019a), few studies investigated this in urban forests.

Functional traits, such as traits related to dispersal and productivity, can be used to evaluate plant ability to survive and colonize in a specific environmental condition (Asanok et al., 2013). For example, seed and fruit traits related to plant dispersal ability are important determinants of animal-plant interactions, evaluating resource availability (McKinney, 2006; Galetti et al., 2013). And, there is growing evidence that seed size and fruit type of species can respond to edge effects (Ewers and Didham, 2006; Magnago et al., 2014; Razafindratsima et al., 2017). Moreover, the decreasing number of large frugivorous dispersers at forest edges limit the regeneration and reproduction of counterpart plants (Laurance et al., 2006; Magnago et al., 2014). As for productive traits, characterizing the traits such as potential maximum height, leaf traits and wood density can reflect plant competition ability and physiological responses to environmental changes (Bolmgren and Eriksson, 2005; Girão et al., 2007; Moran and Catterall, 2010; Ding et al., 2012; Lasky et al., 2013). For instance, specific leaf area has been found to decrease in highly disturbance sites, such as forest edge (Ding et al., 2012; Wright et al., 2013). Besides, some studies have suggested that due to habitat destruction, forest specialists or late-successional species would be replaced by pioneer species, with low wood density (Girão et al., 2007; Tabarelli et al., 2010). Assessing differences of dispersal and productive traits along edge-to-interior gradients can provide more information about understanding patterns of species composition of urban forests in the face of edge effect (Oliveira et al., 2004; Hawes et al., 2020).

Functional diversity, functional trait-based analysis, have been widely used to evaluate the effects of habitat fragmentation on plant community (Magnago et al., 2014; Haddock et al., 2019), changes in that metric resulting from edge disturbance may have implications about species composition in response to ecosystem functioning and environment change (Moran and Catterall, 2010; Razafindratsima et al., 2017; Liu et al., 2020). A study comparing taxonomic diversity and functional diversity suggested that significant edge effects on species community were showed by using functional diversity (Magura, 2017). For example, edge effects can intensify environmental filtering or weaken biological interactions (Guerra et al., 2017), and the community would become functionally more convergent (Razafindratsima et al., 2017; Liu et al., 2019a). Thus, these changes may mirror in urban forests edges where functionally similar species tend to increase over time (Ben et al., 2017).

The plant community structure of urban forests is directly affected by their surrounding habitats, and this effect may be amplified by the plant-animal feedback mechanism (Razafindratsima et al., 2017; d’Albertas et al., 2018). In this regard, the assessment of dispersal and productive traits is an effective approach to explore edge effects of urban forests, and urban forests can be managed and restored with experimental results and policy interventions. In this study, we investigated edge effects on plant community composition and functional diversity in an urban forest. Specifically, we aim to answer two questions: 1) do species composition and taxonomic diversity change from the edge to the interior of urban forest? 2) do dispersal- and productive-related plant traits respond to edge-to-interior gradient? We hypothesized that urban forest should show an increasing tendency of functional diversity and shifts in functions related to dispersal towards species with traits, such as animal dispersion, fleshy-fruited and large seed, in edge-to-interior gradient.