Tree–herb co-existence and community assembly in natural forest-steppe transitions

Csaba Tölgyesi,Orsolya Valkó,Balázs Deák,András Kelemen,Tatyana M Bragina,Róbert Gallé,László Erdős,Zoltán Bátori

doi:10.1080/17550874.2018.1544674

Abstract

ABSTRACTBackground: The effects of trees on understorey communities is a major driver of vegetation composition. However, we have little understanding on how isolated forest patches of the forest-steppe transition affect their herb layer as compared to adjacent grasslands.Aims: Our aim was to test whether trees had a protective effect on understorey herbaceous communities in the most arid regions of the forest-steppe transition, where the stress gradient hypothesis predicts positive net effects.Methods: We surveyed herbaceous cover and species composition in 135 forest-steppe sites and recorded soil moisture, microclimate and canopy cover in northern Kazakhstan.Results: Total cover and species richness were lower in the herb layer of groves than in the steppe stands and the soil of the groves was not moister than that of the steppes. Groves were dominated by grassland specialist plants, while forest specialist species remained scarce. However, these grassland species were different from the ones inhabiting the steppes, leading to little nestedness between groves and steppes.Conclusions: Although the presence of groves greatly increase the landscape level diversity of Middle-Asian forest-steppes, trees in the southern edge of the zone seem to have net negative effect on the understorey vegetation, seemingly contradicting the stress gradient hypothesis.

Highlights

IntroductionMosaic habitats consisting of grassland and forest components often harbour high biodiversity (Scholes and Archer 1997; Manning et al 2006)
Mosaic habitats consisting of grassland and forest components often harbour high biodiversity (Scholes and Archer 1997; Manning et al 2006). Their dynamics and species composition largely depend on the interactions between the trees and their herb layer (Staal and Flores 2015)
We addressed the following questions: (1) Competition or facilitation - is the net effect of trees on the productivity and diversity of the herb layer negative or positive relative to the adjacent grasslands and what ecosystem engineering mechanisms of the trees may contribute to this effect? (2) Community assembly - do forest specialist species comprise the herb layer of the groves or it contains only a filtered subset of the species pool of the surrounding grassland matrix?

Summary

Introduction

Mosaic habitats consisting of grassland and forest components often harbour high biodiversity (Scholes and Archer 1997; Manning et al 2006). Grassland communities would be able to thrive on the forest-ward side of the treelines without any constraining effect of macroclimatic filters, but their composition is altered by biotic constraints (i.e. the presence and shading effect of closed canopy forming trees). This deterministic relationship is affected to some extent by local to regional disturbance factors such as fire or herbivory, frequently making forested and herbaceous patches alternative stable states within the transitional zones (Staver et al 2011; Hoffmann et al 2012). Conclusions: the presence of groves greatly increase the landscape level diversity of Middle-Asian forest-steppes, trees in the southern edge of the zone seem to have net negative effect on the understorey vegetation, seemingly contradicting the stress gradient hypothesis

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