Forest vegetation in western Romania in relation to climate variables: Does community composition reflect modelled tree species distribution?

Steffi Heinrichs,Marius Teodosiu,Karl Heinz Mellert,Any Mary Petrițan,Erwin Bergmeier,Adrian Indreica,Yakov Kuzyakov,Helge Walentowski,Christoph Leuschner

doi:10.15287/afr.2016.692

Abstract

European beech (Fagus sylvatica L.) is the prevailing tree species of mesic forests in Central Europe. Increasing summer temperatures and decreasing precipitation, as climate change scenarios predict, may, however, negatively influence beech growth and induce a shift to more thermophilous forest communities. Temperatures as expected in the future for western Central Europe are currently found in parts of western Romania. In light of this climate analogy we investigated forest vegetation as an indicator for future vegetation changes in five regions of western Romania representing a climatic gradient. We related species composition to climate variables and examined if tree and understorey species composition respond similarly to the climatic gradient. We further analysed if tree species occurrences correspond with their modelled distance to the rear niche edge. We found evidence for climatic effects on vegetation composition among regions as well as within deciduous and pine forests, respectively. This underlines that vegetation composition is a useful indicator for environmental change. Tree and understorey species compositions were closely linked showing that community-based characterization of forest stands can provide additional information on tree species suitability along environmental gradients. Both, vegetation composition and a climatic marginality index demonstrate the rear niche edge occurrence of beech in the studied sites of Romania and can predict the site suitability for different tree species. While vegetation surveys indicate Quercus petraea to be associated to moderately mesic forests, the marginality index suggested an inner niche position of sessile oak along the climatic gradient. Phytosociological releves that differentiate between subspecies (or microspecies) of sessile oak with differing habitat requirements should be considered to complement national forest inventories and species distribution maps when modelling rear distribution edges. We conclude that climate driven forest vegetation composition in western Romania is a suitable analogon and may indicate future forest development in western Central Europe.

Highlights

European beech (Fagus sylvatica L.) is the prevailing tree species of mesic forests across about two thirds of Central Europe
We investigated three basic research questions: (i) Which species compositional shifts can be observed along the climate gradient?; (ii) Are understorey and tree species linked and do they respond to climatic gradients?; (iii) Do the tree species in the survey occur in accordance with their modelled distribution and can it be predicted by climate indices such as the widely applied Ellenberg Quotient (EQ; Ellenberg 1963), the De Martonne aridity index, and the climatic marginality index, which estimates tree species’ distance to the rear niche edge at a particular site (CMI; Mellert et al 2015, 2016)?
Pine and thermophilous deciduous forests had higher environmental indicator values (EIV) for temperature than the mesic deciduous forests, while all three groups showed a gradient in light and nitrogen availability with the mesic and pine forests representing the EIV extremes

Summary

Introduction

European beech (Fagus sylvatica L.) is the prevailing tree species of mesic forests across about two thirds of Central Europe. Future summer temperatures and rainfall in western Central Europe will compare with values currently found in parts of western Romania, more than 1000 km to the South-east This climate analogy prompted us to inspect to what extent species composition and habitat conditions of western Romanian forest sites are useful trend indicators for forest vegetation development further northwest. As predicted by the law of relative habitat constancy (Walter & Breckle 1985), mesic forests occur at low ele vations extrazonally on northern slopes or in valleys with increased humidity (Doniță et al 1992). While in such extrazonal mesic forests thermophilous species from the surrounding territory may occur, they are mainly charac-

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