Microclimatic Tipping Points at the Beech–Oak Ecotone in the Western Romanian Carpathians

Stefan Hohnwald,Helge Walentowski,Christoph Leuschner,Adrian Indreica

doi:10.3390/f11090919

Stefan Hohnwald, Helge Walentowski + Show 2 more

Open Access

https://doi.org/10.3390/f11090919

Copy DOI

Abstract

European beech (Fagus sylvatica) is a drought-sensitive species that likely will retreat from its xeric distribution edges in the course of climate warming. Physiological measurements indicate that the species may not only be sensitive to soil water deficits, but also to high temperatures and elevated atmospheric vapor pressure deficits (vpd). Through microclimatological measurements in the stand interior across near-natural beech forest–oak forest ecotones, we searched for microclimatic tipping points in the contact zone with the aim to define the thermic and hydrometeorological limits of beech more precisely. In three transects in the foothills of the Romanian western Carpathians, we measured in mid-summer 2019 air temperature, relative air humidity, and vpd at 2 m height in the stand interior across the ecotone from pure oak to pure beech forests, and compared the readings to the microclimate in forest gaps. Mean daytime temperature (T) and vpd were by 2 K and 2 hPa, respectively, higher in the oak forests than the beech forests; the extremes differed even more. Especially in the second half of the day, the oak forests heated up and were more xeric than the beech forests. Part of the differences is explained by the elevation difference between oak and beech forests (200–300 m), but species differences in canopy structure, leaf area, and canopy transmissivity enhance the microclimatic contrast. Our T and vpd data point to thresholds at about 30 °C and 25 hPa as maxima tolerated by beech in the lowermost shade canopy for extended periods. In conclusion, the rather sharp stand microclimatic gradient demonstrated here for the xeric distribution limit of beech may well be the decisive factor that hinders the spread of beech into the warmer oak forests.

Highlights

The extraordinary drought and heat in the summers 2018 and 2019 has demonstrated the climatic vulnerability of European beech (Fagus sylvatica L.) in many parts of its Central European distribution range [1,2]
Stand microclimate measurements were conducted in summer 2019 (15 June to 27 July) in three transects spanning from pure oak forest through an oak/beech ecotone to pure beech forest at colline to lower montane elevation in the south-western Carpathians in western Romania
The study region in western Romania has a less continental climate than is experienced by Fagus at its eastern distribution limit in eastern Romania and Poland, where winter cold plays a decisive role [46]. This justifies the focus on summer heat and drought. This gradient study shows that the microclimate in the forest interior is during mid-summer significantly warmer and drier in colline oak forests than in submontane beech forests with a difference in mean daytime temperature by ≈2 K and in mean daytime vpd of ≈2 hPa

Summary

Introduction

The extraordinary drought and heat in the summers 2018 and 2019 has demonstrated the climatic vulnerability of European beech (Fagus sylvatica L.) in many parts of its Central European distribution range [1,2]. There is some evidence that beech is unable to maintain its large and shading leaf area beyond a climatic threshold, which is characterized by an EQ of ≈30, and to successfully suppress its competitors [1,12] This limit has been termed the climatic turning point for beech and oak in Central Europe [13]. Most likely this is not a sharp line, but a continuous ecotone between beech-dominated and oak-dominated forests

Methods

Results

Discussion

Conclusion