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Open AccessArticle

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

1
Faculty of Resource Management, HAWK University of Applied Sciences and Arts, Büsgenweg 1a, 37077 Göttingen, Germany
2
Department of Silviculture, Transilvania University of Braşov, Șirul Beethoven no. 1, 500123 Brasov, Romania
3
Plant Ecology, Albrecht von Haller Institute for Plant Sciences, University of Goettingen, Untere Karspüle 2, 37073 Göttingen, Germany
*
Author to whom correspondence should be addressed.
Forests 2020, 11(9), 919; https://doi.org/10.3390/f11090919
Received: 30 June 2020 / Revised: 8 August 2020 / Accepted: 20 August 2020 / Published: 22 August 2020
(This article belongs to the Special Issue Climatic Variability Anticipation and Adaptation of Forest)
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. View Full-Text
Keywords: air temperature; Fagus sylvatica; Quercus frainetto; Quercus petraea; rear edge; stand microclimate; thermal limit; vapor pressure deficit air temperature; Fagus sylvatica; Quercus frainetto; Quercus petraea; rear edge; stand microclimate; thermal limit; vapor pressure deficit
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Hohnwald, S.; Indreica, A.; Walentowski, H.; Leuschner, C. Microclimatic Tipping Points at the Beech–Oak Ecotone in the Western Romanian Carpathians. Forests 2020, 11, 919.

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