Abstract

Research Highlights: Investigations of evapotranspiration in a mature mixed beech-fir forest stand do not indicate higher resilience towards intensified drying-wetting cycles as compared with pure beech stands. Background and Objectives: Forest management seeks to implement adaptive measures, for example, the introduction of more drought resistant species into prevailing monospecific stands to minimize forest mortality and monetary losses. In Central Europe this includes the introduction of native silver fir (Abies alba) into monospecific beech (Fagus sylvatica) stands. In order to determine, if the introduction of fir would improve the resilience against drier conditions, this study investigates water relations of a mature pure beech and a mature mixed beech-fir stand under natural as well as reduced water availability. Materials and Methods: Sap flow rates and densities were measured in two consecutive years using the heat ratio method and scaled using stand inventory data and modeling. Results: Transpiration rates estimated from sap flow were significantly higher for beech trees as compared with silver fir which was attributed to the more anisohydric water-use strategy of the beech trees. We estimate that stand evapotranspiration was slightly higher for mixed stands due to higher interception losses from the mixed stand during times of above average water supply. When precipitation was restricted, beech was not able to support its transpiration demands, and therefore there was reduced sap flow rates in the mixed, as well as in the pure stand, whereas transpiration of fir was largely unaffected, likely due to its more isohydric behavior toward water use and access to moister soil layers. Thus, we found the rates of evapotranspiration in the mixed beech-fir stand to be smaller during times with no precipitation as compared with the pure beech stand, which was accountable to the severely reduced transpiration of beech in the mixed stand. Conclusions: We conclude that smaller evapotranspiration rates in the mixed beech-fir stand might not be the result of increased water use efficiency but rather caused by restricted hydraulic conductivity of the root system of beech, making mixed beech-fir stands at this site less resilient towards drought.

Highlights

  • Forest ecosystems have to cope with various stresses during their long lifespan

  • In the plots subjected to rain exclusion, soil moisture steadily decreased from mid-May until reaching lowest values in mid-July, and remained low until the first irrigation event in mid-August

  • In a year with higher than average precipitation, we found the water use of the mixed plot exceeded that of the pure plot, indicating a competition reduction of water use for beech in the mixed plot with silver fir

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Summary

Introduction

Forest ecosystems have to cope with various stresses during their long lifespan. Anthropogenic climate change happens at an accelerating speed and implications for forest ecosystems are expected to be severe [1,2]. Prone to disrupt the natural regeneration cycles of forest ecosystems Against this background, the future performance of native tree species in Central Europe is frequently and critically discussed [4,5,6]. The future performance of native tree species in Central Europe is frequently and critically discussed [4,5,6] In this context, forest management concepts have been proposed and implemented in the past decades to mitigate future climate change by improving resistance and resilience of forest ecosystems [7,8,9,10,11]. For the latter purpose, fostering compositional, functional, and structural complexity of forests is a central aim of forest policy in Germany [12,13]

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