Competition for water rather than facilitation in mixed beech-fir forests after drying-wetting cycle

Abstract

In the present study we investigated water use strategies of mature stands of Fagus sylvatica (European Beech) with and without co-cultivation of Abies alba (Silver fir) during a summer drought and subsequent rewetting through simulated heavy precipitation in a temperate forest in south-west Germany. We used a combination of sap flow analysis, water stable isotope labeling and a simple linear mixing model with physiological restrictions to determine whether both species were able to switch water uptake depths during drought and reverse this switch after restoration of water supply. We showed that both species utilized water from relatively deep soil layers (>0.5 m) during the drought phase, though for beech trees these contributions were not able to support the high transpiration demands. In contrast, the transpiration of firs remained unaffected by drought probably due to a mixture of its isohydric water use strategy and taproot system. The reactions towards high intensity irrigation differed among species but also between beeches of the pure and mixed stands. While beech trees in the pure stand, once water uptake from the topsoil was re-established, used ~70% of water from more shallow soil depths and were able to restore transpiration, beech trees in the mixed stand obtained only 50% and transpiration decreased further. Firs used around 70% of the water in shallow soil layers and transpiration rates again remained unaffected. These differences indicate competition between beech trees and firs in the mixed stand upon drought. We conclude that drought is able to disrupt water relations in mixed temperate forested ecosystems in such a way that facilitation can turn into competition for resources like water.