Abstract
Tree species mixing has been widely promoted as a promising silvicultural tool for reducing drought stress. However, so far only a limited number of species combinations have been studied in detail, revealing inconsistent results. In this study, we analysed the effect of mixing Scots pine and oak (pedunculate oak and sessile oak) trees on their drought response along a comprehensive ecological gradient across Europe. The objective was to improve our knowledge of general drought response patterns of two fundamental European tree species in mixed versus monospecific stands. We focused on three null hypotheses: (HI) tree drought response does not differ between Scots pine and oak, (HII) tree drought response of Scots pine and oak is not affected by stand composition (mixture versus monoculture) and (HIII) tree drought response of Scots pine and oak in mixtures and monocultures is not modified by tree size or site conditions. To test the hypotheses, we analysed increment cores of Scots pine and oak, sampled in mixed and monospecific stands, covering a wide range of site conditions. We investigated resistance (the ability to maintain growth levels during drought), recovery (the ability to restore a level of growth after drought) and resilience (the capacity to recover to pre-drought growth levels), involving site-specific drought events that occurred between 1976 and 2015. In monocultures, oak showed a higher resistance and resilience than Scots pine, while recovery was lower. Scots pine in mixed stands exhibited a higher resistance, but also a lower recovery compared with Scots pine in monocultures. Mixing increased the resistance and resilience of oak. Ecological factors such as tree size, site water supply and site fertility were found to have significant effects on the drought response. In the case of Scots pine, resistance was increased by tree size, while recovery was lowered. Resistance of oak increased with site water supply. The observed mixing effect on the tree drought response of Scots pine and oak was in some cases modified by the site conditions studied. Positive mixing effects in terms of resistance and resilience of oak increased with site water supply, while the opposite was found regarding recovery. In contrast, site fertility lessened the positive mixing effect on the resistance of Scots pine. We hypothesise that the observed positive mixing effects under drought mainly result from water- and/or light-related species interactions that improve resource availability and uptake according to temporal and spatial variations in environmental conditions.
Highlights
Climate change is characterised by increased temperatures and modified precipitation patterns (Schär et al, 2004; Della-Marta et al, 2007; Jacob et al, 2014)
We focused on three null hypotheses: (HI ) tree drought response does not differ between Scots pine and oak, (HII ) tree drought response of Scots pine and oak is not affected by stand composition and (HIII ) tree drought response of Scots pine and oak in mixtures and monocultures is not modified by tree size or site conditions
We investigated resistance, recovery and resilience, involving sitespecific drought events that occurred between 1976 and 2015
Summary
Climate change is characterised by increased temperatures and modified precipitation patterns (Schär et al, 2004; Della-Marta et al, 2007; Jacob et al, 2014). Drought occurs under low levels of available water, caused by reduced precipitation and/or increased atmospheric evaporative demand combined with low available soil water (Wilhite, 1993). Trees may reduce stomatal conductance and photosynthesis, and experience carbon starvation (Irvine et al, 1998; McDowell et al, 2008), modified tree allometry (Pretzsch et al, 2012b), enhanced fine root mortality (Deans, 1979; Gaul et al, 2008) or increased defoliation (Carnicer et al, 2011; Poyatos et al, 2013). The large extent of drought and heat-induced mortality currently observed, pays witness to a global vulnerability of trees and forests to climate change (Allen et al, 2010)
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