Abstract
To mitigate negative impacts of drought stress in the face of climate change, mixtures of tree species such as those between European beech (Fagus sylvatica) and silver fir (Abies alba) are assumed to lower risks in forest management. This study investigates the influence of mixing beech and fir on tree growth in general and in particular on tree species responses to the extreme drought event of 2003. For this purpose, we analyzed basal area increment series and carbon isotope composition (δ13C) in the wood of ~160 trees from three mixed-species sites in Germany and one site in Croatia. Overall growth performance for both fir and beech increased with proportions of the admixed species when accounting for the interactions with tree size and competition intensity. Mixing improved growth of large trees for both species irrespective of neighborhood density, whereas smaller trees benefitted only in denser neighborhoods. Positive mixing effects on radial growth were more pronounced in fir compared to beech, yet the latter benefitted by the admixture of fir with regard to growth recovery following drought. Both the resistance of radial growth against reduction during drought as well as the variation of isotopic composition throughout the drought period were not affected by mixing, indicating that water-use in these two species was not complementary under drought stress. Although trees from both species exhibited growth reductions during the drought, fir maintained higher absolute growth levels than beech during the drought. Both species benefited from growing in mixed neighborhoods but complementary effects depended on tree size and neighborhood density. Mixing fir and beech leads to positive or neutral effects on growth performance of trees, also in response to an extreme drought event. Since increasing tree species richness also spreads the risks associated with extreme events, mixtures of beech and fir can be recommended as a possible alternative for more drought-sensitive stands such as spruce monocultures.
Highlights
Climate models predict rising average temperatures and more frequent occurrences of extreme climatic events like storms, floods, heat waves, and droughts for the twenty-first century (Pachauri et al, 2014)
The inconsistent evidence regarding drought response of mixtures may be related to the fact that it is highly dependent on both tree species identity and site conditions because both jointly determine the absence or presence of mechanisms that lead to complementary water use such as differing species-specific rooting depths or different phenology (Forrester and Bauhus, 2016)
Annual mean basal area increment (BAI) was higher in fir than beech in most years at the two higher altitude sites (Conventwald and Croatia, Figure 2) while the two species showed similar absolute growth rates at the two lower altitude sites (Hexental and Freiamt) throughout the period 2000–2016
Summary
Climate models predict rising average temperatures and more frequent occurrences of extreme climatic events like storms, floods, heat waves, and droughts for the twenty-first century (Pachauri et al, 2014). A growing number of studies have reported positive relationships between species diversity and forest productivity (Paquette and Messier, 2011; Vila et al, 2013; Jucker et al, 2014; Forrester and Bauhus, 2016) and tree diversity has been shown to enhance resistance to pest outbreaks (Bauhus et al, 2017a; Jactel et al, 2017). The relationship between diversity and drought tolerance of trees is less straightforward as for example the more commonly reported positive effect of tree species diversity on resistance to pest outbreaks of specialist herbivore insects (Bauhus et al, 2017a). The inconsistent evidence regarding drought response of mixtures may be related to the fact that it is highly dependent on both tree species identity and site conditions because both jointly determine the absence or presence of mechanisms that lead to complementary water use such as differing species-specific rooting depths or different phenology (Forrester and Bauhus, 2016)
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