Abstract
Spruce-fir-beech mixed forests cover a large area in European mountain regions, with high ecological and socio-economic importance. As elevation-zone systems they are highly affected by climate change, which is modifying species growth patterns and productivity shifts among species. The extent to which associated tree species can access resources and grow asynchronously may affect their resistance and persistence under climate change. Intra-specific synchrony in annual tree growth is a good indicator of species specific dependence on environmental conditions variability. However, little attention has been paid to explore the role of the inter-specific growth asynchrony in the adaptation of mixed forests to climate change. Here we used a database of 1790 tree-ring series collected from 28 experimental plots in spruce-fir-beech mixed forests across Europe to explore how spatio-temporal patterns of the intra- and inter-specific growth synchrony relate to climate variation during the past century. We further examined whether synchrony in growth response to inter-annual environmental fluctuations depended on site conditions. We found that the inter-specific growth synchrony was always lower than the intra-specific synchrony, for both high (inter-annual fluctuations) and low frequency (mid- to long-term) growth variation, suggesting between species niche complementarity at both temporal levels. Intra- and inter-specific synchronies in inter-annual growth fluctuations significantly changed along elevation, being greater at higher elevations. Moreover, the climate warming likely induced temporal changes in synchrony, but the effect varied along the elevation gradient. The synchrony strongly intensified at lower elevations likely due to climate warming and drying conditions. Our results suggest that intra- and inter-specific growth synchrony can be used as an indicator of temporal niche complementarity among species. We conclude that spruce-fir-beech mixtures should be preferred against mono-specific forests to buffer climate change impacts in mountain regions.
Highlights
Mountain mixed forests comprised by Norway spruce (Picea abies L.), silver fir (Abies alba Mill.), and European beech (Fagus sylvatica L.) have a substantial share in terms of area in European mountain regions
Through our novel method combining the analysis of intra- and inter-specific growth synchrony at high and low frequency variation we corroborated three of our four hypotheses: H1- we found that intra-specific synchrony was always greater than inter-specific synchrony, suggesting temporal niche complementarity between the species; H3- we identified species-specific and elevation-dependent patterns in both intra- and inter-specific synchronies; and H4-we detected that climate warming is triggering species-specific and elevation-dependent changes in synchrony in tree growth
Our results provide evidence of temporal niche complementarity between all three species, previous studies on competitive interactions in tree growth in spruce-fir-beech mountain forests report mutual positive interactions only for spruce and fir, whereas beech always benefits from growing in this mixture while exerting a stronger competitive effect on spruce and fir (Mina et al, 2018a, 2018b)
Summary
Mountain mixed forests comprised by Norway spruce (Picea abies L.), silver fir (Abies alba Mill.), and European beech (Fagus sylvatica L.) have a substantial share in terms of area in European mountain regions They cover a broad range of site conditions, altitude, and climates (Hilmers et al, 2019). Asynchronous growth rhythms in mixed stands may indicate spatially or temporally complementary resource acquisition and use efficiency (Forrester and Bauhus 2016). This asynchrony can increase stand growth, stand density and stability of stand growth compared with monocultures (Allan et al, 2011; del Río et al, 2017)
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