Abstract
Drought-sensitive European beech forests are increasingly challenged by climate change. Admixing other, preferably more deep-rooting, tree species has been proposed to increase the resilience of beech forests to drought. This diversification of beech forests might also affect soil organic carbon (SOC) and total nitrogen (TN) stocks that are relevant for a wide range of soil functions and ecosystem services, such as water and nutrient retention, filter functions and erosion control. Since information of these effects is scattered, our aim was to synthesize results from studies that compared SOC/TN stocks of beech monocultures with those of beech stands mixed with other tree species as well as monocultures of other tree species. We conducted a meta-analysis including 38 studies with 203, 220, and 160 observations for forest floor (i.e., the organic surface layer), mineral soil (0.5 m depth) and the total soil profile, respectively. Monoculture conifer stands had higher SOC stocks compared to monoculture beech in general, especially in the forest floor (up to 200% in larch forests). In contrast, other broadleaved tree species (oak, ash, lime, maple, hornbeam) showed lower SOC stocks in the forest floor compared to beech, with little impact on total SOC stocks. Comparing mixed beech-conifer stands (average mixing ratio with regard to number of trees 50:50) with beech monocultures revealed significantly higher total SOC stocks of around 9% and a smaller increase in TN stocks of around 4%. This equaled a SOC accrual of 0.1 Mg ha−1 yr−1. In contrast, mixed beech-broadleaved stands did not show significant differences in total SOC stocks. Conifer admixture effects on beech forest SOC were of additive nature. Admixing other tree species to beech monoculture stands was most effective to increase SOC stocks on low carbon soils with a sandy texture and nitrogen limitation (i.e., a high C/N ratio and low nitrogen deposition). We conclude that, with targeted admixture measures of coniferous species, an increase in SOC stocks in beech forests can be achieved as part of the necessary adaptation of beech forests to climate change.
Highlights
European beech (Fagus sylvatica)–the dominant species of the potential natural forest vegetation and the main broad-leaved forest tree species in Central Europe (Ellenberg, 1988; Bolte et al, 2007) is increasingly affected by climate change due to longer, more intense and more frequent summer droughts
The articles had to match the following criteria to be included in the meta-analysis: (1) The experiments used paired stands/designs of European beech and at least one other tree species/tree species mixture with beech; (2) organic C stocks in the forest floor and/or mineral soil were reported or could be calculated; and (3) sampling designs included replicated measurements (n ≥ 3)
We found generally more observations comparing beech monoculture with other monocultures than comparing beech monoculture with mixed-beech stands (Supplement 4)
Summary
European beech (Fagus sylvatica)–the dominant species of the potential natural forest vegetation and the main broad-leaved forest tree species in Central Europe (Ellenberg, 1988; Bolte et al, 2007) is increasingly affected by climate change due to longer, more intense and more frequent summer droughts. This has already led to physiological constrains (Rennenberg et al, 2006) as well as nutrient limitations including impaired ecosystem nitrogen cycling (Geβler et al, 2004; Jonard et al, 2015; Dannenmann et al, 2016). According to the United Nations framework convention on climate change of 1992 and the Kyoto protocol of 1997, C sequestration in terrestrial sinks by silvicultural management, including a change in tree species, has been accepted to offset carbon dioxide emissions for national carbon budgets
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