Mixed-Species Effects on Soil C and N Stocks, C/N Ratio and pH Using a Transboundary Approach in Adjacent Common Garden Douglas-Fir and Beech Stands

Seid Dawud,Karsten Raulund-Rasmussen,Lars Vesterdal

doi:10.3390/f8040095

Abstract

Mixed forest of Douglas-fir and beech has been suggested as one of the possible future forest types in Northwest Europe but the effects of this mixed forest on soil properties relative to monoculture stands are unknown. In a transboundary investigation of adjacent common garden Douglas-fir and beech stands, we determined the effects on topsoil properties. However, responses of C and N stocks, the C/N ratio and pH were site- and soil layer-specific and were mainly single-sided and without synergistic effects. Beech reduced the soil C and N stocks in Douglas-fir at the nutrient-poor site, caused an increase in the C/N ratio in the forest floor and mineral soil at both nutrient-poor and -rich sites, and reduced the acidifying effect of Douglas-fir at the nutrient-poor site. These results do not support the hypothesis that mixture effects would be consistent across sites and soil layers. The lack of synergistic effects may be attributed to the relatively similar litter quality or rooting depth that prevented any larger niche differentiation and complementarity. The results indicate that the transboundary approach within a mature common garden proved useful as a platform to test tree species interactions, and this approach could be explored in soil studies until dedicated mixed-species common gardens reach maturity.

Highlights

It has been suggested that mixtures of tree species rather than monocultures would better support the long-term nutritional sustainability of forests and provision of ecosystem services including productivity [1,2,3] and adaptation to climate change [4,5]
We investigated forest soil C and N stocks, the C/N ratio and pH across the boundary of neighboring stands of Douglas-fir and European beech with the objective to test whether the boundary will exhibit synergistic or additive effects on soil properties
We found no site-specific effects of the two tree species, in line with previous studies of beech and Douglas-fir forest floor C stocks across seven common garden experimental sites including CHR and LØV [18]

Summary

Introduction

It has been suggested that mixtures of tree species rather than monocultures would better support the long-term nutritional sustainability of forests and provision of ecosystem services including productivity [1,2,3] and adaptation to climate change [4,5]. One of the introduced species in this region is Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) that is native to Northwest America and Canada [6,7]. Douglas-fir has been viewed as a promising species to European forestry owing to its fast growth, valuable timber and most importantly because of its adaptability to the ecological and climatic settings of the region [8]. It grows well together with other conifers and broadleaved species [3]. These characteristics have made Douglas-fir a potential tree species for mixtures with native species such as European beech (Fagus sylvatica L.) [9,10]

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Results

Discussion

Conclusion