Abstract
BackgroundSpecies-specific genotypic features, local neighbourhood interactions and resource supply strongly influence the tree stature and growth rate. In mixed-species forests, diversity-mediated biomass allocation has been suggested to be a fundamental mechanism underlying the positive biodiversity-productivity relationships. Empirical evidence, however, is rare about the impact of local neighbourhood diversity on tree characteristics analysed at a very high level of detail. To address this issue we analysed these effects on the individual-tree crown architecture and tree productivity in a mature mixed forest in northern Germany.MethodsOur analysis considers multiple target tree species across a local neighbourhood species richness gradient ranging from 1 to 4. We applied terrestrial laser scanning to quantify a large number of individual mature trees (N = 920) at very high accuracy. We evaluated two different neighbour inclusion approaches by analysing both a fixed radius selection procedure and a selection based on overlapping crowns.Results and conclusionsWe show that local neighbourhood species diversity significantly increases crown dimension and wood volume of target trees. Moreover, we found a size-dependency of diversity effects on tree productivity (basal area and wood volume increment) with positive effects for large-sized trees (diameter at breast height (DBH) > 40 cm) and negative effects for small-sized (DBH < 40 cm) trees. In our analysis, the neighbour inclusion approach has a significant impact on the outcome. For scientific studies and the validation of growth models we recommend a neighbour selection by overlapping crowns, because this seems to be the relevant scale at which local neighbourhood interactions occur. Because local neighbourhood diversity promotes individual-tree productivity in mature European mixed-species forests, we conclude that a small-scale species mixture should be considered in management plans.
Highlights
Species-specific genotypic features, local neighbourhood interactions and resource supply strongly influence the tree stature and growth rate
Neighbourhood definition effect The two neighbour selection approaches yielded highly variable results regarding the number of neighbours (NN) and Neighbour species richness (NSR): the median ΔNN was 3, spanning a range from min = 0 to max = 18 and the median ΔNSR was 0, with a range of min = 0 and max = 2
Neighbourhood diversity effects on crown architecture and wood volume Besides the strong positive effect of the basal area (BA), NSR and Exponential Shannon-Index (eHS) significantly positively influenced the tree crown dimensions (CV, crown projection area (CPA), crown surface area (CSA)), branch length sum and the wood volumes based on the neighbour selection approach “crown overlap” (Table 3 and Fig. 3)
Summary
Species-specific genotypic features, local neighbourhood interactions and resource supply strongly influence the tree stature and growth rate. Empirical evidence is rare about the impact of local neighbourhood diversity on tree characteristics analysed at a very high level of detail To address this issue we analysed these effects on the individual-tree crown architecture and tree productivity in a mature mixed forest in northern Germany. Photomorphogenetic responses of individual trees result in considerable crown plasticity, i.e. the environmentally driven intraspecific variability in crown architecture (Schröter et al 2012; Pretzsch 2014) These light-related tree interactions for physical niche partitioning in canopy space have been found to be severely impacted by species mixing and are thought to be an important process in explaining the positive biodiversity – productivity relationships (BPRs) in forests (Morin et al 2011; Huang et al 2018; Kunz et al 2019)
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