Size-structure dynamics in mixed versus monospecific stands

Abstract

The community of mixed tree species is conceptualised in this chapter by the dynamics of its size distribution. We use experiments of mixed and neighbouring monocultures to show how mixture changes the size-structure dynamics and thereby the productivity of mixed versus monospecific stands. As the main cause of this modification, we identify the predominantly more size-asymmetric partitioning of the growth in favour of dominant trees in mixed stands. Furthermore, mixing can modify the growth allocation at the tree level in favour of the crown size. These differences in the inter- and intra-individual growth distribution may just slightly favour the size of tall trees at the expense of small ones and may slightly increase the canopy density in mixed compared to monospecific stands in a single growth period. However, via the feedback functioning!structure!environmental conditions!functioning, small initial and continuous advantages may result in a significantly different canopy structure, denser stocking, and higher productivity of mixed versus monospecific stands, especially in the advanced and mature phase. We characterise the size and growth distribution pattern emerging in mixed stands using various measures, indices, and relationships. The denser canopy space filling by trees with complementary light ecology may contribute to the general pattern of overyielding of mixed versus monospecific stands. The community of mixed tree species is conceptualised in this chapter by the dynamics of its size distribution. Analysing the evolution of tree size distributions will provide essential insight into mixed-species population dynamics and provide a link between the stand level addressed in Chap. 4 and the tree level presented in Chap. 6. Mixing effects at the stand level are relevant for decision making and forest planning and indicate that advantageous species interactions are worth further exploration but are insufficient for revealing the underlying causes. Analyses at the tree level may reveal basic growth response patterns for better understanding competition and facilitation. They may also reveal how competition and facilitation result from a modification in the supply, capture, and use efficiency of resources. But the findings at individual tree level are barely sufficient for scaling up to stand-level behaviour because of system properties emerging through interindividual interactions. The size distribution dynamics provide a link between the stand and tree level and a transition between both levels of organisation.