Effects of local neighbourhood structure on radial growth of Picea crassifolia Kom. and Betula platyphylla Suk. plantations in the loess alpine region, China

Abstract The species diversity-forest productivity relationship has been extensively studied, but few studies have systematically examined the relationship between forest structure and forest productivity. As one of the most important ecosystem establishment areas in China, the loess alpine region is in need of research to guide vegetation restoration and afforestation. This study aimed to elucidate the relationship between forest structure and tree growth at the local neighbourhood level and to guide the establishment and management of plantations in the loess alpine region. We investigated 1243 living trees in Qinghai Province and analysed the effects of initial size, species diversity, competition, spatial distribution and size heterogeneity on annual growth in basal area (BA) of Picea crassifolia Kom. and Betula platyphylla Suk. using mixed-effect models. The responses of individual growth to local forest structure varied between species and were strongly related to initial size which was expressed by the DBH of focal tree in 2015. The growth of both target species decreased when the neighbourhood competition became more intense. BA growth of P. crassifolia increased with the increase in local species diversity under the same competition intensity, while that of B. platyphylla did not. The uniform spatial distribution in local communities had a negative association with the radial growth of B. platyphylla, but had no significant effect on that of P. crassifolia. Neighbourhood size heterogeneity was positively related to the growth of both target species; P. crassifolia individuals were sensitive to DBH heterogeneity, while B. platyphylla individuals were sensitive to height heterogeneity. This study indicates that complex and heterogeneous forest structures are more conducive to improving forest productivity and highlights the need to fully consider species’ functional traits during afforestation and forest management.