Forest demography depends on stand structure, functional traits, and climate in the eastern region of the Qinghai-Tibet Plateau

Forest demographic dynamics are a key process in forest restoration, but how multiple variables may affect forest demography remains unclear. With data from 1,399 permanent forest plots containing 125 species and 103,773 individual trees in the Eastern Qinghai-Tibet Plateau, we examined the patterns of forest demography from 1979 to 2017 as affected by climate, functional traits and forest structure. We found that forest stand age, tree density and wood density (WD) all had strong negative effects on demographic rates. Additionally, climate factors and leaf functional traits indirectly affected growth rate (GR) and recruitment rate (RR) via tree density and specific leaf area (SLA). Tree turnover (recruitment and death) was faster in places with higher temperatures, with species in these places tending to be replaced by the species with higher SLA. Moreover, relationships between demographic rates and functional traits were modulated by site climate and forest structure. GR-SLA relationships varied with tree density, becoming more negative at higher density sites. There was no effect of WD on RR at colder sites, but the relationships were increasingly negative at warmer sites and at higher tree density. This suggests that forest quality could be improved via adjusting the composition of forest species with different functional traits, or via adjustments to tree density, or stand age. We propose that forest improvement projects can be optimised by 1) thinning the species with high SLA in sites with high tree density, increasing the RR of species with high WD in the Eastern Qinghai-Tibet Plateau; and 2) prioritizing species with high SLA in warm sites, and species with higher WD for growth and RR in cool sites.