Modeling dynamics of stand density by linking ingrowth and mortality with stand structural and environmental factors

Stand density, the number of trees per unit area, is a critical metric affecting resource competition between trees, tree biodiversity, and forest succession. Stand density is the balance between ingrowth and mortality that interact with stand structural and environmental factors. However, understanding of the dynamics of stand density remains uncertain because research on ingrowth and mortality was done apart from one another and thereby overlooked the replacement of existing trees with new trees. This study investigated the impacts of forest type (three forest types dominated by Pinus sylvestris, Picea abies, and Betula spp.), mean diameter at breast height (DBH), and stand density as well as mean annual air temperature and aridity on the number of ingrowth and mortality by using a generalized linear mixed model. The dataset included repeated measurement data in 5,358 permanent plots through the fourth, fifth, and sixth Swedish forest inventory. The changes in stand density and the number of tree replacement over time were smallest in P. sylvestris forests because of a smaller number of ingrowth and mortality than those of the other forest types. The main drivers of the change in stand density over time were mean DBH and stand density. The number of ingrowth and mortality decreased with the increase in mean DBH, and this decreasing trend was accelerated when stand density is high. Warmer stands had a greater number of ingrowth and a lower number of mortality, while drier stands showed the opposite trend. This indicates the decrease in stand density over time would be large in regions with low temperature and high aridity. These findings would broaden our knowledge of the dynamics of stand density and hence contribute to vegetation dynamics modeling and contribute sustainable forest management under changing climates.

*Acknowledgement: This work was supported by Korea Environment Industry & Technology Institute (KEITI) through "Climate Change R&D Project for New Climate Regime (RE202201934)", funded by Korea Ministry of Environment (MOE).