Abstract: Stand density is a critical metric in forest ecology and management and it changes over time as trees grow and compete with each other for limited resources. However, it should be noted that the change in stand density is the balance between the number of evolutions of new trees (i.e., ingrowth) and the number of deaths of existing trees (i.e., mortality). In this study, we investigated (1) the dependence between ingrowth and mortality, (2) the impacts of stand structural (mean diameter at breast height (DBH) and stand density) and climatic factors (mean annual temperature and annual precipitation) on ingrowth and mortality, and (3) the change in stand density over time by integrating ingrowth and mortality. We studied Pinus sylvestris and Picea abies forests across the whole Swedish forests. The dataset included the repeated measurement data in the Swedish National Forest Inventory plots without histories of tree cutting. Zero-inflated negative binomial bivariate Copula model, a statistical method analyzing the dependence between two count variables showing excess zeros was used. There was a negative dependence between ingrowth and mortality, and the dependence parameters were ˗0.402 and ˗0.382 for the P. sylvestris and P. abies forests, respectively. Ingrowth was mainly determined by mean DBH, showing greater ingrowth in a stand with a small mean DBH. Mortality was mainly determined by stand density, showing greater mortality in a stand with a high stand density. These trends in ingrowth and mortality led the greater ingrowth than mortality, i.e., increases in stand density over time, in a stand with a small mean DBH and greater mortality than ingrowth, i.e., decreases in stand density over time, in a stand with a high stand density. This resulted in and mimicked the early increasing and later decreasing trends in stand density, which is usually observed as long-term trends in the natural forests. A warmer stand had a greater ingrowth for both P. sylvestris and P. abies forests. P. sylvestris forests had greater mortality in a warmer stand, while P. abies forests had greater mortality in a colder stand. However, the impacts of annual precipitation on ingrowth and mortality were marginal. These findings would broaden our knowledge of the changes in stand density over time and hence contribute to sustainable forest management under changing climate. * This study was supported by the Korea Forestry Promotion Institute (2021363A00-2223-BD01), funded by the Korea Forest Service.
