Higher competition intensified tree mortality in natural Larix gmelinii and Betula platyphylla forests in Northeast China

Abstract

Tree mortality is a crucial ecological process that is strongly linked to dynamic changes in vegetation and ecosystem function. Given that global climate change increases the risk of mortality, understanding the specific causes that lead to tree mortality at the local level is crucial. However, the contributions of factors explaining tree mortality patterns of Dahurian larch (Larix gmelinii) and White birch (Betula platyphylla) need exploration. Based on comprehensive tree census data (2010–2015) from a fully mapped 0.06-hectare permanent forest dynamics plot in the Da Xing’an Mountains, Northeast China, encompassing 32,565 individual trees, we classified forest stands into three distinct forest types according to species composition: mixed birch and larch forest, pure larch forest, and pure birch forest. A generalized linear mixed model was used to analyze the effects of the factors influencing the mortality of birch and larch in mixed and pure forests. Our research indicates that (1) competition is typically the primary cause of tree mortality. For the two main tree species, birch mortality shows greater sensitivity to increased conspecific competition, whereas larch mortality is more affected by higher hetero-specific competition. (2) For different stand types, species diversity had a more negative effect on the mortality of the two tree species in the mixed forest than in the pure forest. (3) Larch and birch mortality with varying tree diameter classes are found in different stands, and the significance of the factors influencing tree mortality patterns varies significantly. Our research demonstrated significant differences in the relative importance of variables driving tree mortality between pure and mixed forests and emphasized the role of both conspecific and hetero-specific competition in tree mortality. These results offer crucial insights for future studies on forest management in this area and improve our comprehension of the factors leading to individual tree mortality in natural secondary forests in cold temperate zones.