Mortality and growth patterns of habitat trees in montane forests

Habitat trees, large, old trees that support microhabitats, are retained as life-boating structures to supply resources for certain forest-dwelling species in forests managed for wood production. Once selected, living habitat trees are expected to fulfil that objective in the long term. However, especially large trees suffer from the impacts of climate change such as more frequent and intense drought events. In addition, the presence of certain tree-related microhabitats (TreMs) that are derived from wounds and decay can also affect tree vitality and contribute to mortality risks. Thus, it is crucial to understand the interactions between TreM occurrence and extreme droughts on vitality of habitat trees. For this purpose, this study investigated mortality rates and temporal growth patterns of habitat trees in continuous cover forests, using remote sensing images and tree-ring data. Our findings revealed relatively low mortality rates among habitat trees compared to other trees in relation to the extreme drought event of 2018. Yet, coniferous habitat trees were most susceptible. Habitat trees that died exhibited a long-lasting growth decline and lower tree ring growth and variance prior to their death when compared to surviving trees. Our pioneering study on relationships between TreM richness, TreM abundance and mortality of habitat trees, found no significant relationship between past growth patterns, mortality rates and TreMs. Trees with TreMs were not necessarily more susceptible to death; rather, their early growth patterns played more significant roles in determining their risk of mortality. Early warning signals were identified as potential predictors of tree mortality and could be further incorporated in forest growth models and decision-making tools. • Habitat trees in managed forests had low mortality rates after the 2018 drought. • P. abies and A. alba proved more vulnerable to drought than broadleaved trees. • Tree-related microhabitats’ occurrence did not enhance tree mortality risk. • Declining and variable ring width can signal tree mortality risk.