Stem rot significantly reduces the basic density of wood and increases its nitrogen content in living deciduous trees, while the carbon content appears irresponsive; The effect of the distance from the pith on the basic density and nitrogen content of wood varies, depending on presence of discoloration or decomposition in the wood.
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While numerous studies have focused on analyzing various aspects of the carbon (C) budget in forests, there appears to be a lack of comprehensive assessments specifically addressing the impact of stem rot on the C budget of broadleaf tree species, especially in old-growth forests where stem rot is prevalent. One of the main challenges in accurately quantifying C losses caused by stem rot is the lack of precise data on the basic density and C content of decayed wood, which are crucial for converting decayed wood volume into biomass and C stocks. Using linear mixed-effects models, we examine the variability of wood basic density, C content, and nitrogen (N) content. Discolored and decomposed wood was collected from the stems of 136 living deciduous trees common in hemiboreal forests in Latvia. Our research indicates a noticeable reduction in the wood basic density, coupled with an increase in the N content within the stem wood throughout the decomposition process in birch (Betula spp.), European aspen (Populus tremula L.), grey alder (Alnus incana (L.) Moench), and common alder (Alnus glutinosa (L.) Gaertn.). While aspen wood showed a decreasing trend in C content as decay progressed, a pairwise comparison test revealed no significant differences in C content between discolored and decomposed wood for the studied species, unlike the findings for basic density and N content. This study emphasizes the need to account for stem rot in old-growth forest carbon budgets, especially in broadleaf species, and calls for more research on stem rot-induced carbon losses.