Tree species traits are the predominant control on the decomposition rate of tree log bark in a mesic old-growth boreal forest

Decomposition of coarse woody debris (CWD) bark is characterized by complex and poorly understood dynamics with unclear implications for carbon and nutrient cycling and biodiversity. We examined changes in cover and physical parameters through decomposition of bark attached to logs of the main tree species in an old-growth middle boreal forest. In a 66 yrs long chronosequence after tree death and fall, we analyzed changes in the following parameters of log bark: cover, moisture, area-specific mass, total mass, dry bulk density, thickness and proportion of phloem. The percent of bark left on the sampled stems decreased with time since tree death and averaged 38%, 61% and 86% for Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and silver and downy birch (Betula pubescens and Betula pendula) = aspen (Populus tremula), respectively. Bark moisture increased along with succession of epixylic vegetation on logs that progressed similarly for all studied tree species. On average, no vegetation was recorded on logs 3 yrs after tree death. In 9 yrs, logs were characterized by the first stage of sparse vegetation cover. The closed groups of the second stage, with cover of not less than 70%, consisted mainly of non-epigeous species, and developed an average of 19 yrs after tree death. The third stage was dominated by ground cryptogam species without a significant contribution of vascular plants, and the fourth stage, when the wood was completely overgrown by the establishment and spread of vascular plants, was observed and average of 30 yrs after tree death. The exponential rate of total mass loss of bark increased at rates of 0.068, 0.110, 0.197 and 0.312 yr−1 for birch, aspen, spruce, and pine, respectively. The highest rate of bulk density loss was recorded for aspen (0.024 yr−1) and did not differ for birch, pine and spruce (0.009 yr−1). The decomposition rate was expressed as a rate of bark mass loss divided by initial volume (integrating losses due to bark mineralization, peeling, consumption by insects and sloughing from logs). It averaged 0.147 yr−1 for birch, aspen and spruce and 0.291 yr−1 for pine, independent of stem section, log diameter and decay class. In old-growth forests, where CWD volumes may reach hundreds of cubic meters, the accurate portrayal of bark decomposition patterns is crucial for estimating the role of CWD in carbon and nutrient cycles and the diversity of CWD-dependent organisms with different habitat requirements.