Mineralization and fragmentation rates of bark attached to logs in a northern boreal forest

Predicting the characteristics of coarse woody debris (CWD) and its importance for biodiversity, carbon and nutrient cycling requires narrowing the uncertainties in bark decomposition rate assessments. We estimated bark decomposition rates and compared them to wood decomposition rates of birches (Betula pubescens and Betula pendula), Siberian fir (Abies sibirica), Siberian pine (Pinus sibirica) and Siberian spruce (Picea obovata) in the northern boreal forest of the Komi Republic, Russia. Estimates were made based on the mass loss of bark attached to above-ground fallen and leaning logs dated to have fallen from 1 to 168 years previous to sampling. A single-exponential model was used to estimate the mass loss of bark attached to the logs. Decomposition rate estimates of all log bark included mass loss due to fragmentation. Mineralization rate as mass loss per surface area was estimated for non-fragmented bark pieces. The initial bark mass of the tree base was higher compared to that of the rest of the log; it did not depend neither on the tree species nor on the log size. The proportional rate of bark mineralization was the same for Siberian fir, Siberian pine and Siberian spruce logs - 0.040 yr(-1). The mineralization rate of birch bark was 0.009 yr(-1). Bark fragmentation accelerated mass loss. Variation in bark decomposition rates was explained by tree species and log diameter and did not depend on tree mortality mode. The bark turnover time (t(95)) was 302, 224, 149, 140 and 117 years for birch, fir, spruce, Siberian pine with diameter more than 41 cm, and Siberian pine with diameter less than 40 cm, respectively. Bark decomposed faster than wood for fir (0.034 vs. 0.026 yr(-1)), spruce (0.051 vs. 0.030 yr(-1)) and Siberian pine with diameters less than 40 cm (0.054 vs. 0.008 yr(-1)) and (0.065 vs. 0.008 yr(-1)) for logs with diameters more than 41 cm. Birch bark decomposed slower than birch wood (0.025 vs. 0.032 yr(-1)). The different decomposition rates of wood and bark suggest that considering wood and bark together as one substrate can result in a less accurate portrayal of decomposition patterns. These bark decomposition rates can be used for modelling carbon dynamics in similar ecosystems. Knowing the turnover time of log bark for these tree species also facilitates the prediction of the quality of CWD in biodiversity studies in boreal forests. (C) 2013 Elsevier B.V. All rights reserved.