Decay of Scots pine coarse woody debris in boreal peatland forests: Mass loss and nutrient dynamics

Although boreal forests have been the setting for numerous studies on the decomposition of coarse woody debris (CWD), these have been limited to mineral soils. In boreal peatland forests, CWD decay rates and nutrient dynamics have not been previously quantified. Therefore, we investigated the decomposition of Scots pine (Pinus sylvestris L.) logs incubated for 10-15 years in three climatically and nutritionally different, managed peatland forests in the European boreal zone. This information is needed for, e.g., estimations of the ecosystem carbon balance. Further, our motivation for studying nutrient dynamics was to evaluate if deliberate retention of large pieces of CWD can make a difference in the nutrient management of the post-harvest stand. We focused on the wood component, which, unlike pine bark, is known to decay slowly, and may thus be important as a long-term nutrient store and source. Changes in wood nutrient concentrations and contents were studied at our primary experimental site (south boreal), which included both control (i.e., forested) and clearcut treatments. Based on mass loss from logs over time, decay rates (k, year−1) decreased with increasing latitude as follows: hemiboreal (0.081), south boreal (0.068), and north boreal (0.062); however, the difference between the latter two was not significant. The hemiboreal site, which had the mildest climate and highest nutrient status, thus accounted for the fastest rate of CWD decay. Generally, smaller logs decayed faster than larger ones. At the south boreal site, the clearcut environment accelerated CWD decomposition (k, 0.085 year−1) compared to that under forest canopy. After fifteen years of decay, logs retained N, and accumulated P to more than double initial amounts at our P-limited site. The loss of carbon typically surpassed the loss of nutrients, even base cations. Nutrient trends were mostly similar with versus without forest canopy, but faster decay in the clearcut consequently expedited these trends (e.g., P accumulated faster). However, at CWD quantities of 5-100 m3 ha−1, the nutrient store in the wood component of CWD was small relative to that in the peat. Hence, CWD retention is apparently not a long-term solution for conserving nutrients in managed pine-dominated peatland forest.