Coarse woody debris legacies and their dynamics in retained forest patches

Retaining trees in small patches at final harvest is a common forest conservation measure to maintain structural and biological diversity through the young forest phase. Long-term studies of coarse woody debris (CWD, breast height diameter ≥ 10 cm) changes in retention patches remain uncommon, especially in relation to different types of patches with varying tree species composition, ground moisture, size, shape, and exposure. In the present study we re-inventoried CWD three times (1–3, 5–7 and 18–20 years after harvesting) in 60 small tree retention patches (0.03–0.54 ha) of six commonly created retention patch types, in central boreal Sweden. Most retention patch types, despite being very small (3 ha−1 and buffer zones to rocky outcrop the highest average volume of 32 m3 ha−1, when summarised over the whole study period. Very few patch-level environmental variables (except type) related to the amount of CWD in retention patches; only the retained living tree volume of patches during creation was clearly positively correlated with higher post-harvest CWD volumes. Patch characteristics such as size and shape index did not relate to the CWD volumes in retention patches, and patch exposure only related to higher CWD volumes within the first 1–3 years after harvest. Retention patches generally supported a variety of living trees, CWD volumes and qualities over time. Several retention patch types; such as free-standing coniferous and deciduous patches, rocky outcrop buffers, wet forest patches and buffers to water, supported average CWD volumes between 19–41 m3 ha−1 after 18–20 years, under remaining canopy cover. Our findings challenge the traditional management principles aimed at minimizing severe windthrow and CWD input (i.e., creating larger patches (>0.5–1 ha), choosing topographically sheltered areas, selecting specific tree species and ground conditions). However, our study was too small in scale to investigate multiple within-patch-type interacting environmental variables. Future larger-scale studies over extended time periods are needed to disentangle such interactions for the dynamics of CWD and associated biodiversity in retention patches