Nicholas Kruys

A stage-based matrix model for decay-class dynamics of woody debris

Dead trees contribute to the structural diversity in forested ecosystems, providing habitat, shelter, and substrate for a wide range of organisms. Forecasting the dynamics of dead trees is a key to incorporating woody debris in managed forests to conserve those species dependent on dead trees. We present a new model for predicting woody-debris dynamics over time using a stage-based matrix, with transition rates between decay classes. It is constructed for use with the type of classification systems commonly used for assessments of dead trees as a plant and animal habitat. Dendrochronological measurements of time since death are the basis for transition rates between decay classes. Calculating mean residence time in decay classes from a single time point sample, rather than using longitudinal long-term data, tends to overestimate residence time due to a higher probability of inclusion of slow-decaying trees. A new method for correcting this bias is presented, and incorporated into the model. The stage-based decay-class model approach is suggested as a general model, applicable to many tree species and forest regions. We illustrate how such a model can be used for management planning for woody debris by forecasting decay-class distributions of woody debris over time at five-year intervals. In combination with functions for growth and mortality, different ways to reach woody-debris goals can be explored. The model is parameterized for Picea abies (L.) Karst. (Norway spruce) in mid-northern Sweden. Because there is a lack of data from long-term studies in similar conditions, model validation is difficult. However, comparisons with available independent data support the conceptual basis for the model. Future studies should concentrate on the relative importance of factors that influence residence time in decay classes, e.g., tree size, site productivity, and decomposer communities.

LOCAL DISPERSAL SOURCES STRONGLY AFFECT COLONIZATION PATTERNS OF WOOD-DECAYING FUNGI ON SPRUCE LOGS

Coarse woody debris (CWD) is an important habitat for many species in forest ecosystems. However, forestry has decreased the abundance of CWD so that many wood-dependent species have become threatened. To alleviate this problem, guidelines for a more biodiversity-oriented forestry focus on increasing CWD in managed forests. Un- fortunately, how this increase is to be allocated on a landscape scale is not well understood. The present study reports an experiment in which freshly cut logs of varying sizes were placed in stands with contrasting abundance of natural CWD and subsequently varying pools of wood-inhabiting species. The first six years of colonization by wood fungi show that local abundance and composition of the fungal flora strongly influenced colonization. Higher species richness was observed in CWD-rich sites, and several species were more frequent on the experimental logs at CWD-rich sites. The strong within-site effect is in- terpreted as resulting from high spore deposition from the local species pool. This is supported by spore deposition estimates of Fomitopsis rosea, a red-listed species that only occurred on experimental logs at the CWD-rich sites. F. rosea had a 9-180 times higher spore deposition at the CWD-rich sites compared to the CWD-poor sites. The species richness and composition on small logs differed from that of large logs with higher richness on the latter. The results strongly suggest that restoration efforts would be more efficient if directed toward sites close to CWD-rich sites and that preferably large logs should be created.