A STAGE-BASED MATRIX MODEL FOR DECAY-CLASS DYNAMICS OF WOODY DEBRIS

Abstract

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.