Cost-effective methods for monitoring coarse woody debris in northeastern forests

Abstract

Across boreal and temperate biomes, the area of old forests is in global decline, with the consequent extinction of dependent species posing a major threat to biodiversity. As such, current sustainable forestry certification programs position management for biodiversity as a fundamental goal. Yet, to do so necessitates both the use of effective indicators, of which downed coarse woody debris (CWD) is well-established, and the establishment of reference levels, which are most often based on comparable old growth systems. However, the extreme spatial variability of CWD makes inventorying and monitoring this structural attribute problematic. Trade-offs exist between costs, sampling methods, sample area, and the statistical ability to detect change. Faced with vast uncertainty regarding the effectiveness of monitoring approaches, large-scale inventories of CWD are largely neglected in the Northeast. The objectives of this project were two-fold: 1) to develop cost-effective methods for monitoring coarse woody debris volume at a scale appropriate to northeastern forest management and 2) to discern the potential impacts of forest management on CWD attributes. A systematic sampling approach was used to inventory CWD in a managed and an old growth forest in Northern Maine. Two promising methods for measuring CWD—line intersect sampling and perpendicular distance sampling—were compared in the managed landscape, using different sampling areas for each approach. Perpendicular distance sampling exhibited high sampling costs and poor statistical efficiency relative to line intersect sampling. As such, it cannot be recommended for large-scale forest inventories. Only line intersect sampling was used in the old growth forest. Doing so enabled comparison of the statistical efficiencies of varying transect length and CWD attributes between landscapes. Power analyses were conducted to determine the tradeoffs between statistical precision and sample effort in using a particular transect length for inventorying CWD in managed and unmanaged forests. Variance was reduced with increasing transect length. However, choosing the appropriate transect length for a large-scale inventory depends on the level of precision required and the sensitivity to change in CWD volume for a given landscape. Land managers can consult these graphs to determine the appropriate minimum sample size required on average to detect a specific change in CWD volume at an accepted power and alpha level. Further, the old growth forest had more than twice the mean CWD volume than in the managed landscape, and landscapes differed in how this volume was distributed across size and decay classes, suggesting insight into the impacts of forest management on CWD.