Response of forest vegetation to varying levels and patterns of green-tree retention: An overview of a long-term experiment

Timber harvest with retention of live ('green') trees, snags, and logs is now a standard practice on federal 'matrix' lands within the range of the northern spotted owl. Although specific guidelines have been adopted for the levels and spatial configurations of retained structures, neither the ecological assumptions that underlie these recommendations nor the outcomes of these practices have been rigorously tested. The Demonstration of Ecosystem Management Options (DEMO) study examines the responses of forest ecosystems in the Pacific Northwest to varying levels (percentage of basal area) and patterns (dispersed versus aggregated) of green-tree retention. In this paper we describe vegetation studies that form the foundation of this long-term, interdisciplinary experiment. We review the results of retrospective analyses and simulation models which suggest that even minimal levels of retention may have important effects on forest development. We describe the characteristics of the DEMO sites, the experimental design, and the principal variables of interest (stand structure, tree regeneration and growth, and understory composition and diversity). We speculate about the silvicultural and ecological responses of forests to varying levels and patterns of retention and focus in particular on the dynamics of the forest understory. We anticipate strong contrasts among treatments in the establishment of early-seral, open-site species, and in the persistence of shade-tolerant plants associated with older forests or forest-interior environments. Short-term responses are likely to be driven by variation in the distribution and intensity of harvest disturbance. Longer-term trends are expected to reflect the effects of contrasting patterns of canopy retention. We conclude by discussing some of the scientific challenges faced in designing and implementing large-scale, interdisciplinary experiments.