Effects of timber harvest and forest edges on abundance, viability, and physiology of understory plants in Pseudotsuga forests of western Washington

Thesis (Ph. D.)--University of Washington, 2004There is widespread interest in the ecological effects of timber harvest, the resultant fragmentation of forest habitat, and the increased area of forest edges. However, there has been surprisingly little research devoted to the consequences of these management activities for understory plants. I investigated short-term responses of understory plants to timber harvest and creation of edges in aggregated retention harvest units at two sites in the western Cascade Range of Washington. Pre- and post-treatment abundance of vascular plants and ground-layer bryophytes was measured in four, 1-ha aggregates (patches of intact forest) and in surrounding harvest areas along 16 transects placed perpendicular to the edges of these aggregates. For three late-seral herbs, Asarum caudatum, Clintonia uniflora, and Pyrola picta, demographic data (ramet survival, clonal growth, flowering rate, and seedling density) were collected before and for 2 yr after treatment within nine plots in the harvest area and nine in adjacent undisturbed forest. For these same species, morphological and physiological acclimation to removal of the forest overstory was assessed by comparing leaf mass per unit area and leaf chlorophyll content from 20 leaves of plants representing each environment (harvest area and undisturbed forest). Two years after logging, 25% of common vascular plants and 60% of common bryophytes showed significant harvest-related declines in abundance. Forest aggregates retained populations of species that disappeared from or declined substantially in harvest areas, but showed edge-related changes in plant abundance. Within aggregates, herbaceous species showed larger declines in abundance with proximity to edge than did shrubs or bryophytes, with declines becoming more prominent over time. Asarum, Clintonia, and Pyrola showed different demographic and physiological responses to timber harvest, suggesting that late-seral species that are assumed to respond similarly to timber harvest and associated environmental stresses employ different strategies for re-establishment and long-term recovery. Forest aggregates, which are integral components of current harvest prescriptions, may be especially important as refugia for those plant species that require long periods of time to recover from harvest-related declines