PLANT SPECIES DIVERSITY IN NATURAL AND MANAGED FORESTS OF THE PACIFIC NORTHWEST [Review]

摘要

With the exception of the tropics, nowhere has the relationship between resource management and conservation of biological diversity been more controversial than in the Pacific Northwest region of the United States. Widespread loss and fragmentation of old-growth ecosystems have stimulated critical review and revision of existing forest management policies. However, studies of the consequences of forest management for plant species diversity are sorely lacking. We present data from permanent-plot and chronosequence studies in managed and unmanaged forests of western Oregon and Washington to describe the early responses of understory communities to forest harvest, and to suggest how post-harvest practices that alter natural successional processes may influence longterm patterns of diversity and species occurrence. Permanent-plot studies of early succession in old-growth Pseudotsuga forests suggest that changes in understory diversity are fairly short-lived following clear-cut logging and slash burning. Populations of most vascular plant species recover to original levels prior to canopy closure. However, diversity may remain depressed for more than two decades on severely burned sites, and some species may experience local extinction. Evidence of the effects of post-harvest practices on vascular plant diversity is limited by an absence of community-level studies in older, managed forests. Chronosequence studies of natural forest stands indicate that, following canopy closure, vascular plant species diversity tends to increase with time, peaking in old growth. Few understory species are restricted to, or absent from, any stage of stand development (i.e., young, mature, or old growth). However many species differ significantly in their abundance among stages. A majority of these showed greatest abundance in old growth. Changes in levels of resources (increased shade), changes in the spatial variability of resources and environments (increased horizontal and vertical heterogeneity), and species' sensitivity to fire and slow rates of reestablishment/growth may drive these trends during natural stand development. Silvicultural prescriptions that maintain or foster spatial and temporal diversity of resources and environments will be most effective in maintaining plant species diversity. Practices associated with intensive, short-rotation plantation forestry, that preclude or de:ay the development of old-growth attributes, may result in long-term loss of diversity. Ultimately, it may be necessary to manage some stands on long rotations (150-300 yr) Co maintain understory species that require long periods to recover from disturbance. [References: 131]