Fire-Excluded Relict Forests in the Southeastern Klamath Mountains, California, USA

摘要

The rare relict ponderosa pine (Pinus ponderosa C. Lawson) mixed-conifer forests of northern California’s Whiskeytown National Recreation Area (WNRA), USA, present a classic example of fire exclusion. Altered fire regimes in this biologically unique area have been documented, but the resulting changes in forest composition and structure have not previously been described. A fully randomized, park-wide sampling of relict forest structure at WNRA reveals a high degree of topographic variability in tree species composition, but strikingly similar changes in recent structural development. A distinct cohort of encroachment trees initiated approximately 64 yr to 67 yr ago with little age variation (2 yr SE), with distinct strata now distinguishing the relict and encroachment cohorts. Over the past five decades, the average periodic annual basal area growth of relict trees has remained at a virtual constant of 24 cm2 to 27 cm2 per tree. In contrast, the annual basal area increment of encroachment trees has been steadily increasing, from 3 cm2 per tree in 1955 to 16 cm2 per tree in 2005. Whereas relict trees are comprised primarily of pine species (76%), they represent just 17% of encroachment trees. In contrast, white fir, Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco.), and tanoak (Lithocarpus densiflorus [Hook. & Arn.] Rehder) are rare among relicts (16%) but are the most predominant species among encroachment trees (64%). This study’s findings should inform the planning of restoration activities to reduce threats to relict forests at the WNRA as well as similar forests in the southeastern Klamath Mountains.