PARENT SESSION
Oral Session #62: Vegetation change: Succession, recovery, facilitation. Presiding: S. Franklin.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas G.
Reproductive role of remnant old-growth trees in mature Douglas-fir forests, southern Washington Cascade Range.
Keeton, William1, Franklin, Jerry2, 1 2
ABSTRACT- We tested the hypothesis that remnant old-growth Tsuga heterophylla (western hemlock) and Thuja plicata (western red cedar) trees enhance the re-establishment of shade-tolerant conifers by increasing the availability of seed. Reestablishment of shade-tolerant conifers is critical for the development of late-successional characteristics in mature Pseudotsuga menziesii (Douglas-fir) forests because it leads to vertical differentiation of the canopy and co-dominance of shade-tolerant species. Two study areas were selected in the southern Washington Cascades. Both had an unfragmented, mature forest cover that was regenerated naturally following wildfire. Twelve study sites were selected, including sites with and without remnant Thuja and Tsuga. Overstory structure and composition, microsite variables, and conifer regeneration were systematically sampled using nested belt transects and quadrats. Sites with remnant Tsuga and Thuja had significantly higher densities of conspecific seedlings. Remnant Tsuga and Thuja presence and density were the strongest predictors of seedling densities, although the basal area of mature conspecific trees, relative density, aspect, stand age, and microsite characteristics were strong secondary predictors. Seedling densities were strongly correlated with proximity to remnant trees, exhibiting a negative exponential decline with distance. Shade-tolerant conifers are likely to reestablish faster at sites with remnant seed trees, but canopy disturbances are probably required to elicit subsequent vertical development. Remnant shade-tolerant conifers are an important biological legacy and seed source influencing rates of ecological succession in mature Pseudotsuga stands. Forestry practices that retain shade-tolerant conifers may accelerate rates of late-successional forest development.
KEY WORDS: ecological succession , old-growth, biological legacies, seed source