Development of two coniferous stands impacted by multiple, partial fires in the Oregon Cascades : establishment history and the spatial patterns of colonizing tree species relative to old-growth remnant trees

Graduation date: 1997Trees that survive disturbances can form a prominent legacy which may influence\ud post-disturbance successional pathways. The effects of biological legacies on community\ud dynamics is a critical question in ecology. In the present study, I examined two mapped\ud stands in which old-growth remnant trees, survivors of partial fires, emerge above a lower\ud canopy of mature trees which had regenerated after these fires. In the first part of this\ud study, I reconstructed the history and patterns of the most recent fires and the establishment\ud history of the post-fire regeneration. At the Eagle Rock study site, fires occurred in 1848,\ud 1870 and 1892. At the Wolf Rock site, a fire burned in 1892, and fires in 1829 and 1896\ud appeared likely. Fires burned under the remnant trees, and no area remained unburned\ud during the nineteenth century. Cohorts were layered and interspersed among each other\ud rather than juxtaposed as discrete patches. Regeneration of both early seral Pseudotsuga\ud menziesii and late seral Tsuga heterophylla was initiated by the fire events, and neither\ud displayed continuous recruitment. At both sites, Pseudotsuga regenerated more quickly\ud than Tsuga. The median establishment time for both Pseudotsuga and Tsuga was longer at\ud Eagle Rock (south-facing) than at Wolf Rock (northwest-facing). In the second part of this\ud study, I described the spatial patterns of colonizing tree species relative to the remnant trees. The spatial patterns of post-fire species were clearly dependent upon the remnant tree\ud pattern. Species were typically dispersed away from remnant trees, but several species,\ud Castanopsis and Cornus nuttallii at Eagle Rock and Tsuga at Wolf Rock, were aggregated\ud around remnant trees. Tsuga patterns differed between sites. Consistent with its shade\ud tolerance, Tsuga was independent of or aggregated around remnant trees at Wolf Rock,\ud but was unexpectedly dispersed away from remnants at south-facing Eagle Rock. The\ud within-group patterns of remnant trees were clustered, as were the patterns of almost all\ud post-fire species. The clustering of post-fire species may reflect the spatial dependence of\ud each species' pattern upon the clustered remnant tree pattern which ,itself, is the outcome of\ud the patchy pattern of partial fire. The aggregation and dispersion of different post-fire\ud species relative to remnant trees suggests that remnant trees or remnant-associated features\ud from the pre-disturbance community differentially facilitate or inhibit colonizing species,\ud thus influencing the direction of post-disturbance succession