Effects of Bark Fragmentation on Plant Succession on Conifer Logs in the Picea-Tsuga Forests of Olympic National Park, Washington

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Abstract
Plant succession on Picea (spruce), Pseudotsuga (Douglas fir) and Tsuga (hemlock) logs in the Hoh Rain Forest, Washington, was examined using the chronosequence method. Bark fragmentation patterns differed among these species; Picea and Tsuga logs lost their bark in 60 years and Pseudotsuga in 190 years. Bark of logs of all species was 85-90% covered by bryophytes within 11-19 years after tree fall. Live and dead bryophytes reached steady-state masses on bark of 324 and 684 g/m2 within 91 and 150 years, respectively. Humus mass on bark of logs of all three species increased slowly the first decade and rapidly after 20 years; a steady-state humus mass of 4400 g/m2 was predicted after 190 years. Tree density on bark peaked at 140/m2 at 15-25 years and then thinned at a rate of 8%/yr. Bark fragmentation influenced succession by removing plants and reinitiated the sere. Modeling indicates differences in succession patterns among species of logs were caused by differences in bark fragmentation patterns. Although logs are a major seedbed in Picea-Tsuga forests, self-thinning, bark fragmentation and toppling of trees greatly reduced long-term survival on logs.