Early forest succession following clearcuts in western Oregon: Patterns and abiotic controls

Historical aerial photos were used to examine the early phase of forest succession after stand replacement disturbance, covering the Coast Ranges Province (CRP) and the western Cascades Province (WCP) of western Oregon. The study consisted of two components: characterizing the pattern of forest succession in western Oregon; analyzing the influence of climatic and physiographic factors on forest succession.; Succession has many dimensions. In this study, I examined succession after stand replacement disturbance in terms of canopy cover change of different life forms: shrubs, hardwood trees, and conifer trees. Canopy cover changes from 1959 to 1997 for selected sample stands were obtained by photo interpretation. Canopy cover growth curves for each sample stand were developed based on a Chapman-Richards growth function. Seven successional pathways were defined to characterize forest successional processes based on projected canopy cover at age 50 and the process of canopy composition change over this 50 years period. In addition, seven parameters from the canopy cover growth curves were derived for the purpose of comparison of successional patterns in the CRP and WCP. These include time to reach 5% and 70% of canopy cover, weighted mean absolute growth rate, weighted mean relative growth rate, maximum absolute growth rate, time to reach maximum absolute growth rate, and active growth period. Results indicated that a wide range of variation in forest succession exists in western Oregon. Successional patterns for the CRP and WCP were different in terms of conifer development.; Possible abiotic controls of two successional parameters were examined by stepwise regression analysis. Only 23%, 37%, and 29% of the total variation in the delay, and 29%, 25% and 57% of the total variation in rate parameters can be explained by abiotic factors for the WCP, CRP, and across provinces, respectively. Unexplained variation was likely due to stochastic, biotic, and management factors, as well as experimental error due to photo interpretation and derivation of successional parameters. The most important influential climatic factor was temperature, and the most important physiographic factor was elevation. Depending on location, interactions among climatic and physiographic factors also influenced successional delay and rate.