THE DISTURBANCE REGIME OF AN OLD-GROWTH FOREST IN COASTAL CALIFORNIA

This study deals with the disturbance regime of an old-growth, mixed-evergreen forest with a canopy composed of Arbutus menziesii, Lithocarpus densiflora, Pseudotsuga menziesii, Quercus chrysolepis, Q. wislizenii, and Sequoia sempervirens. 80 canopy gaps were randomly selected from throughout a 230-ha watershed. Of the land area sampled, 11.1 to 16.6 % was within gaps. Gap area had a mode of < 50 M2 and a range of 6 - 3437 M2. Gaps were formed by snags, snaps, tips, and slope failures. Although < 10 % of the gaps sampled were due to slope failures, these accounted for 43 % of the total land area within gaps. All snags resulted from the death of a Pseudotsuga or Arbutus individual, the widely branched trunks of Arbutus individuals accounted for most of the irregularly shaped gaps, and larger gaps resulted from the death of Pseudotsuga and Sequoia individuals, averaging 119 and 111 m2 respectively, than from the death of Lithocarpus or Arbutus, averaging 54 and 52 M2. Gaps were more frequent over concave sections of slopes and large gaps were more frequent on north-facing slopes. The creation of a gap increased disturbance to the adjacent canopy, with half of the gaps formed through more than one disturbance. The relationship of disturbance regime to topography, the influence of canopy species biology on gap properties, and the repeated events involved in gap formation all indicate a fine scale variation in the pattern of gaps and their characteristics.