Canopy gaps in Douglas-fir forests of the Cascade Mountains

Types and rates of mortality were measured and canopy gap formation rates were estimated from 5- to 15-year recordsof mortality in 34 permanent plots in mature (100- to 150-year-old) and old-growth (>200-year-old) Douglas-fir(Pseudotsuga menziesii (Mirb.) Franco)/western hemlock ( Tsuga canadensis (Raf.) Sarg.) forests in western Oregonand Washington. Gap surveys were conducted in a mature and an old-growth stand, and characteristics of 40 gapsand regeneration were measured. Most canopy trees died without disrupting the forest in both mature (87.6%) andold-growth stands (73.3%). The amount of forest area per year representing new gaps was 0.7% in mature stands and0.2% in old-growth stands. The gap survey found a higher proportion of gaps in the mature stand than in the old-growth stand. Most regeneration ( > 1 m tall) in gaps was western hemlock; Douglas-fir regeneration did not occur.The ratio of seedling density in gaps to density under canopies was about 3 for the mature stand and about 9 for theold-growth stand. Seedling density was correlated with measures of gap age but not gap size. The study suggests thatgap disturbances and vegetative responses are important processes in the dynamics of these forests. However, gap for-mation rates and vegetative responses appear to be slow relative to other forest types. In addition to gap size, canopystructure and disturbance severity are important determinants of gap response.