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Structure and dynamics of a coastal dune forest at Humboldt Bay, California
Forest stands occurring on coastal dunes along the western margin of Humboldt Bay, California are isolated fragments of a regional forest type that stretches discontinuously along the Pacific Coast from northern California to Alaska. I used aerial photographs to stratify the forest at Lanphere Dunes into 3 stand types. Using fixed-radius (13.2 m) circular plots, I sampled these types to determine their composition (overstory and understory), size structure (basal area, height, and sapling, seedling, snag and tree density), and age structure. Beach pine, Sitka spruce, and mixed-species types exhibit significant compositional and structural differences. The beach pine type is distinguished not only by the importance of beach pine in the overstory, but by high stem density, low basal area, and the overall importance of bearberry in the understory. The Sitka spruce type is characterized by the importance of Sitka spruce in the overstory, low stem density, high basal area, and the importance of twinberry and wax myrtle in the understory. In the mixed-species type, beach pine and Sitka spruce are almost equally important among an overstory layer that often includes grand fir. This type exhibits stem density and basal area values intermediate to those of the other two types. Age structure differences among the types are non-significant, suggesting that each type has a similar disturbance history as the others. Within each type, beach pine and/or Sitka spruce populations consist of numerous age classes that are normally distributed. Age class chronologies correspond (in part) among types and among spatially disjunct plots of the pine type. Small patches of forest (<0.04 ha) typically include trees representing numerous age classes, and age class distributions (i.e. number and size) vary over larger areas. Patch dynamics are apparently complex and affect forest development at a variety of spatial and temporal scales. These age and spatial patterns support the hypothesis that small-scale windfall events have been the most important disturbance factor underlying seedling establishment over the last 150 years. Though regeneration levels are currently low, beach pine stands generally appear to be self-replacing over a period of many decades.