Stand and tree growth characteristics of Quercus garryana and Quercus kelloggi woodlands in northwestern California

Oregon white oak (Quercus garryana) and California black oak (Q. kelloggii) woodlands are unique ecosystems in the Pacific West that support high levels of biodiversity, yet little is known about their current and historic stand establishment patterns, nor the variability of stand structure and its effect on oak-tree growth. With concerns of local extirpation due to native Douglas-fir (Pseudotsuga menziesii) encroachment, my research objectives were to characterize the variability of age and stand structure, current tree regeneration, and oak growth in 10 mixed oak-conifer woodlands from xeric to mesic site conditions in the North Coast region of California. Each site varied from open canopy oak-dominant woodland to closed canopy conifer-dominant forest. Most white and black oak trees established from 1850 to 1910 with minimal ( 0.05%) establishment after 1950. Conversely, most Douglas-fir trees established after ~ 1950. All sites exhibited high proportions of oak seedling mortality in open stands while closed stands supported lower densities of oak seedlings and greater proportions of shade tolerant tree species. White oak had slower growth (p-value 0.001) and higher proportions of mortality in conifer-dominant stands in comparison to oak-dominant stands. White oak growth was more sensitive to overstory Douglas-fir than black oak, whose shade tolerance and mortality appeared to be influenced by environmental gradients and potentially past conifer harvests. The high proportion of Douglas-fir establishment, and the lack of successful oak recruitment, suggests the effects of altered disturbance regimes enabling these ecosystems to transition into conifer-dominant stands.