Regeneration of Oaks and Tanoak in Phytophthora ramorum-Affected Forests

Recent mortality in coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and tanoak (Lithocarpus densiflorus) exceeds historical levels in forests affected by the pathogen Phytophthora ramorum. We assessed the balance between recent mortality and seedling populations in these species to examine the status of regeneration in stands with varying levels of mortality caused by P. ramorum and other agents. Regeneration data were collected from two sets of permanent 0.02 ha plots. In a set of 150 plots distributed across 12 locations where P. ramorum is present, seedling counts and mature tree condition were rated annually from 2000 through 2005. In a second set of 250 plots distributed across 11 locations with and without P. ramorum in Sonoma County, data were collected in 2001 and 2004. Data from plots assessed annually showed that coast live oak seedling populations fluctuated more widely from year to year than did tanoak seedling populations. Across both plot sets, tanoak seedlings were consistently present in plots with tanoak trees. Consequently, nearly all plots with tanoak mortality had tanoak seedlings, which could potentially grow to replace dead trees. Coast live oak seedlings were present in about 80 percent of all plots with coast live oak trees. About 6 to 8 percent of plots with coast live oak trees had mortality but no coast live oak seedlings. Less than half of all plots with California black oak trees had California black oak seedlings. Three-quarters of the plots with California black oak mortality lacked seedlings of this species. In these forests, regeneration of California black oak appears inadequate to maintain stand density even without the additional mortality due to P. ramorum.