| Forest invasion by nonnative pathogens, and the resulting decline in native species, is an important ecological, economic, and evolutionary process that may alter ecosystem structure and function, with devastating impacts on natural resources and aesthetics. Sudden oak death (SOD), caused by the recently discovered pathogen, Phytophthora ramorum, has already killed tens of thousands of coast live oaks and tanoaks in California. Little is known about the potential short and long-term impacts of SOD on coast live oak/bay laurel forest structure and composition, particularly in these early stages of the disease. I studied overstory composition and structure, woody tree regeneration, and understory herbaceous vegetation at eight sites across a gradient of SOD-impacted forests.; Stand reconstruction results indicated that from the early 1990's to 2004, sites heavily impacted by P.ramorum lost 55% of coast live oak basal area, while sites with no P. ramorum lost only 4%, or natural background levels of mortality. Combining past loss with projected loss allowed for an estimate of coast live oak mortality over a span of roughly 20 years. Over this period, a total of 66% of coast live oak basal area will be lost in high infection sites. The relative importance of bay laurel, the codominant tree and an important foliar host of P. ramorum, goes from 25% to over 45% in one highly infected site over this time period.; Understory herbaceous composition, as well as drivers of differences in extent and composition of the understory, were studied over the SOD-impact gradient. Along with site factors, canopy cover was the main predictor of herbaceous cover. Canopy cover also influenced the degree of nativeness of understory forbs and grasses. Denser canopy led to a more native understory in two ways: Grass cover was predominantly nonnative (64%), and these grasses appeared to be highly light-limited, growing poorly under canopy. In contrast, forb cover was predominantly native (81.5%), and appeared less limited by overhead canopy, although all forms of ground cover grew scant as shade increased. Furthermore, even within grass and forb classes, nativeness increased in a linear fashion as tree canopy cover increased. Increases in nonnative annual grasses have often been cited as an interfering factor in oak regeneration in California. SOD, as a canopy remover, may facilitate the invasion of annual grasslands into oak-canopied areas, with detrimental impacts on oak seedlings and saplings.; SOD infected coast live oak/bay laurel forests have already seen great compositional and structural change in recent years. The interplay of many factors will determine what happens in these forests. If host resistance does not occur in coast live oak, if P. ramorum sporulation continues unabated on bay laurel leaves, and if favorable weather years continue to foster new SOD infection, coast live oaks may become an increasingly small component of these stands. The combination of few oak saplings, long regeneration time of oaks, and the proximity of adjacent annual grassland, suggests that these forests are moving away from oak-dominance.; Based on my data, it is possible to imagine at least two scenarios within the parameters above. In the best case, rapid infill of canopy gaps by mature trees preserves the native, forb-dominated understory. This understory is hospitable to coast live oak seedlings, and would allow for recruitment. Gaps would contain increased downed woody debris, increased nurse site shrubs, and lighting conditions favoring coast live oak recruitment and release. In this case, some degree of coast live oak regeneration would be maintained, and forest change would be slowed. (Abstract shortened by UMI.)... |
