Summary1 Phytophthora ramorum (causal agent of sudden oak death) is an emerging generalist pathogen in coastal forests of California and Oregon, USA, that causes lethal stem infections on oaks ( Quercus spp.) and tanoak ( Lithocarpus densiflorus ) as well as nonlethal foliar infections on a broad range of trees and shrubs. 2 We studied P. ramorum over its known range in coastal redwood forests to determine forest compositional variables that are important to its epidemiology within the geographical area that it has already invaded. Redwood forests are dominated by coast redwood ( Sequoia sempervirens ), tanoak and California bay laurel ( Umbellularia californica ). 3 A total of 120 permanent plots (500 m 2 each) were established in redwood forests at 12 sites within the main epidemic area in California. Over 5000 trees were mapped and examined for the presence of P. ramorum during spring 2002 and resampled in spring 2003. 4 Mean incidence of P. ramorum across all plots was 0.17 ± 0.01 in 2002 and 0.24 ± 0.02 in 2003. The highest infection levels by P. ramorum were found on California bay laurel (range 0.42-0.69) and tanoak (0.32-0.45). The highest levels of mortality were associated with tanoak and ranged from 0 to 66%, with 62.5% of that mortality associated with P. ramorum infection. 5 Disease incidence above 30% was most often associated with bay laurel importance value. In plots with few bay laurel stems, high disease levels were associated with the presence of understorey tanoaks. Bay laurel and small tanoaks are thought to represent the main source of inoculum for further spread of P. ramorum . 6 Differential host mortality due to this emerging generalist pathogen will exert considerable influence on redwood forest dynamics, with potentially dramatic shifts in forest composition and structure and subsequent cascading ecological and evolutionary effects.

Substantial genetic variation in development time is known to exist among mountain pine beetle (Dendroctonus ponderosae Hopkins) populations across the western United States. The effect of this variation on geographic patterns in voltinism (generation time) and thermal requirements to produce specific voltinism pathways have not been investigated. The influence of voltinism on fitness traits, body size, and sex ratio is also unclear. We monitored mountain pine beetle voltinism, adult body size, sex ratio, and air temperatures at sites across latitudinal and elevational gradients in the western United States. With the exception of two sites at the coolest and warmest locations, the number of days required to complete a generation was similar. Thermal units required to achieve a generation, however, were significantly less for individuals at the coolest sites. Evolved adaptations explain this pattern, including developmental rates and thresholds that serve to synchronize cohorts and minimize cold-sensitive life stages in winter. These same adaptations reduce the capacity of mountain pine beetle at the warmest sites to take full advantage of increased thermal units, limiting the capacity for bivoltinism within the current realized distribution. Temperature was not correlated with adult size and sex ratio, and size was greatest in host trees other than lodgepole pine (Pinus contorta Dougl.). Our results provide baseline information for evaluating population responses in a changing climate.

The finding of Phytophthora ramorum -the pathogen that causes sudden oak death in four California native trees -on rhododendron in Europe led us to hypothesize that its host range in California's natural forests was much greater than previously suspected. In addition to the affected oak species, we have now identified an additional 13 species from 10 plant families that act as hosts for P. ramorum in California. Our data indicates that nearly all of the state's main tree species in mixedevergreen and redwood-tanoak forests -including the coniferous timber species coast redwood and Douglas fir -may be hosts for P. ramorum. The broad host range of P. ramorum, the variability of symptoms among different hosts and the ability of the pathogen to disperse by air suggests that it may have the potential to cause long-term, landscape-level changes in California forests.

We described 38 relictual old-growth stands -with data on the mortality, regeneration, floristic richness, fuel load and disease incidence in our study area in the Tahoe Basin of California and Nevada. The stands are within the lower and upper montane zones (1900-2400 m a.s.l.) and they are rare, occupying < 2% of the land in the Basin's watershed. Correlation matrices and ANOVAs of forest types and conifer species with environmental gradients revealed significant relationships with elevation, distance east of the Sierran crest, slope aspect, annual precipitation, date of complete snow melt, litter depth and degree of soil profile development. Pathogens, parasites and wood-boring insects were present on 23% of living trees; 16% of all trees were dead. We compared these stands to a reconstruction of pre-contact Basin forests and to ecologically analogous old-growth forests of Baja California that have never experienced fire suppression management. Currently, overstorey trees (> 180 yr old) in the Basin stands have ca. 33% cover, 54 m 2 .ha -1 basal area and 107 individuals.ha -1 , values very similar to reconstructions of pre-contact Basin forests and to modern Baja California forests. Understorey trees (60-180 yr old), however, are several times more dense than historic levels and species composition is strongly dominated by A. concolor, regardless of the overstorey composition. The ratio of Pinus : Abies has increased -and the age structure of extant stands predicts that it will continue to increase -from approximately 1:1 in pre-contact time to 1:7 within the next century. Disease incidence and mortality in Baja forests were lower. Although we quantitatively defined current Basin oldgrowth forests -in terms of stand structure -we realize that our definition will differ from that of both past and future oldgrowth forests unless management protocols are changed.