Hotter Drought Escalates Tree Cover Declines in Blue Oak Woodlands of California

California has, in recent years, become a hotspot of interannual climatic variability, recording devastating climate-related disturbances with severe effects on tree resources. Understanding the patterns of tree cover change associated with these events is vital for developing strategies to sustain critical habitats of endemic and threatened vegetation communities. We assessed patterns of tree cover change, especially the effects of the 2012–2016 drought within the distribution range of blue oak (Quercus douglasii), an endemic tree species to California with a narrow geographic extent. We utilized multiple, annual land-cover and land-surface change products from the U.S. Geological Survey (USGS) Land Change Monitoring, Assessment and Projection (LCMAP) project along with climate and wildfire datasets to monitor changes in tree cover state and condition and examine their relationships with interannual climate variability between 1985 and 2016. Here, we refer to a change in tree cover class without a land-cover change to another class as “conditional change.” The unusual drought of 2012–2016, accompanied by anomalously high temperatures and vapor pressure deficit, was associated with exceptional spikes in the amount of both fire and non-fire induced tree cover loss and tree cover conditional change, especially in 2015 and 2016. Approximately 1,266 km2 of tree cover loss and 617 km2 of tree cover conditional change were recorded during that drought. Tree cover loss through medium to high severity fires was especially large in exceptionally dry and hot years. Our study demonstrates the usefulness of the LCMAP products for monitoring the effects of climatic extremes and disturbance events on both thematic and conditional land-cover change over a multi-decadal period. Our results signify that blue oak woodlands may be vulnerable to extreme climate events and changing wildfire regimes. Here, we present early evidence that frequent droughts associated with climate warming may continue to affect tree cover in this region, while drought interaction with wildfires and the resulting feedbacks may have substantial influence as well. Consequently, efforts to conserve the blue oak woodlands, and potentially other vegetation communities in the Western United States, may benefit from consideration of climate risks as well as the potential for climate-fire and vegetation feedbacks.

Extremely low fecundity and highly female-biased sex ratio in nest-living spider mite Schizotetranychus brevisetosus (Acari: Tetranychidae)

Spider mites show various life types characterized by spinning behavior, web structure, and sociality. Individuals of the woven-nest (WN) species construct silken nests on the undersurface of host leaves, inside which they develop and reproduce. This nesting behavior may be related with the defense mechanism and life history traits of mites in the stable habitat (e.g., evergreen trees). If the WN life type affects the life-history traits, these traits may be similar within WN species. The WN species are known to be fragmentarily distributed in diverse genera, Stigmaeopsis, Schizotetranychus, Eotetranychus, and Oligonychus, and their life types are suspected to have secondarily converged. However, their life histories have not been elucidated except for several species in specific genera. To supply the information in Schizotetranychus, we investigated the demographic traits and the sex ratio of Schizotetranychus brevisetosus, which shows the WN life type and lives on the evergreen Japanese blue oak Quercus glauca. We estimated the development time of females as 22.6 ± 3.1 days (mean ± SD, n = 22) and the fecundity of fertilized females as 13.7 ± 5.9 (n = 37) at 25°C. The sex ratio of males to the total number of adults at emergence was low (0.072). The intrinsic rate of natural increase (rm) was estimated as 0.060 day-1, one of the lowest ever reported for spider mites at the same temperature. The present results were similar to other WN species in that fecundity and male ratio were low.

The Ancient Blue Oak Woodlands of California: Longevity and Hydroclimatic History

Ancient blue oak trees are still widespread across the foothills of the Coast Ranges, Cascades, and Sierra Nevada in California. The most extensive tracts of intact old-growth blue oak woodland appear to survive on rugged and remote terrain in the southern Coast Ranges and on the foothills west and southwest of Mt. Lassen. In the authors' sampling of old-growth stands, most blue oak appear to have recruited to the canopy in the middle to late nineteenth century. The oldest living blue oak tree sampled was over 459 years old, and several dead blue oak logs had over 500 annual rings. Precipitation sensitive tree-ring chronologies up to 700 years long have been developed from old blue oak trees and logs. Annual ring-width chronologies of blue oak are strongly correlated with cool season precipitation totals, streamflow in the major rivers of California, and the estuarine water quality of San Francisco Bay. A new network of 36 blue oak chronologies records spatial anomalies in growth that arise from latitudinal changes in the mean storm track and location of landfalling atmospheric rivers. These long, climate-sensitive blue oak chronologies have been used to reconstruct hydroclimatic history in California and will help to better understand and manage water resources. The environmental history embedded in blue oak growth chronologies may help justify efforts to conserve these authentic old-growth native woodlands.