Treesearch

Spread of the plant pathogen Phytophthora ramorum, causal agent of the forest disease sudden oak death, is driven by a few competent hosts that support spore production from foliar lesions. The relationship between traits of a principal foliar host, California bay laurel (Umbellularia californica), and susceptibility to P. ramorum infection were investigated with multiple P. ramorum isolates and leaves collected from multiple trees in leaf-droplet assays. We examined whether susceptibility varies with season, leaf age, or inoculum position. Bay laurel susceptibility was highest during...

At present, negative impacts of air pollution on California ecosystems are caused mainly by elevated levels of ozone and nitrogen deposition. Generally, ozone air pollution in California has been improving significantly since the 1970s; however, it still causes serious ecological and human health effects. The most serious ecological effects occur in mixed conifer forests of southern California and on the southwestern slopes of the Sierra Nevada. Ozone can be transported long distances, affecting remote areas. Nitrogen deposition to California ecosystems is dominated by dry deposition of...

The effects of thermal radiation and moisture content on the pyrolysis and gas phase ignition of a solid fuel element containing high moisture content were investigated using the coupled Gpyro3D/FDS models. The solid fuel has dimensions of a typical Arctostaphylos glandulosa leaf which is modeled as thin cellulose subjected to radiative heating on one side. We incorporated a five-step extended Broido-Shafizadeh reaction model for thermal degradation, moisture evaporation, and pyrolysis of cellulose in Gpyro3D. The solid-phase model was successfully validated against published data....

Understorey vegetation plays a significant role in the structure and function of forest ecosystems. Controlling understorey vegetation has proven to be an effective tool in increasing tree growth and overstorey development. However, a long-term consequence of the practice on plant diversity is not fully understood. Here, we analyzed early development of overstorey and understorey vegetation of four ponderosa pine plantations established decades ago in northern California, USA. These experiments were intended to examine the effect of understorey control on tree growth, including additional...

Studies of resource selection at multiple scales are critical to understanding ecological and evolutionary attributes of a species. We analyzed relative abundance, habitat use, and oviposition site selection of Foothill Yellow-Legged Frogs (Rana boylii) at 11 localities across two geographic regions in California (northern Coast Range and Sierra Nevada) over 16 yr. We found narrow ranges for oviposition microhabitat characteristics (water depth, water velocity, and stream substrate) among study localities. At the Main and South forks of the Trinity River, variances of the habitat traits...

This study uses a nonmarket valuation method to investigate the recreation values of the San Jacinto Wilderness in southern California. The analysis utilizes survey data from a stated-choice experiment involving backcountry visitors who responded to questions about hypothetical wildfire burn scenarios. Benefits of landscape preservation are derived using a Kuhn-Tucker (KT) demand system. Model results suggest that recreationists are attracted to sites with recent wildfires that can be viewed up-close. For example, recreational welfare estimates increased for sites that were partially...

The walnut twig beetle (WTB), Pityophthorus juglandis Blackman (Coleoptera: Curculionidae), vectors a fungus, Geosmithia morbida Kolarík, Freeland, Utley, and Tisserat (Ascomycota: Hypocreales), which colonises and kills the phloem of walnut and butternut trees, Juglans Linnaeus (Juglandaceae). Over the past two decades, this condition, known as thousand cankers disease (TCD), has led to the widespread mortality of Juglans species in the United States of America. Recently the beetle and pathogen were discovered on several Juglans species in northern Italy. Little is known about the...

Ethical issues are intrinsic to conducting research regarding society and natural resources, but they often become poignant when engaging in Participatory Action Research. We compiled common and persistent challenges into a list of "Questions That Won't Go Away" or "QTWGAs" that are relevant to people interested in conducting participatory research to benefit communities. We discuss these questions in the context of previously published literature and share examples from the experience of graduate student Fellows who conducted research with various communities around the United States. The...

The subalpine forests of California comprise the highest elevation ecosystems that are dominated by upright trees. They are defined as a zone influenced primarily by abiotic controls, including persistent snowpack, desiccating winds, acute and chronic extreme temperatures, soil moisture and evapotranspirative stresses, and short growing seasons. Bounded at the upper elevation by the forest-alpine treeline ecotone, the forests persist under conditions of deep snows and exposure to severe winds and high solar radiation. Disturbances such as fire and biotic interactions are less important...

Alpine ecosystems are typically defined as those areas occurring above treeline, while recognizing that alpine ecosystems at a local scale may be found below this boundary for reasons including geology, geomorphology, and microclimate. The lower limit of the alpine ecosystems, the climatic treeline, varies with latitude across California, ranging from about 3500 m in the southern California mountains and southern Sierra Nevada to 3200 m in the Yosemite region, 3000 m near Donner Pass, 2800 m at Lassen Peak, and finally 2700 m on Mount Shasta. Alpine ecosystems extend beyond the typically...