California Agriculture

Lake Tahoe is a high-altitude (6,227 feet) lake located in the northern Sierra Nevada at the California-Nevada border. During the second half of the 20th century, the decline in Lake Tahoe’s water clarity and degradation of the basin’s air quality became major concerns. The loading of gaseous and particulate nitrogen, phosphorus and fine soil via direct atmospheric deposition into the lake has been implicated in its eutrophication. Previous estimates suggest that atmospheric nitrogen deposition contributes half of the total nitrogen and a quarter of the total phosphorus loading to the lake, but the sources of the atmospheric pollutants remain unclear. In order to better understand the origins of atmospheric pollutants contributing to the decline in Lake Tahoe’s water clarity, we reviewed a series of studies performed by research groups from the U.S. Department of Agriculture’s Forest Service, UC Davis and the Desert Research Institute. Overall, the studies found that the pollutants most closely connected to the decline in Lake Tahoe’s water quality originated largely from within the basin.

In the Lake Tahoe Basin, the retention of native ecosystems within urban areas may greatly enhance the landscape’s ability to maintain biotic diversity. Our study of plant, invertebrate and vertebrate species showed that many native species were present in remnant forest stands in developed areas; however, their richness and abundance declined in association with increasing development across all taxonomic groups. Species richness for land birds and mammalian carnivores declined with development, whereas ant richness and small mammal abundance peaked at intermediate levels of development. Vegetation structure simplified with increasing development and the exotic plant species increased. The results of this study, the first to consider the effects of urbanization and remnant forests on biotic diversity in the Lake Tahoe Basin, can be used to guide land-use planning in order to maintain and enhance biodiversity in the face of increasing urbanization.

The long-term trend toward decreased water clarity in Lake Tahoe is well documented, and is strongly linked to increased nitrogen and phosphorus loading from surrounding watersheds. Recent research has detected very high concentrations of biologically available nitrogen and phosphorus in overland/litter interflow from Sierra ecosystems. The objective of this study was to assess the effect of a localized wildfire on the nutrient content of such runoff. The wildfire increased the frequency and magnitude of elevated nutrient concentrations in discharge runoff for all three parameters studied (nitrate nitrogen, ammonium nitrogen and phosphate phosphorus). Although the mobilization of nutrients was increased due to wildfire, the lack of O horizon material (the surface organic layers of mineral soils) after burning may ultimately reduce discharge concentrations over time.

Sediment in hillslope runoff from disturbed soils in the Lake Tahoe Basin is the source of many fine suspended particles that transport nutrients and contribute to a loss in lake clarity. In previous studies, we used rainfall simulation to assess and quantify infiltration, runoff and erosion rates from hillslope soils. Building on this research, our current study evaluates the relationship between particle sizes in runoff sediments and slope, and compares the relationships between restored soil treatments and bare and undisturbed (native) forest soils. Soil restoration combined with pine needle mulch treatments substantially reduced sediment yields in runoff water, and increased the size of runoff particles when compared to that from bare soils. Very little, if any, runoff and erosion occurred from relatively undisturbed “native” soil plots at similar slopes.

An overstocked montane, mixed-conifer forest on the west shore of the Lake Tahoe Basin was thinned using a Fecon masticator, leaving woodchips and tree shreddings on-site as mulch. No significant differences in soil compaction were found in 13 of 15 comparisons of soil-profile resistance values at several distances from the machine track and varying depths. We then watered the mastication sites with a rainfall simulator, and measured runoff, infiltration and erosion. The treatments included woodchip-covered and bare-soil plots corresponding to mulched track, as well as native grass, bare soils and relatively undisturbed soil plots. Sediment yields were greatest from the bare soils, followed by the undisturbed, grass and woodchip plots. Mastication appears to be an effective thinning treatment for overstocked forests with few discernible negative impacts on soil compaction or lake-polluting runoff.

Yellow starthistle is an exotic invasive weed that is estimated to infest over 14 million acres in California and is considered the most common exotic weed statewide. We reviewed several previous studies and conducted a township survey to provide an up-to-date analysis of the weed’s rapid spread throughout the state. A county-by-county comparison between 1985 and 2002 showed increases in yellow starthistle in all regions of the state except for northeast California and the southeast desert region. Currently, most infestations occur in Northern California, but future invasions and spread will likely occur in the coastal counties of Southern California.

In an 8-year study from 1997 to 2004, we evaluated the use of 186 bat houses in rural areas of California’s Central Valley. We considered the bat houses’ size, color, height and location, and found that location was the main factor affecting bat use. Colonies of bats (generally mothers and their young) preferred houses mounted on structures such as buildings, shaded or exposed only to morning sun, and within one-quarter mile of water. In contrast, individual bats (generally males and nonreproductive females) were less selective in where they roosted. The overall occupancy rate for bat houses in our study was 48% for colonies and 28% for individual bats. Mexican free-tailed and Myotis bats were the main species using the houses, with occasional sightings of pallid and big brown bats. Bats occupied most houses within the first 2 years of placement.

Processing tomato yields have increased by 53% over the past 35 years, but the current seasonal crop-evapotranspiration requirements that growers use to schedule irrigation are based on 1970s-era data. We updated this data and developed new crop coefficients for processing tomatoes using the Bowen ratio energy balance method in eight commercial fields from 2001 to 2004. Today’s evapotranspiration rates are similar to those of the early 1970s, indicating a substantial increase in water-use efficiency by processing tomatoes during the past 35 years. In addition, we collected data in both furrow- and drip-irrigated fields, but no statistical differences were found between them.

Foliar spraying was found to be an effective method to rapidly increase boron levels in most vegetative and reproductive tissues in grapevines. The reduction of fruit-set deficiency symptoms with a pre-bloom or bloom spray was immediate but not complete. Foliar sprays applied during the previous fall were more effective in reducing such symptoms than pre-bloom or bloom sprays. This may be due to the earlier incorporation of boron in reproductive tissues, especially dormant buds. Grapevine foliage is also more tolerant to boron postharvest in the fall, when 1 pound per acre of actual boron can be safely applied. Spring and summer sprays of boron should be limited to 1/2 pound per acre per application to avoid phytotoxicity.