A prominent forest type in the Sierra Nevada is that of Jeffrey pine, and on the eastern slopes at mid and low elevations where the soil is typically dry during the growing season, Jeffrey pine forests dominate. Because of extensive harvesting of this species during the Comstock era, a reflection of a high commercial value that still persists, coupled with fire exclusion during much of the 20^th century, many Jeffrey pine stands today are of poor quality with large numbers of small stems, little spacing between trees, and relatively high canopy closure. This retards the growth of healthy stands, diminishes wildlife habitat, and reduces water yield.

Forest health in the Jeffrey pine is of major concern because crowding in this species predisposes it to bark beetle attack, a condition related to an inability to obtain sufficient water. In turn, excessive mortality due to bark beetles elevates fuel loading, and particularly of the critical ladder fuels. Consequently, wildfire behavior in the eastern Sierra Nevada has tended to change over time from low-intensity ground fire, the historical norm, to catastrophic stand-replacement fires today.

Thinnings in Jeffrey pine forests, in which weak trees are removed so that desirable stems have greater access to water and nutrients, can potentially provide the benefits of improved forest health, wildfire mitigation, increased water yield, and enhanced wildlife habitat. Also, because this practice produces wood fiber of commercial value and/or biomass that is suitable as an alternative energy feedstock, a monetary return on the investment can be realized that offsets the costs of implementation.

Approaches to thinning forest stands in the Sierra Nevada have improved substantially in recent years due to the introduction of a variety of machines that traverse steep slopes with little disturbance, affording such choices as cut-to-length systems, which feature extremely low soil impacts, and whole-tree harvesting, an approach noted for reducing fuel loads. Prescribed fire in the form of controlled underburning in Jeffrey pine stands may, with the exception of the fiber and biomass yield, provide many of the same benefits as thinning.

Prior to widespread use of either of these practices in the eastern Sierra Nevada, however, an assessment of their impacts on stand health and productivity, nutrient cycling, and water quality is required to optimize their use for maximum benefit. Also, an understanding of the historical fire regime is needed to aid in the development of prescribed fire prescriptions that mimic the natural fires of the past.

Prominent among the forest cover types of the Sierra Nevada is that of Jeffrey pine (Pinus jeffreyi Grrev. & Balf.), which becomes the dominant cover type descending down the eastern slopes as xeric sites become commonplace at mid and low elevations (Jenkinson, 1980; Bom et al., 1992). Extensive timber harvesting during the Comstock Lode mining era (Beesley, 1996), combined with the effects of subsequent fire exclusion, have resulted in stands that often consist of large numbers of small trees relative to historic norms (Laudenslayer et al., 1989). High stand densities retard the development of healthy, high-quality individual trees, increase the risk of catastrophic stand-replacement wildfire, diminish wildlife habitat, and reduce water yields (Jenkinson, 1990; Overend, 1997; Amo, 2000; Walker et al., 2003). Ecophysiological implications include impaired water relations (Brix and Mitchell, 1986; Donner and Running, 1986; Aussenac and Granier, 1988) that can predispose stress-related bark beetle attack (Kozlowski et al., 1991). The role of fire in the Sierra Nevada has historically been a key factor in the successional development of its forests. Prior to the suppression efforts of the 20th century, the natural fire regime was likely one of frequent low-severity surface fires, with crown fires relatively uncommon, occurring at short intervals which kept the stands open and fuel loading minimized (Caprio and Swetnam, 1995; Stephens, 2001). Thus, effective management of Sierra Nevada forests must include prescribed fire (Brennan and Herman, 1994; Donaldson et al., 1999; Bradley and Tueller, 2001), but safety concerns and its impacts on air quality will likely necessitate that it be supplemented, and in some cases replaced, with mechanical thinnings. Furthermore, more comprehensive information is needed to develop benchmarks for the presettlement fire regime that can be used to guide prescribed fire and stand density restoration treatments (Cissel et al., 1999; Swetnam et al., 1999) and to assess how long-term fire suppression effected nutrient cycling processes in Sierran watersheds.

Compared to most forested regions, very little is known regarding nutrient cycling through the vegetation and soils of the eastern Sierra Nevada (Reuter and Miller, 2000). The relationship between nutrient cycling and transport of nutrients and fine sediments in surface runoff, stream flow, and shallow subsurface water flows is clearly important, and these processes are undoubtedly affected by changes in vegetation, such as those resulting from natural succession or imposed through managed manipulations, and by surface disturbances such as soil erosion and compaction. Nutrient cycling processes in natural watersheds, particularly in terms of the transport of biologically available N, P, and S, must be better understood in order to formulate effective management strategies for the Sierra Nevada. Furthermore, an understanding of the impacts of forest management practices on site C balance is now imperative due to concerns about atmospheric CO2 buildup and its potential impacts on climate change (Johnson and Curtis, 2001).

It is generally held that surface runoff from forest ecosystems is minimal and unimportant (Fisher and Binkley, 2000). Our studies to date, however, have detected substantial, highly nutrient laden overland flow from Sierran watersheds. We therefore argue that the importance of even small amounts of overland flow cannot be ignored at the watershed scale because of the high concentrations ofbiologically available N and P (Reuter and Miller, 2000). Central to this argument is the widespread belief that Sierran forests long protected from wildfire and mechanical disturbance contribute little in the way of natural nutrient discharge. To the contrary, our data indicate that water is seeping down slope within these watersheds that contains biologically available N and P well in excess of that in snowmelt or soil solution. The implications of this finding in the pollution of surface water bodies is apparent; fire suppression may be indirectly contributing to water quality deterioration by allowing the buildup of forest floor litter that is likely the source of this N and P.

1. Evaluate the potential of mechanical thinnings and prescribed fire, individually and interactively, to enhance long-term stand health, productivity, and water relations in east side Sierra Nevada Jeffrey pine.
2. Reconstruct the fire history in east side Sierra Nevada Jeffrey pine by quantifying past fire frequency, severity, and changes over time in different topoclimatic settings for purposes of guiding prescribed fire prescription development in this cover type.
3. Evaluate the long-term effects of mechanical thinnings and prescribed fire, individually and interactively, on stand and soil nutrient status and on site C, N, P, and S budgets in the east side Sierra Nevada Jeffrey pine cover type.
4. Evaluate the long-term effects of mechanical thinnings and prescribed fire, individually and interactively, on temporal (event-based, rain vs. snow) and spatial (site-specific locations) water balance components in the east side Sierra Nevada Jeffrey pine cover type.