Forest Management, wildfire, and climate impacts on the hydrology of Sierra Nevada mixed-conifer watersheds by Philip Saksa. Ph.D. Environmental Systems. University of California, Merced. Committee Chair: Roger Bales. The research presented in this dissertation aims to 1) assess the water balance of headwater catchments in the Sierra Nevada and determine if fuel treatments implemented in 2012 impacted runoff, 2) use a hydro-ecologic model to simulate the effects of fuel treatments and modeled wildfire at a larger fireshed scale, and 3) to investigate the interaction of vegetation disturbance and projected temperature increases through 2100 to determine relative impacts on hydrologic fluxes. The high variability in annual precipitation, combined with low post-treatment precipitation, masked any detectable changes in headwater catchment runoff from fuel treatments. Model results, however, do show the potential of increased runoff with treatments at both the headwater and fireshed scales, particularly in the high precipitation region of the American River Basin, where vegetation is less water-limited. While the potential for increasing runoff with fuel treatments exists, and may be a co-benefit of reduced fire risk, high-precision equipment for measuring stream discharge may be necessary to verifiable detect these increases. Although increasing temperatures adversely affect snowpack storage, changes in runoff and evapotranspiration are limited to the highest potential temperature increases towards the end of the century, and have less of an impact than vegetation disturbances.

A framework is presented for defining the present atmospheric chemistry of West Antarctica and the photochemistry of the atmosphere during the early- to mid-Holocene transition (9,000-6,000 yr BP). Little is known on the contributions of local and regional sources to the photochemical composition of the current atmosphere of West Antarctica and the early- to mid-Holocene atmosphere. Measurements over the West Antarctic continent were scarce in regards of exploring its present photochemical composition, while the contribution of the atmospheric oxidative capacity, namely OH radicals, to the methane decline over the early- to mid-Holocene has been assumed negligible. In this study, investigation of the present atmospheric chemistry was run during Antarctic summer at the WAIS Divide site, through multi-week continuous measurements of photochemical active species: atmospheric NO, O₃, H₂O₂, MHP, and snow NO₃^-, NO₂^- and H₂O₂. Then, estimation of the mixing ratios of past atmospheric oxidants from ice-core records was made possible from the latest findings on air-ice partition equilibrium of reversibly deposited species, HCHO and H₂O₂. Results from the West Antarctic campaign show that the levels of short-lived species above this region, such as NO_X, are driven by local sources, mainly snowpack emissions, whereas regional air mass transport controls the atmospheric concentrations of species with longer lifetime, such as O₃. Atmospheric estimates from the early- to mid-Holocene reveal that both methane sources and sink (low-latitude emissions and oxidation with OH) contributed to the CH₄ budget between 12,000-9,000 yr BP, whereas only the sources drove the decline of CH₄ between 9,000-6,000 yr BP.

Forest ecosystems are a significant faction of the Earth’s landscape, and accurate estimates of forest structures are important for understanding and predicting how forest ecosystems respond to climate change and human activities. Light detection and ranging (LiDAR) technology, an active remote sensing technology, can penetrate the forest canopy and greatly improve the efficiency and accuracy of mapping forest structures, compared to traditional passive optical remote sensing and radar technologies. However, currently, LiDAR has two major weaknesses, the lack of spectral information and the limited spatial coverage. These weaknesses have limited its accuracy in certain forestry applications (e.g., vegetation mapping) and its application in large-scale forest structure mapping. The aim of research described in this dissertation is to develop data fusion algorithms to address these limitations. In this dissertation, the effectiveness of LiDAR in estimating forest structures and therefore monitoring forest dynamics is first compared with aerial imagery by detecting forest fuel treatment activities at the local scale. Then, a vegetation mapping algorithm is developed based on the fusion of LiDAR data and aerial imagery. This algorithm can automatically determine the optimized number of vegetation units in a forest and take both the vegetation species and vegetation structure characteristics into account in classifying the vegetation types. To extend the use of LiDAR in mapping forest structures in areas without LiDAR coverage, a data fusion algorithm is proposed to map fine-resolution tree height from airborne LiDAR, spaceborne LiDAR, optical imagery and radar data in regional scale. Finally, this dissertation further investigates the methodology to integrate spaceborne LiDAR, optical imagery, radar data and climate surfaces for the purpose of mapping national- to global-scale forest aboveground biomass. The proposed data fusion algorithms and the generated regional to global forest structure parameters will have important applications in ecological and hydrologic studies and forest management.

The work presented here was motivated by a need to better understand components of the water balance in the Sierra Nevada with respect to a warming climate. I present two case studies preceded by a summary of a climate data set prepared during the course of this work. The first case study examined the implications of climate warming on snowpack storage in the Merced and Tuolumne River watersheds, which encompass Yosemite National Park. The second study focused on estimating the impact of forest treatment and fire on forest evapotranspiration (ET) at the patch to watershed scale in the American and Kings River basins.

Modeled April 1st snowpack storage in the Yosemite area declined 38, 73, and 90% for +2, 4, 6°C dry-year warming scenarios. Seasonal snowpack disappears below 2000 m elevation with 4°C warming in both wet and dry years. Assuming vapor pressure remains constant with increasing temperature resulted in up to 100 mm more late-spring snow water equivalent than the respective constant-relative-humidity scenario with 6°C warming. Reduced snowpack and snow-cover duration will mean less summer melt input to meadow and forest soils, resulting in loss of wetlands and longer forest fire seasons. Other management implications include reduced late-season streamflow for dependent downstream communities and wildlife.

Water-limited forests exhibited smaller changes in the normalized difference vegetation index (NDVI) per respective changes in basal area than less water-limited areas. Intensive thinning projects, which reduced forest density 40-50%, resulted in NDVI reductions of 0.09-0.12 units from control plot values of 0.7-0.8, corresponding to ET reductions of 152-216 mm yr-1. The minimum observed change for treated areas was 0.07 NDVI units from an initial value of 0.7 units, associated with a basal area reduction of 9%. Net ET reduction due to forest fires in 1990-2008 in the American River watershed was about 5 times that in the Kings River (65 vs. 14 million m3 yr-1). In addition to reducing the multiple impacts of catastrophic forest fire, forest thinning and managed fire offer the potential to offset water losses due to increased ET in a warming climate.

Much of the Sierra Nevada forests are overstocked with too many trees, are at risk of burning in stand-replacing wildfires due to the fuel loading, contain almost complete canopy coverage over the landscape and intercept a substantial portion of the snowpack, and in need of thinning. New research shows that forest thinning patterns allowing for forest structures comprising of clumps of varying density, single individual trees, and forest gaps are what the historic stands in Sierra Nevada were comprised of. In addition to returning the forest to historic stand structure, forest thinning will also allow for greater amounts of snowpack to reach the forest floor and persist longer into the season before complete meltout. This study set out to see if this new thinning spacing pattern would affect snow accumulation or ablation rates as compared with the current standard even-tree spacing. Data from four snow surveys, distributed light/temperature sensors, hemispherical photographs, and a small distributed network of snow-depth sensors were analyzed from three different forest treatments in the Sierra Nevada. The first treatment consisted of variable thinning, which reduced the stems per acre by 80%, the basal area per acre by 40%, and created a heterogeneous forest structure with small gaps and clumps of trees of varying density. The second forest treatment reduced the stems per acre and the basal area per acre stand down to similar levels as the variable thinning, however the residual trees were left to replicate a homogenous, even spacing. An unthinned control provided a comparison to pre-treatment conditions. Snow-survey data showed that approximately 10-20% more snow accumulated in the two thinning treatments than in the control. Differences in snow depth between the two thinning treatments were statistically insignificant. Snow-survey results also indicated that the variable treatments kept snow on the ground longer than the even treatments in the 2013 water year, which had a peak snow water equivalent (SWE) of 17.9 cm, and melted out at approximately the same time as the control units. In 2014 the peak SWE recorded was only 6.3 cm, both the variable and even treatments melted out at the same time, and the control units kept a steady snowpack for approximately 1-3 days longer. Data from snow-surveys, distributed light/temperature sensors and snow-depth sensors showed that more snow accumulated in the open than at the drip edge and under the forest canopy, and that melt rates are faster in the open than under the canopy. Our data showed that the variable treatment held the snowpack longer and exhibited a slower melt rate than did the even treatments indicating a possibility to retain substantial amounts of snow in the mountains longer if this thinning structure were to be implemented on a landscape level.