Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Charles T. Driscoll

Second Advisor

Jason D. Fridley

Subject Categories

Engineering

Abstract

Projection of ecosystem functions and biogeochemical cycling of elements under future climate change requires a quantitative understanding of both ecosystem processes and site-specific climate change scenarios. Biogeochemical and ecological studies over the last decades have provided the intellectual basis for these projections, especially at the small watershed scale. Recent developments in biophysical sciences and computationally based meteorology coupled with advanced downscaling techniques have made it possible to project future climate change scenarios at the small watershed scale. Using a biogeochemical model, PnET-BGC, which has been extensively applied to the forest ecosystems in the northeastern United States, the interactive effects of multiple environmental factors on ecosystem function and element dynamics can be investigated.

In this dissertation, I applied PnET-BGC to three ecosystems, including one in Oregon (The H. J. Andrews Experimental Forest) and two in Colorado (Niwot Ridge and Loch Vale Watershed) to evaluate the effects of climate change at the intensively studied watersheds with distinct climate and vegetation type. Results from these three sites were compared and contrasted with projections conducted in the northeastern U.S. using PnET-BGC to identify which sites are vulnerable to future climate change and what factors contribute to this vulnerability. Future climate considered in this study was developed from two radiative forcing scenarios under the Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathways (RCPs). The site specific climate inputs are statistically downscaled from outputs of four general circulation models (GCMs) to drive PnET-BGC. To more accurately depict different types of ecosystems in this study, updated parameters and improved algorithms were incorporated into PnET-BGC, taking advantage of findings from recent studies. This study expands the type of ecosystem from which PnET-BGC is applied. It also provides a basis for future studies on these ecosystems to examine the interactive effects of climate change with other disturbances, such as changes in atmospheric deposition or land disturbance. In this research, I tested the hypotheses that 1) climate change at high elevation watersheds in the western U.S. will result in physiological stress on vegetation that is adapted to its native climate, and alter future dynamics of water, carbon, and nitrogen in these ecosystems; 2) other aspects of global change such as elevated atmospheric CO2 concentrations and an extended growing season will alleviate the impacts of physiological stress on ecosystem function and element dynamics; and 3) ecosystem responses to climate change will vary among the three sites in the western U.S. and are distinct from patterns in the northeastern U.S. due to the differences in vegetation type and site specific current and future climate conditions. This work improved understanding of the effects of climate change on element dynamics and the function of different types of ecosystems. It complements existing literature on response of ecosystem structure and function to future climate change scenarios.

I conducted my research in this dissertation in four phases. In phase one, I applied the model at Watershed 2 in H. J. Andrews Experimental Forest, an old-growth Douglas-fir forest located in the western Cascade Range of Oregon. The model algorithm on calculation of vapor pressure deficit was improved for the Pacific Northwest. Parameters on plant functional traits and soil characteristics were also updated using local observations. Simulation outputs were validated against local observations. Seasonal and long-term projections show large increases in stomatal conductance throughout the year from 1986-2010 to 2076-2100 and increases in leaf carbon assimilation between October and June over the same period, but future dynamics of water and carbon under the RCP scenarios are largely affected by a reduction in foliar biomass resulting from severe air temperature and humidity stress to the forest in summer. Projected future decreases in foliar biomass in the old-growth Douglas-fir forest results in 1) decreases in transpiration and increases in summer and fall soil moisture; 2) decreases in photosynthesis, plant biomass, and soil organic matter under the high radiative forcing scenario; and 3) altered foliar and soil stoichiometry of carbon to nitrogen.

In phase two, I developed the first alpine tundra version of PnET-BGC and applied the model at the Saddle of Niwot Ridge in Colorado. Projections indicate that in the future this watershed will become more energy-limited on an annual basis, and the seasonal distribution of the water supply will become decoupled from energy inputs due to advanced snowmelt, causing soil moisture stress to plants during the growing season. The model simulations suggest that future shortened snow-covered periods may cause decreases in winter soil decomposition by 9% to 16% due to limitations in subnivean microbial activity; while the associated extended growing season is projected to result in only slight decreases in carbon sequestration of 8% under the high radiative forcing scenario, despite a 33% reduction in leaf production due to the soil moisture stress. The analyses demonstrate that future nitrogen uptake by alpine plants is regulated by nitrogen supply from mineralization, but plant nitrogen demand may also affect plant uptake under the aggressive RCP8.5 scenario. In addition, PnET-BGC simulations suggest that potential CO2 fertilization effects on alpine plants are projected to cause larger increases in concentrations of non-structural carbohydrates and lipids than leaf and root production.

In phase three, PnET-BGC model was applied at Loch Vale watershed, a subalpine forest near Niwot Ridge in the southern Rocky Mountains of Colorado. Necessary improvements of the model were made on processes that are important in subalpine forests but negligible in other ecosystems such as soil evaporation. The analyses using the Budyko curve suggest that future evapotranspiration may become more water-limited in the subalpine forest. From 1986-2010 to 2076-2100, evapotranspiration increases at the start and end of the growing season. Recurring plant soil moisture stress is projected between July and September which reduces foliar biomass by 5% to 16%. However, the annual rate of photosynthesis and wood biomass are projected to increase by up to 29% and 76%, respectively, due to the increasing temperature under the RCP8.5 scenario. Unexpectedly, an extended growing season had little contribution to the dynamics of water and carbon. Fertilization by elevated atmospheric CO2 concentrations is projected to result in 16% to 27% higher rates of annual photosynthesis under RCP4.5 and RCP8.5 scenarios, respectively, and increasing carbon accumulation in wood biomass.

In the fourth phase, I conducted a cross-site analysis of the three western sites in Oregon and Colorado with Hubbard Brook Experimental Forest in New Hampshire which was simulated in a previous study. Various ecosystem responses from the four sites under the RCP scenarios were attributed to the differences in vegetation type and site specific current and future climate conditions. Projections in the western and northeastern U.S. suggest water-use efficiency and soil water holding capacity may largely determine the type of physiological stress that plants experience in the future, while foliar retention time and wood turnover rate may largely affect the storage and decomposition of soil organic matter in forest ecosystems. Although foliar nitrogen contents have large variation among the four sites, their future changes were not projected to be large in any sites, therefore having little impact on carbon or water dynamics of the watersheds. Projections also suggest future increases in temperature may impact ecosystem and biogeochemical processes to a smaller extent during the winters of alpine tundra ecosystems than other seasons and sites in which the temperature is above or close to freezing. An extended growing season was projected in all sites under the RCP scenarios, but showed distinct impacts on ecosystem functions at different sites. Potential CO2 fertilization effects on carbon dynamics were mainly manifested in enhanced wood growth from forest ecosystems but result in large increases in non-structural carbohydrates in the alpine tundra ecosystem.

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