Dynamics of water, carbon, and nitrogen in forest and alpine tundra ecosystems in the Pacific Northwest and the Rocky Mountains of the U.S. under future climate change

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

Responses and adaptation strategies of terrestrial ecosystems to climate change

Terrestrial ecosystems are likely to be affected by climate change, as climate change-induced shift of water and heat stresses patterns will have significant impacts on species composition, habitat distribution, and ecosystem functions, and thereby weaken the terrestrial carbon (C) sink and threaten global food security and biofuel production. This thesis investigates the responses of terrestrial ecosystems to climate change and is structured in four main chapters.;The first chapter of the thesis is directed towards the impacts of snow variation on ecosystem phenology. Variations in seasonal snowfall regulate regional and global climatic systems and vegetation growth by changing energy budgets of the lower atmosphere and land surface. We investigated the effects of snow on the start of growing season (SGS) of temperate vegetation in China. Across the entire temperate region in China, the winter snow depth increased at a rate of 0.15 cm•yr-1 (p=0.07) during the period 1982-1998, and decreased at a rate of 0.36 cm•yr-1 (p=0.09) during the period 1998-2005. Correspondingly, the SGS advanced at a rate of 0.68 d•yr-1 (p\u3c0.01) during 1982 to 1998, and delayed at a rate of 2.13 d•yr-1 (p=0.07) during 1998 to 2005, against a warming trend throughout the entire study period of 1982-2005. Spring air temperature strongly regulated the SGS of both deciduous broad-leaf and coniferous forests; whilst the winter snow had a greater impact on the SGS of grassland and shrubs. Snow depth variation combined with air temperature contributed to the variability in the SGS of grassland and shrubs, as snow acted as an insulator and modulated the underground thermal conditions. Additionally, differences were seen between the impacts of winter snow depth and spring snow depth on the SGS; as snow depths increased, the effect associated went from delaying SGS to advancing SGS. The observed thresholds for these effects were snow depths of 6.8 cm (winter) and 4.0 cm (spring). The results of this study suggest that the response of the vegetation\u27s SGS to seasonal snow change may be attributed to the coupling effects of air temperature and snow depth associated with the soil thermal conditions.;The second chapter further addresses snow impacts on terrestrial ecosystem with focus on regional carbon exchange between atmosphere and biosphere. Winter snow has been suggested to regulate terrestrial carbon (C) cycling by modifying micro-climate, but the impacts of snow cover change on the annual C budget at the large scale are poorly understood. Our aim is to quantify the C balance under changing snow depth. Here, we used site-based eddy covariance flux data to investigate the relationship between snow cover depth and ecosystem respiration (Reco) during winter. We then used the Biome-BGC model to estimate the effect of reductions in winter snow cover on C balance of Northern forests in non-permafrost region. According to site observations, winter net ecosystem C exchange (NEE) ranged from 0.028-1.53 gC•m-2•day-1, accounting for 44 +/- 123% of the annual C budget. Model simulation showed that over the past 30 years, snow driven change in winter C fluxes reduced non-growing season CO2 emissions, enhancing the annual C sink of northern forests. Over the entire study area, simulated winter ecosystem respiration (Reco) significantly decreased by 0.33 gC•m-2•day -1•yr-1 in response to decreasing snow cover depth, which accounts for approximately 25% of the simulated annual C sink trend from 1982 to 2009. Soil temperature was primarily controlled by snow cover rather than by air temperature as snow served as an insulator to prevent chilling impacts. A shallow snow cover has less insulation potential, causing colder soil temperatures and potentially lower respiration rates. Both eddy covariance analysis and model-simulated results showed that both Reco and NEE were significantly and positively correlated with variation in soil temperature controlled by variation in snow depth. Overall, our results highlight that a decrease in winter snow cover restrains global warming through emitting less C to the atmosphere.;The third chapter focused on assessing drought\u27s impact on global terrestrial ecosystems. Drought can affect the structure, composition and function of terrestrial ecosystems, yet the drought impacts and post-drought recovery potential of different land cover types have not been extensively studied at a global scale. Here, we evaluated drought impacts on gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE) of different global terrestrial ecosystems, as well as the drought-resilience of each ecosystem type during the period of 2000 to 2011. We found the rainfall and soil moisture during drought period were dramatically lower than these in non-drought period, while air temperatures were higher than normal during drought period with amplitudes varied by land cover types. The length of recovery days (LRD) presented an evident gradient of high (\u3e 60 days) in mid- latitude region and low (\u3c 60 days) in low (tropical area) and high (boreal area) latitude regions. As average GPP increased, the LRD showed a significantly decreasing trend among different land covers (R 2=0.53, p\u3c0.0001). Moreover, the most dramatic reduction of the drought-induced GPP was found in the mid-latitude region of north Hemisphere (48% reduction), followed by the low-latitude region of south Hemisphere (13% reduction). In contrast, a slightly enhanced GPP (10%) was showed in the tropical region under drought impact. Additionally, the highest drought-induced reduction of ET was found in the Mediterranean area, followed by Africa. The water use efficiency, however, showed a pattern of decreasing in the north Hemisphere and increasing in the south Hemisphere.;The last chapter compared the differences of performance in trading water for carbon in planted forest and natural forest, with specific focus on China. Planted forests have been widely established in China as an essential approach to improving the ecological environment and mitigating climate change. Large-scale forest planting programs, however, are rarely examined in the context of tradeoffs between carbon sequestration and water yield between planted and natural forests. We reconstructed evapotranspiration (ET) and gross primary production (GPP) data based on remote-sensing and ground observational data, and investigated the differences between natural and planted forests, in order to evaluate the suitability of tree-planting activity in different climate regions where the afforestation and reforestation programs have been extensively implemented during the past three decades in China. While the differences changed with latitude (and region), we found that, on average, planted forests consumed 5.79% (29.13mm) more water but sequestered 1.05% (-12.02 gC m-2 yr -1) less carbon than naturally generated forests, while the amplitudes of discrepancies varied with latitude. It is suggested that the most suitable lands in China for afforestation should be located in the moist south subtropical region (SSTP), followed by the mid-subtropical region (MSTP), to attain a high carbon sequestration potential while maintain a relatively low impact on regional water balance. The high hydrological impact zone, including the north subtropical region (NSTP), warm temperate region (WTEM), and temperate region (TEM) should be cautiously evaluated for future afforestation due to water yield reductions associated with plantations

Ecosystem modeling to understand global change effects to terrestrial and fresh water systems

2013 Spring.Includes bibliographical references.Concurrent changes in climate, atmospheric nitrogen (N) and sulfur (S) deposition, and increasing levels of atmospheric carbon dioxide (CO2) affect ecosystems in complex ways. Atmospheric deposition of S and N species have the potential to acidify terrestrial and aquatic ecosystems, but nitrate and ammonium are also critical nutrients for plant and microbial productivity and are a potential cause of eutrophication. Climate change and CO2 fertilization, with or without changes in N deposition, may affect rates of plant growth, water availability, soil organic matter decomposition rates, and net greenhouse gas flux. I developed a non-spatial biogeochemical model to simulate soil and surface water chemistry by linking the daily version of the CENTURY ecosystem model (DayCent) with a low temperature aqueous geochemical model, PHREEQC. The coupled model, DayCent-Chem, simulates the daily dynamics of plant production, soil organic matter, cation exchange, mineral weathering, elution, stream discharge, and solute concentrations in soil water and stream flow. The model was first validated against a rich data set for an alpine watershed in Rocky Mountain National Park, then for seven other forested montane and alpine watersheds in the United States. I modeled how much nitrogen deposition it takes to acidify an alpine watershed, and whether the rate at which deposition increases matters. I also used the model to investigate the combined effects of N deposition, warming, and increasing CO2 over the period 1980-2075 at seven forested montane and two alpine watersheds by looking at changes to net ecosystem production, soil organic C, soil nitrous oxide (N2O) emissions, and stream nitrate. I found that N was the main driver of change to net ecosystem greenhouse gas flux with warming and CO2 fertilization playing lesser roles. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change. Both the ecological response and the hydrochemical response to these human-caused drivers of global change are of interest to scientists as well as regulatory and land management agencies. This model is appropriate for accurately describing the ecosystem and surface water chemical response of small watersheds to atmospheric deposition and climate change

Adding tree rings to North America's National Forest Inventories: an essential tool to guide drawdown of atmospheric CO2

Tree-ring time series provide long-term, annually resolved information on the growth of trees. When sampled in a systematic context, tree-ring data can be scaled to estimate the forest carbon capture and storage of landscapes, biomes, and-ultimately-the globe. A systematic effort to sample tree rings in national forest inventories would yield unprecedented temporal and spatial resolution of forest carbon dynamics and help resolve key scientific uncertainties, which we highlight in terms of evidence for forest greening (enhanced growth) versus browning (reduced growth, increased mortality). We describe jump-starting a tree-ring collection across the continent of North America, given the commitments of Canada, the United States, and Mexico to visit forest inventory plots, along with existing legacy collections. Failing to do so would be a missed opportunity to help chart an evidence-based path toward meeting national commitments to reduce net greenhouse gas emissions, urgently needed for climate stabilization and repair.Published versio

Management and site effects on carbon balances of European mountain meadows and rangelands

We studied carbon balances and carbon stocks of mountain rangelands and meadows in a network of 8 eddy covariance sites and 14 sites with biomass data in Europe. Net ecosystem exchange of pastures and extensively managed semi-natural rangelands were usually close to zero, while meadows fixed carbon, with the exception of one meadow that was established on a drained peatland. When we accounted for off-site losses and inputs also the carbon budget of meadows approached zero. Soil carbon stocks in these ecosystems were high, comparable to those of forest ecosystems, while carbon stocks in plant biomass were smaller. Since soil carbon stocks of abandoned mountain grasslands are as high as in managed ecosystems, it is likely that the widespread abandonment of mountain rangelands used currently as pastures will not lead to an immediate carbon sink in those ecosystems

Response of European Mountain Forests to Abrupt Climate Change

Paleoclimatic data reveals large abrupt climate changes such as the Younger Dryas and the 8.2 ka event. Alpine forests are characterized by distinct ecotonal borderlines and their vulnerability to such events is potentially high, although there are hardly any assessments made up to now. Both paleo-records and climate-model projections are used to conduct a model simulation with the forest ecosystem model PICUS v1.3. Impacts on Norwegian spruce-dominated mountain forests along an altitudinal transect are investigated. Ecosystem-specific thresholds are identified, the potential magnitude of loss is quantified and possible negative feedbacks to the carbon cycle are assessed

Synthesis of Knowledge: Fire History and Climate Change

This report synthesizes available fire history and climate change scientific knowledge to aid managers with fire decisions in the face of ongoing 21st century climate change. Fire history and climate change (FHCC) have been ongoing for over 400 million years of Earth history, but increasing human influences during the Holocene Epoch have changed both climate and fire regimes. We describe basic concepts of climate science and explain the causes of accelerating 21st century climate change. Fire regimes and ecosystem classifications serve to unify ecological and climate factors influencing fire, and are useful for applying fire history and climate change information to specific ecosystems. Variable and changing patterns of climate-fire interaction occur over different time and space scales that shape use of FHCC knowledge. Ecosystem differences in fire regimes, climate change, and available fire history mean that using an ecosystem-specific view will be beneficial when applying FHCC knowledge