Steven C. Wofsy

Biosphere/atmosphere CO2 exchange in tundra ecosystems: Community characteristics and relationships with multispectral surface reflectance

The spatial and temporal patterns of many of the factors controlling CO2 exchange are related to characteristics of the vegetated surface which can potentially be monitored using multispectral remote sensors. Realization of this potential depends, in part, on an improved understanding of ecosystem processes and their relationship to variables which are accessible to remote sensing techniques. We examined these relationships using portable, climate-controlled, instrumented enclosures to measure CO2 exchange rates in selected tundra sites near Bethel, Alaska. Rates were related to vegetation community type and climatic variables. Exchange rates in enclosures were compared to exchange measurements obtained by eddy correlation on a 12-m micrometeorological tower. For an average light input of 37 einsteins/day during 20 midsummer days, the empirically modeled exchange rate for a representative area of vegetated tundra was 1.2 ±1.1 (95% confidence interval) g CO2 m−2 d−1. This was comparable to a tower measured exchange over the same time period of 1.1 ±1.1 (95% confidence interval) g CO2 m−2 d−1. Net exchange in response to varying light levels was compared for two major community types, wet meadow and dry upland tundra, and to the net exchange measured by the micrometeorological tower technique. Portable radiometers were used to measure the multispectral reflectance properties of the sites. These properties were then related to exchange rates with the goal of providing a quantitative foundation for the use of satellite remote sensing to monitor biosphere/atmosphere CO2 exchange in the tundra biome. The Normalized Difference Vegetation Index (NDVI) and the near-infrared/red reflectance ratio (SR) computed from surface reflectance were strongly correlated with net CO2 exchange for both upland and wet meadow vegetation. However, the form of the relationship was distinct from measured correlations in other ecosystems, suggesting that global surveys may require adjustment for geographical differences in exchange processes.

Detection of long-term trends in carbon accumulation by forests in Northeastern U. S. and determination of causal factors: Final report

The overall project goal was to quantify the trends and variability for Net ecosystem exchange of CO{sub 2}, H{sub 2}O, and energy by northeastern forests, with particular attention to the role of succession, differences in species composition, legacies of past land use, and disturbances. Measurements included flux measurements and observations of biomass accumulation using ecosystem modeling as a framework for data interpretation. Continuation of the long-term record at the Environmental Measurement Site (EMS) Tower was a priority. The final quality-assured CO{sub 2}-flux data now extend through 2010. Data through 2011 are collected but not yet finalized. Biomass observations on the plot array centered on the tower are extended to 2011. Two additional towers in a hemlock stand (HEM) and a younger deciduous stand (LPH) complement the EMS tower by focusing on stands with different species composition or age distribution and disturbance history, but comparable climate and soil type. Over the period since 1993 the forest has added 24.4 Mg-C ha{sup -1} in the living trees. Annual net carbon uptake had been increasing from about 2 Mg-C ha{sup -1}y{sup -1} in the early 1990s to nearly 6 Mg-C ha{sup -1}y{sup -1} by 2008, but declined in 2009-2010. We attribute the increasing carbon uptake to a combination of warmer temperatures, increased photosynthetic efficiency, and increased influence by subcanopy hemlocks that are active in the early spring and late autumn when temperatures are above freezing but the deciduous canopy is bare. Not all of the increased carbon accumulation was found in woody biomass. Results from a study using data to optimize parameters in an ecosystem process model indicate that significant changes in model parameters for photosynthetic capacity and shifts in allocation to slow cycling soil organic matter are necessary for the model to match the observed trends. The emerging working hypothesis is that the pattern of increasing carbon uptake over the early 2000s represents a transient pulse that will eventually end as decomposition of the accumulated carbon catches up.

Simulating Peatland Methane Dynamics Coupled to a Mechanistic Model of Biogeochemistry, Hydrology, and Energy: Implications to Climate Change

Northern peat lands have a strong potential to modify climate through changes in soil organic carbon (SOC) and methane (CH4). A dynamic interaction among climate, soil physical properties (e.g., temperature and moisture), and biogeochemistry (e.g., quantity and quality of SOC) determines the peat land system. Due to this interaction, CH4 production, oxidation, and transport dynamics changes dramatically under climate change. To appropriately study the future CH4 in a predictive manner, a simulation model must be able to reproduce the inherent dynamic interaction in the peat land system. Here, these complex interactions were simulated simultaneously in a biogeochemical peat land model coupled with mechanistic soil hydrology and thermal dynamics (ED2.0-peat). The model successfully reproduced soil physical profiles and the resultant SOC and CH4 observed in a poor fen of northern Manitoba. With an experimental simulation of 4°C warming, a significant long-term decline in CH4 emission was found, caused by a loss in substrate and prevalence of aerobic conditions. However, there was a transient increase in CH4 emission shortly after warming because of time lag between the temperature dependence of microbial activity (a fast response to climate change) and the loss in peat depth (a slow response). 17 Climate Change and Variability 328