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Chapter 6. Center for the Study of Earth from Space (CSES), 1985-2002

Ecosystem Science at Landscape and Regional Scales

Both natural and anthropogenic disturbances drive the dynamics and structure of natural systems and their potential to respond to climate and other large-scale environmental changes. Perturbations through physical (e.g. conversion to agriculture) or chemical (nitrogen deposition) change can alter natural systems in composition and/or functioning. The goal of Wessman's lab is to contribute to the understanding of (i) the dynamic (two-way) links between landscape/regional spatial heterogeneity and biogeochemical processes, and (ii) how perturbations can influence and perhaps alter trajectories of natural system processes. The group's approach is to link extensive field work with remote sensing methodologies that characterize temporal and spatial heterogeneity in natural ecosystems, and ultimately incorporate these data in landscape and ecosystem simulation models to estimate biogeochemical flux rates.

Wessman's group has helped define the role of remote sensing as a principal means to extrapolate field-measured variables based on structural heterogeneity of vegetation and the landscape. Contributions have been in two main areas, use of radiative transfer model inversion to estimate land surface properties, and in hyperspectral remote sensing. Ann Bateson (CSES) and Wessman have been pioneers in the study of nonlinearities in spectral mixture analysis and natural variation in reflectance of landscape components. Other work by students has focused on biogeochemical processes in disturbed systems and the pattern-to-process connections that may assist large-scale assessment through remote sensing. Specific projects include the following:

Carbon and Nitrogen Dynamics in Arid and Semi-arid Regions

Dryland ecosystems store 30 percent of global soil organic carbon (C), cover half the terrestrial surface, support 20 percent of the human population, and produce 70 percent of the world's livestock. Exponential growth of the human population accompanied by fire suppression and intensification of land uses over the past century has led to dramatic and widespread increases in tree and shrub cover in what were once primarily grassland ecosystems. Under NASA funding within the Earth Observing System Interdisciplinary Science and Land-Cover/Land-Use Change Programs, Wessman and colleagues at NCAR, Texas A&M and Carnegie-Stanford have been particularly concerned with measurements of structural and biophysical attributes which will most effectively link to underlying ecosystem processes, particularly in association with contrasting land management practices. The team has developed novel and viable approaches for coupling field data, isotope biogeochemistry, remote sensing, and modeling to quantify the impact of woody plant encroachment on aboveground biomass and carbon and nitrogen pools and primary production at spatially complex local and regional scales. Techniques in multi-angular and hyperspectral data inversions were developed by Greg Asner (Ph.D. 1997) to quantify canopy characteristics (e.g. leaf area index) and related ecosystem processes (e.g. aboveground carbon allocation) of rangelands in Texas. Andrew Hudak (Ph.D. 1999) combined isotope techniques, spectral mixture analysis and historical photo analyses to provide insights into South African woody plant encroachment rates and extent, and suggest consequent changes in carbon dynamics as grasslands transition to woodlands.

Disturbance Impact on Biogeochemical Cycling

It naturally follows that disturbance to an ecosystem will disrupt biogeochemical pathways and cycling. However, the extent and duration of the alterations are often poorly known and the implications for the long-term integrity of the system can be difficult to predict. Jon Carrasco (Ph.D. 2002) determined that the litter chemistry, and therefore the soil organic matter chemistry, of two different montane forest types (lodgepole pine and trembling aspen) results in different patterns of carbon and nitrogen cycling and transport. Increased nitrogen deposition to the Front Range montane forest may cause significant alteration of the ambient cycling and transport patterns for these forest types. Cristina Rumbaitis-del Rio (Ph.D. 2003) currently studies the impacts on biogeochemical cycling and forest stand structure of massive inputs of wood-based carbon resulting from windthrow, and the subsequent effects of management practices such as salvage logging. Her work will help elucidate the connections between forest stand structure and processes of response to disturbance; i.e. forest resilience and the regeneration process. Nancy Golubiewski (Ph.D. 2002) studied the integrated effects of urban/surburban development on carbon sequestration, phenology and aboveground net primary productivity within the Colorado Front Range. The process of urbanization in this semi-arid region appears to increase regional primary production above that of the pre-developed system, but the trajectory of long-term carbon storage is less clear.

Arctic Temperature Regulation by Convective Processes over Open Ocean

An observed lower extreme of mid-tropospheric (500 millibar) temperatures in the Arctic of approximately 45 degrees Celsius during the winter season is the basis for a hypothesized regulatory mechanism for high latitude temperature. The coldest air masses in the Arctic reach 45 degrees Celsius during the fall months but seldom get much colder even into late winter despite a continued net radiative loss. Chase's research at CSES has demonstrated that mid-tropospheric temperatures are significantly skewed toward warmer temperatures indicating a regulatory mechanism at work. Preliminary evidence indicates that moist convective processes controlled to a large extent by high latitude sea surface temperatures are responsible for this regulation of minimum temperature. This research is pertinent to global change issues because climate model simulations of the effects of increased greenhouse gasses hypothesize that high, northern latitude regions should warm at a faster rate than the globe as a whole and mostly in winter; a hypothesis that does not appear to have strong observational support at present.

Climatic Effects of Global-scale Vegetation Changes

Landcover changes due to human activity have large and demonstrable effects on both regional and global climate. However, research attempting to quantify accurately the feedbacks between landcover change and a changing climate system is in its early stages. Vegetation affects the surface energy balance, the hydrological, carbon and other biogeochemical cycles, generates aerosols and affects soil structure, all of which are interacting systems. Present research in CSES includes modeling the effects of observed changes in vegetation on the global climate and attempting to detect those effects in observational data. Past model simulation research is being extended to interactions with other natural and human-caused effects on the climate system including changes in atmospheric composition (carbon dioxide increases) or changes in circulation due to El Niño/La Niña cycles. Chase and Gupta are also working on the re-interpretation of results from the most simple to the most complex Earth-system models and exploring how results from differing models can be used in conjunction to better understand feedbacks between the biosphere, the hydrological cycle and other components of the Earth system.

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