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John Harte The Effects of Climate Warming on Ecosystem Processes Global climate change may alter terrestrial systems on local to landscape scales in ways that, in aggregate, can affect climate. Such potential feedbacks include a climate-induced shift in the amount of carbon sequestered in terrestrial ecosystems, a change in the rate of methane consumption by methanogenic bacteria, and alteration of the land surface albedo as a consequence of altered plant species composition. The goal of this research is to improve ability to reliably forecast the sign and magnitude of such feedbacks across a range of spatial and temporal scales. Our empirical strategy consists of combining experimental field manipulations with observations along landscape-scale natural climatic gradients. Combined with relatively simple mechanistically-based models, manipulation and gradient analyses provide a basis for understanding short and long term effects of climate change over a range of spatial scales. Our study sites consist of montane meadows and conifer forest on the Western Slope of the Colorado Rockies and montane grazing land in the Tibetan Plateau.
Our research has shown that: There are significant physical, biogeochemical, and biotic responses of montane meadow ecosystems to manipulated climate change. Some of these responses result in feedback effects, which on a larger scale could further alter climate. Many ecological responses to manipulated climate are transient, species-specific, and/or contingent on ambient annual climate. The combination of manipulation experiments and analysis of patterns along natural climate gradients provides a useful means of understanding ecosystem responses to climate change on temporal and spatial scales longer than that accessible under manipulation experiments.
Our findings refute the naive notion that a simple "space-for-time" substitution allows prediction of ecological responses to climate change based solely on observation of spatial patterns of ecological variables along climate gradients. At the same time, however, our combined observational, manipulative, and theoretical approach is pointing the way to how such gradient analyses can be usefully exploited to predict both short- and long-term ecosystem responses to climate change.
Future research will extend these insights into ecosystem-climate feedback and scaling along several directions: 1. Extending radiometric data on plant species albedo to landscape and regional scales and estimating the importance of "plant-albedo"feedback to global warming. 2. Extending results to additional habitat types and testing our understanding of how climate and other factors control the quantity of carbon stored as soil organic matter. 3. Understanding how biodiversity modulates ecosystem response to climate change and how climate change will affect biodiversity (see following item).
Biodiversity and Climate Change. Anthropogenic climate warming and loss of biodiversity are two major concerns of our time. At their interface lie three critical questions: How will climate warming alter the diversity of life on earth? What is the relationship between ambient species diversity and ambient ecosystem processes? What is the relationship between ambient species diversity and the magnitude and direction of ecosystem responses to climate warming? These questions are important because of our intrinsic and economic concern for the preservation of diverse forms of life on earth, because biological effects of climate warming have the potential to generate feedbacks to the climate system, and because of inherent interest in understanding the role of ecological diversity and the relation between diversity and stability. Our research is providing insight into each of these questions for subalpine meadow ecosystems. By combining investigations of ecosystem responses to a long-term climate manipulation experiment and to interannual climate variability with observations of patterns of ecosystem characteristics along natural climate gradients, responses of ecosystems to climate change are studied at a variety of time scales and under a variety of environmental contingencies. Biodiversity, as measured by the species richness of plants and arthropods, is studied as both a factor that may contribute to the resilience of ecosystem functions under climate change and as an ecosystem characteristic that may be altered by climate change. Our work is carried out in subalpine meadow habitat at four sites that have been instrumented for automated microclimate measurement and at which we have a rich data base characterizing vegetation phenology, community structure and above ground biomass, the species abundance and distribution of arthropods, and a variety of biogeochemical measures including soil carbon and nitrogen and nitrogen turnover.
The Distribution and Abundance of Species We have previously demonstrated the empirical success of a simple statistical theory (HEAP: Harte et al., 2005, Ecological Monographs 75, 179-197) of patterns in the distribution and abundance of species across multiple spatial scales in natural ecosystems. The theory predicts, with no adjustible parameters,
Current research focuses on understanding the mechanisms that generate the fundamental statistical assumption on which the theory is based. Two leading distinct candidates for the underlying mechanism are 1. the domination of ecological community assembly by dispersal limitation rather than by habitat or niche limitations, and 2. the "maximum entropy principle" of Shannon and Jaynes combined with simple constraints on the average abundance of species and the relationship between size and abundance of individuals within species. The latter approach has the advantages that it predicts one additional macroecological metric, the relative abundance distribution of species, and also at finer spatial scales its predictions for the metrics listed above deviate slightly from, and improve on those of, the HEAP theory.
Mutualism and Competition Between Plants and Microorganisms Theoretical ecological are continuing on the biogeochemical and evolutionary
implications of the simultaneous mutualism and competition that exists between
plants and microorganisms. The research to date reveals that across a diversity
of ecosystems, patterns of nitrogen allocation among microbial, vegetative,
and nonliving pools can be readily explained by assuming that microorganisms
allocate nitrogen in such a way as to promote plant nitrogen uptake at a
rate that optimizes microbial biomass. Although this allocation strategy
appears altruistic and evolutionarily impossible in a homogenous environment,
in that microbial consumption of nitrogen at higher rates would increase
microbial reproduction, an evolutionary model of the plant-microbe system
in a spatially heterogeneous environment yields this optimum strategy. |