The Harte lab studies the effects of human actions on, and the linkages among, biogeochemical processes, ecosystem structure and function, biodiversity, and climate. Research spans a range of scales from plot to landscape to global, and utilizes field investigations and mathematical modeling. A long term goal of the group is to understand the dependence of human well being on the health of ecosystem processes.  

Two broad areas of research are currently under investigation.

1.       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 our 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.
  

•       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 alone.
  

•       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 community
          composition and 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.

4.       Investigating how vegetation change and erosion events influence carbon sequestration and the surface energy balance in the Marin           headlands of California.

Some recent publications from the Harte lab on climate-ecosystem linkages are:

J. Klein, Zh. Xinquan, and J. Harte, "Climatic and Grazing Controls on Vegetative
Aboveground Biomass: Implications for the Rangelands on the North-eastern Tibetan
Plateau". In Yak Production in Central Asian Highlands, pp. 21 - 35, edited by H. Jianlin, C. Richard, O. Hanotte, C. McVeigh and J.E.O. Rege. International Livestock Research Institute, Nairobi Kenya, (2003).

T. Perfors, J. Harte, and S. Alter, "Enhanced growth of sagebrush (Artemisia tridentata)
in response to manipulated ecosystem warming. Global Change Biology 9: 736-
742(2003).

F. Saavedra, D. Inouye, M. Price and J. Harte, "Changes in Flowering and Abundance of Delphinium nuttalianum (Ranunculaceae) in Response to a Subalpine Climate Warming Experiment" Global Change Biology 9: 885-894 (2003).

J. Weltzin, M. Loik, S Schwinning, D. Williams, P. Fay, B. Haddad, J. Harte, T. Huxman,
A. Knapp, G. Liu, W. Pickman, M. Shaw, E. Small, M. Smith, S. Smith, D. Tissue, and J. Zak, "Assessing the Response of Terrestrial Ecosystems to Potential Changes in Precipitation", Bioscience 53(10):941-952(2003).

J. Dunne, S. Saleska, M. Fischer, and J. Harte, "Integrating Experimental and Gradient Methods in Ecological Climate Change Research", Ecology 85(4):904-916(2004).

M. Loik, C. Still, T. Huxman, and J. Harte, "In Situ Photosynthetic Freezing Tolerance for Plants Exposed to a Global Warming Manipulation in the Rocky Mountains, Colorado, USA". New Phytologist 162: 331-341 (2004).

L. Kueppers, J. Southon, P. Baer, and J. Harte, "Dead Wood Biomass and Turnover Time, Measured by Radiocarbon, along a Subalpine Elevation Gradient". Oecologia 141: 641-651 (2004).

L. Kueppers, P. Baer, J. Harte, B. Haya, L. Koteen, and M. Smith, "A Decision Matrix Approach to Evaluating the Impacts of Land-Use Activities Undertaken to Mitigate Climate Change", Climatic Change 63: 247-257(2004).

J. Klein, J. Harte, and Zh Xinquan, "Experimental Warming Causes Large and Rapid Species Loss, dampened by Simulated Grazing, on the Tibetan Plateau". Ecology Letters 7: 1170-1179 (2004).

T. Huxman, M. Smith, P. Fay, A. Knapp, M. Shaw, M. Loik, S. Smith, D. Tissue, J. Zak, J. Weltzin, W. Pockman, O. Sala, B. Haddad, J. Harte, G. Koch, S. Schwinning, E. Small, and D. Williams. "Convergence across Biomes to a Common Rain-use Efficiency", Nature 429: 651-654(2004).

J. Klein, J. Harte, and Zh Xinquan, "Dynamic and Complex Microclimate Responses to Warming and Grazing Manipulations". Global Change Biology 11: 1440-1451 (2005).

2.       The Distribution and Abundance of Species

We have previously shown that the single fundamental assumption of self-similarity in the distribution of species across a landscape is mathematically equivalent to an empirical relationship in ecology, the widely cited and tested power-law form of the species-area relationship (SAR). The SAR relates the number of species found in a patch of habitat to the area of that patch and the power-law form expresses the relationship with the equation S=cAz.

We also shown that under specified assumptions self similarity leads to:

•       A "commonality" formula that describes the fraction of species in common to two patches as a function of patch size and
        inter-patch distance.
  
  
•       An endemics-area relationship characterizing the dependence of the number of species unique to patch on the area of that patch.
  
  
•       A formula for the dependence of species richness on the shape of censused patches. 
  
  
•       A unique species-abundance distribution.
  
  
•       A power-law relationship between species range and abundance.
  
  
•       The dependence on abundance of an aggregation index describing how clumped are the individuals within each species.

  The theory of self-similarity thus provides an overarching framework linking numerous ecological relationships to one another. However, a power law behavior for the SAR or for the other measures of spatial pattern listed above is only observed at best over limited scale ranges and for some, but not all, taxa and habitats. Recently, the group has been developing a more comprehensive theory of spatial pattern in ecology that can encompass the limiting case of self similarity but also illuminates the circumstances under which power-law behavior is not expected. In this new theory, knowledge of the species-abundance distribution completely determines the shape of the SAR and all of the other patterns itemized above. Tests of this more comprehensive theory using data from a serpentine grassland system in California and wet and a dry tropical forest sites in Central America are underway. Ongoing work seeks to derive the fundamental statistical assumption of the theory from a dynamical model describing birth, death, and dispersal of individuals.

Some recent publications from the Harte lab on the distribution and abundance of species are:

A. Ostling, J. Harte, J. Green and A. Kinzig, "A Community-Level Fractal Property Produces Power-Law Species-Area Relationships", Oikos 103: 218-224 (2003).

J. Green, J. Harte, and A. Ostling, "Species Richness, Endemism, and Abundance
Patterns: Tests of Two Fractal Models in a Serpentine Grassland" Ecology Letters
6:919-928(2003)

J. Harte, "Tail of Death and Resurrection", Nature 424:1006-1007(2003)

A. Ostling, J. Harte, J. Green, and A. Kinzig, "Self Similarity, the Power Law Form of the Species-Area Relationship, and a Probability Rule: A Reply to Maddux". American Naturalist 163: 627-633(2004)

J. Harte, "The Value of Null theories in Ecology". Ecology 85(7):1792-1794(2004).

R. Krishnamani, A.Kumar, and J.Harte, "Estimating Species Richness at Large Spatial Scales Using Data from Discrete Plots". Ecography 27: 637-642(2004).

Roy, B., Gusewell, S. and Harte, J. "Response of Plant Pathogens and Herbivores to a Warming Experiment" Ecology 85(9): 2570-2581(2004)

J. Harte, A. Ostling, J. Green, and A. Kinzig, "Climate Change and Extinction Risk". Nature
02718; doi:10.1038(2004).

J. Harte, E. Conlisk, A.Ostling, J. Green, A. Smith, "A theory of Spatial Structure in Ecological
Communities at Multiple Spatial Scales". Ecological Monographs 75(2): 179-197(2005).