Our research program consists of three integrated studies designed to: (1) develop an understanding of the abiotic and biotic controls on forest distribution and productivity as a basis for assessing potential vegetation change for a range of projected climate scenarios; (2) project a range of possible scenarios of future climate change for the Colorado Rockies; and (3) evaluate potential responses of hydrologic and aquatic ecosystem processes to climate change at watershed, drainage basin and regional scales. The ongoing synthesis of these studies is assessing the interaction between land-use change, regional vegetation distribution, mesoscale climate, and hydrology. Our goal is to develop a better understanding of regional climate and hydrologic patterns and of species-environment relationships to determine which species and ecosystem processes are most sensitive to rapid environmental change. Large, natural areas (e.g., National Parks), are ideal outdoor laboratories for this work, and research results are immediately useful in resource management, interpretation, and public outreach.

Evidence of Vegetation Change

• Krummholz (wind-trimmed low-growing trees) in the forest-tundra ecotone of Rocky Mountain National Park is growing vertically at an average rate of about 1 m per 27 yrs, and, if this continues unabated, krummholz may become patchy forest on certain sites. There is abundant and widespread tree invasion into openings between patches of forest in the so-called "patch forest" zone, below the krummholz zone in the Park. If this invasion persists and the trees continue to grow up, the patch forest zone will become closed, dense forest. This will reduce understory plant diversity and habitat for subalpine/alpine wildlife species. For more information see Baker et al. 1995, Weisberg, P.J. and W.L. Baker. 1995.

• Field studies in Rocky Mountain National Park indicate that forested ecotones (i.e., boundaries) are sensitive to changes in regional climate. Environmental factors (temperature, soil moisture) appear to be more significant in controlling forest distribution than soil characteristics (texture, depth, rockiness). Soil factors will not inhibit changes in vegetation distribution, while shorter term changes in climate could affect vegetation dominance. We established 14 long-term vegetation transects (20m x 200m+) to provide a means to monitor subtle vegetation change (10 year trends) and long-term species shifts (30-100 year trends). Data synthesis and models created over the next few years will provide land managers with a framework for assessing resource management problems in a rapidly changing environment including wildlife habitat loss and the invasion of exotic plant species. For more information see Stohlgren and Bachand 1997, Stohlgren et al. 1997a)
• No simple pattern in total biomass was found in 200- to 400-year-old forests relative to aspect; local topographic position is very important. Current productivity may not be very limited by the N supply, and any human-induced change in the soil N supply (i.e., existing levels of air pollution) may not have a large effect on tree growth in the spruce/fir zone. For more information see Binkley et al. 1997, Olsson et al. 1997).

• Analysis of the effects of climatic variation on fire regimes in the montane zone of the Front Range over the past 400 years demonstrates that fire occurrence is extremely sensitive to climatic variability. During severe droughts that occur approximately once per century, nearly half of the entire montane zone burns during a single year. Fire in the montane zone is likely to be increased due to increased climatic variability. Tree-ring studies show that when years of above average precipitation are followed by springs and summers of below average precipitation there is an increase in the area burned. Fire suppression since approximately 1915 constitutes a major change in Front Range ecosystems related to changes in land use and has created forest structures that are much more susceptible to catastrophic wildfires that are more likely to occur under future scenarios of warmer and/or more variable climates. For more information see Mast 1993, Veblen et al. 1994.

Evidence of Climate Change

• Mesoscale climate models suggest that the Colorado Rockies, with extreme elevation and vegetation gradients, are very sensitive to regional and global climate change. For more information see Copeland et al. 1996a,b.

• Land use practices in the plains of Colorado influence regional climate and vegetation in adjacent natural areas in the Rocky Mountains in predictable ways. Mesoscale climate model simulations, using the Colorado State University Regional Atmospheric Modeling System (RAMS), projected that modifications to natural vegetation in the plains, primarily due to agriculture and urbanization, could produce a regional cooling effect expressed as lower summer temperatures in the mountains (Fig. 1). We corroborate the RAMS simulations with three independent sets of data: (1) climate records from sixteen weather stations, which showed significant trends of decreasing July temperatures in recent decades; (2) the distribution of seedlings of six dominant conifer species in Rocky Mountain National Park, Colorado, which suggested that cooler, wetter conditions occurred over roughly the same time period; and (3) increased hydrologic runoff, normalized for changes in precipitation, during the summer months in four river basins, which also indicated cooler summer temperatures and lower transpiration at landscape scales. Combined, the mesoscale atmospheric/land-surface model, short-term trends in regional temperatures, forest distribution changes, and hydrology data indicate that the effects of land use practices on regional climate may overshadow larger-scale temperature changes commonly associated with observed increases in CO2 and other greenhouse gases. Regional model simulations on monthly time scales demonstrate a significant sensitivity of regional weather, and therefore, climate to land use change. For more information see Pielke et al. 1994, 1997, Stohlgren et al. 1997b.

Evidence of Hydrologic Changes

• When the ground is snow-covered, daytime net radiation is the single most important factor driving snowmelt, and snowmelt drives the hydrologic cycle of the Rocky Mountains and the rivers that drain from them. Once ground is exposed in spring, soil heat fluxes become equally important to melting snow. For more information see Cline 1997a,b.

• RHESSys model structure results are extremely robust when tested against validation data. They point out the importance, however, of a priori knowledge of ecosystem processes and function. Without long-term data and understanding of natural processes, this model (and probably all models) would not present an accurate portrayal of hydro-ecological processes. For more information see Lammers et al. 1997.

• The amount of surficial debris (glacial till) is very important to flowpath length and duration. This will cause two watersheds next to each other to have completely different responses to changing climates or pollutants. Continued work with natural isotope tracers will tell us how long water resides in till, and current estimates are on the order of years to decades. For more information see Sueker 1996.

• Ecosystem/hydrology models suggest land cover change and climate change influence different parts of the Rocky Mountain and adjacent plains in complex ways, depending on location, climate, and topography. Land cover change exerted stronger controls on plant productivity and water fluxes to the atmosphere than temperature changes at lower elevations (foothills and grasslands). In contrast, temperature was a more important influence on water fluxes and plant productivity in higher elevation coniferous forests and tundra. In high elevation watersheds, cooling or warming had a great influence on the timing of snowmelt, and this influenced the amount of soil water available for tundra and forest growth later in the growing season. It did not greatly influence the amount of water released from the snowpack. For more information see Baron et al. 1997a, b, c.

• Paleo-climate records contained in lake sediments provide evidence that biological and geomorphological processes in high elevations of the Rocky Mountains have been very responsive to climate variability in the past 12,000 years. Evidence of changes caused by temperature variability as well as shifts in the amount and seasonality of precipitation are found in debris flow frequency and movement of vegetation up and down an elevational gradient. For more information see Menounos 1994, Reasoner 1996.

SELECTED REFERENCES

Baker, W.L., J.J. Honaker, and P.J. Weisberg. 1995. Using aerial photography and GIS to map the forest-tundra ecotone in Rocky Mountain National Park, Colorado, for global change research. Photogrammetric Engineering & Remote Sensing 61: 313-320.

Baron J, Ojima DS, Hartman MD, Kittel TGF, Lammers RB, Band LE, Pielke, RA (1997) The influence of land cover and temperature change on hydrological and ecosystem dynamics in the South Platte River Basin. AWRA Proceedings, Water Resources Training, Education, and Practice: Opportunities for Next Century. (In Press).

Baron, J.S., D.S. Ojima, M.D. Hartman, T.G.F. Kittel, R.B. Lammer, L.E. Band, and R.A. Pielke. 1997a. The influence of land cover and temperature change on hydrological and ecosystem dynamics in the South Platte River Basin. pp. 279-287 in: J.W. Warwick, ed. Water Resources for Education, Training, and Practice: Opportunities for the next Century. AWRA , Herdon, VA.

Baron, J.S., M.D. Hartman, T.G.F. Kittel, L.E. Band, D.S Ojima, and R.B. Lammer. 1997b. Land cover, water redistribution, and temperature: factors influencing ecosystem processes and land-atmosphere fluxes in the South Platte River Basin. Ecol. Applications (in review).

Baron, J.S., M.D. Hartman, L.E. Band, and R.L. Lammers. 1997c. Sensitivity of high elevation Rocky Mountain watersheds to climate change. Proceedings of the 5th National Watershed Coalition. in press.

Binkley, D. U. Olsson, and T, Stohlgren. 1997. Nitrogen cost of production in Rocky Mountain Forests: 1. Old-growth spruce/fir. Canadian Journal of Forest Research (In Press).

Cline, D.W. 1997a. Effect of seasonality of snow accumulation and melt on snow surface energy exchanges at a continental alpine site. J. Appl. Meteorol. 36: 32-51.

Cline, D.W. 1997b. Snow surface energy exchanges and snowmelt at a continental midlatitude Alpine site. Wat. Resour. Resear. 33:689701.

Copeland J, Pielke RA, Kittel TGF (1996a) Potential climatic impacts of vegetation change: A regional modeling study. Journal of Geophysical Research, 101, 7409-7418.

Copeland JH, Chase TN, Baron J, Kittel TGF, Pielke RA (1996b) In: Regional Impacts of Global Climate Change: Assessing Change and Response at the Scales that Matter. pp. 199-212. Battel Press, Richland, Washington.

Lammers, R.B., L.E. Band, and C.L. Tague. 1997. Scaling behavior of watershed processes. in: P. van Gardingen, G. Foody, and P. Curran, eds. Scaling Up. Cambridge University Press. in press.

Mast, J.N. 1993. Climatic and disturbance factors influencing Pinus ponderosa stand structure near the forest/grassland ecotone in the Colorado Front Range, Ph.D. thesis, University of Colorado, Boulder.

Menounos, B.P. 1994. A Holocene, debris-flow chronology for an alpine catchment, Colorado Front Range. M.S. Thesis. University of Colorado Boulder. 160 pp.

Olsson, U., D. Binkley, and F.W. Smith. 1997. Nitrogen cost of production in Rocky Mountain Forests: 2. Patterns with stand age in lodgepole pine. Canadian Journal of Forest Research (In Press).

Pielke, R.A., T.J. Lee, T.G.F. Kittel, T.N. Chase, J.M. Cram, and J.S. Baron, 1994: Effects of mesoscale vegetation distributions in mountainous terrain on local climate. In: Mountain Environments in Changing Climates, M. Beniston, Ed., Routledge Publishing Company, London and New York, 121-135.

Pielke, RA Sr., Liston GE, Lu L, Vidale PL, Walko RL, Kittel TGF, Parton WJ. (1997) Coupling of Land and Atmospheric Models over the GCIP Area - Century, RAMS, and SiB2C. In: Proc. 13th Ann. Conf. on Hydrology, 77th AMS Annual Meeting, Long Beach, California (In Press).

Reasoner, M.A., 1996. Late Quaternary alpine and subalpine lacustrine records: Canadian and Colorado Rocky Mountains. Ph.D. Dissertation, University of Alberta, Edmonton, Alberta, Canada. 132 pp. Stohlgren TJ, Bachand RR (1997) Lodgepole pine (Pinus contorta) ecotones in Rocky Mountain National Park, Colorado, USA. Ecology 78:632-641.

Stohlgren TJ, Bachand RR, Onami Y, Binkley, D. 1997a. Species-environment relationships and vegetation patterns: effects of scale and tree life-stage. Plant Ecology (In Review).

Stohlgren, T.J., T. N. Chase, R.A. Pielke, T.G. F. Kittel, and J.Baron. 1997b. Evidence that local land use practices influence regional climate and vegetation patterns in adjacent natural areas. Submitted to Global Change Biology (In Review).

Sueker, J.K. 1996. Isotopic and chemical flowpath separation of streamflow during snowmelt and hydrogeologic controls of surface-water chemistry in six alpine-subalpine basins, Rocky Mountain National Park, Colorado. Ph.D Dissertation. University of Colorado Boulder. 202 pp.

Veblen, T.T., K.S. Hadley, E.M. Nel, T. Kitzberger, M. Reid, and R. Villalba. 1994. Disturbance regime and disturbance interactions in a Rocky Mountain subalpine forest. Journal of Ecology, Vol. 82, pp. 125-135.

Weisberg, P.J. and W.L. Baker. 1995. Spatial variation in tree seedling and krummholz growth in the forest-tundra ecotone of Rocky Mountain National Park, Colorado. Arctic and Alpine Research,.v.27.

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