Fire, vegetation and ecosystem processes in Yellowstone National Park
Lodgepole pine, spatial variability, Yellowstone National Park, forest fire, disturbance, net primary production, nitrogen mineralization
The size and severity of the fires that burned through Yellowstone National Park (YNP) and surrounding lands (Figure 1.) during the summer of 1988 surprised scientists, park managers and the general public. The 1988 fires were a large, infrequent but natural disturbance that created a complex landscape mosaic. Since the fires, Turner and colleagues, especially Dr. William H. Romme (Colorado State University) have been studying the effects of fire size and pattern on postfire vegetation and ecosystem processes. Natural disturbances are key sources of heterogeneity in many ecosystems, yet the causes and consequences of disturbances that are large, severe and infrequent are not well understood.
Our initial studies focused on understanding how the complex landscape mosaic of fire severities (Figure 2.) influenced the patterns of post-fire succession. Plant re-establishment following the 1988 fires was rapid (Figure 3.). The fires did not burn deeply into the soil, averaging 14 mm in areas of stand-replacing burn, and the “biotic legacies” that remained after the fires dominated post-fire recovery and generated plant communities similar to those present before the fires. Native perennial plants re-sprouted from surviving roots and rhizomes in 1989 and flowered abundantly in 1990, resulting in a large pulse of seedling recruitment of numerous wild flowers, grasses and sedges within the burned area (FIGURE). Local dispersal from surviving individuals, rather than long-distance dispersal from unburned forest, appeared to be the most important process. Non-native invasive plant species largely did not expand into the burned areas, counter to our initial expectations.
Seedlings of the dominant tree, lodgepole pine (Pinus contorta var. latifolia) established abundantly in 1989 and 1990. The spatial variability in the density of lodgepole pine seedlings was particularly noteworthy. Some burned forests had few if any tree seedlings (Figure 4.), whereas others had >500,000 per hectare (Figure 5.). We have documented these patterns at a variety of scales and identified the important role played by variation in serotiny (Figure 6.) and burn severity across the landscape. In addition, we have documented an influence of the time between fires on post-fire vegetation and the long-term changes in stand structure and function that occur through successional time. The “footprint” of the 1988 fires is likely to be detectable for nearly 200 years, at which times stands converge to similar density, basal area, and growth rates.
Trembling aspen (Populus tremuloides) is a tree species of concern in YNP and throughout the Intermountain West. In 1989, the year after the fires, there was widespread and locally abundant establishment of seedling aspen only in burned forests and well beyond the pre-fire distribution of aspen (Figure 7.). This appeared to be an infrequent seedling-recruitment event in a long-lived species, and genetic diversity in the seedling populations increased relative to mature aspen stands in Yellowstone. As of 2000, the seedling aspen were persisting in many locations, although most stems were not very tall (averaging 30 cm) because of sub-optimal environmental conditions and browsing by native ungulates, primarily elk (Cervus elaphus).
Given the spatial variation we observed in the young stands of lodgepole pine, we also investigated the consequences of this heterogeneity for ecosystem processes. We studies a number of functional indicators, including aboveground net primary production (ANPP), leaf area index (LAI), the accumulation of downed coarse wood (fallen dead trees) after the fires, and rates of decomposition, nitrogen cycling, and microbial activity. For details of these results, please view completed research projects.
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Erwin, E.E., M.G. Turner, R.L. Lindroth and W.H. Romme. 2001. Secondary plant compounds in seedling and mature aspen in Yellowstone National Park, Wyoming. American Midland Naturalist 145:299-308.
Kashian, D.M., D.B. Tinker, M.G. Turner and F.L. Scarpace. 2004. Spatial heterogeneity of lodgepole pine sapling densities following the 1988 fires in Yellowstone National Park, Wyoming, USA. Canadian Journal of Forest Research 34 2263-2276.
Kashian, D.M., M.G.Turner, and W.H. Romme. 2005. Changes in leaf area and stemwood increment with stand development in Yellowstone National Park: Relationships between forest stand structure and function. Ecosystems 8:48-61.
Kashian, D. M., M. G. Turner, W. H. Romme and C. J. Lorimer. 2005. Variability and convergence in stand structure with forest development on a fire-dominated landscape. Ecology 86:643-654.
Romme, W. H., M. G. Turner, R. H. Gardner, W. W. Hargrove, G. A. Tuskan, D. G. Despain and R. A. Renkin. 1997. A rare episode of sexual reproduction in aspen (Populus tremuloides) following the 1988 Yellowstone fires. Natural Areas Journal 17:17-25.
Romme, W. H., M. G. Turner, G. A. Tuskan and R. A. Reed. 2005. Establishment, persistence and growth of aspen (Populus tremuloides) seedlings in Yellowstone National Park. Ecology 86:404-418.
Schoennagel, T., M. G. Turner and W. H. Romme. 2003. The influence of fire interval and serotiny on postfire lodgepole pine density in Yellowstone National Park. Ecology 84:1967-1978.
Schoennagel, T., D. M. Waller, M. G. Turner and W. H. Romme. 2004. The effect of fire interval on understory communities in Yellowstone National Park (USA). Journal of Vegetation Science 15:797-806.
Schoennagel, T., M.G. Turner, A. Fall and D. M. Kashian. 2006. Influence of fire regimes on lodgepole pine stand age and density across the Yellowstone National Park (USA) landscape. Landscape Ecology 21:1281-1296.
Stevens, M. T., M. G. Turner, G. A. Tuskan, W. H. Romme, and D. M. Waller. 1999. Genetic variation in postfire aspen seedlings in Yellowstone National Park. Molecular Ecology 8:1769-1780.
Turner, M. G., W. H. Hargrove, R. H. Gardner and W. H. Romme. 1994. Effects of fire on landscape heterogeneity in Yellowstone National Park, Wyoming. Journal of Vegetation Science 5:731-742.
Turner, M. G., W. H. Romme, R. H. Gardner and W. W. Hargrove. 1997. Effects of fire size and pattern on early succession in Yellowstone National Park. Ecological Monographs 67:411-433.
Turner, M. G., W. H. Romme, and R. H. Gardner. 1999. Prefire heterogeneity, fire severity and plant reestablishment in subalpine forests of Yellowstone National Park, Wyoming. International Journal of Wildland Fire 9:21-36.
Turner, M. G., W. H. Romme, R. A. Reed and G. A. Tuskan. 2003. Postfire aspen seedling recruitment across the Yellowstone (USA) landscape. Landscape Ecology 18: 127-140.
Turner, M. G., W. H. Romme and D. B. Tinker. 2003. Surprises and lessons from the 1988 Yellowstone fires. Frontiers in Ecology and the Environment 1:351-358.
Turner, M. G., D. B. Tinker, W. H. Romme, D. M. Kashian and C. M. Litton. 2004. Landscape patterns of sapling density, leaf area, and aboveground net primary production in postfire lodgepole pine forests, Yellowstone National Park (USA). Ecosystems 7:751-775.
Sources of research funding are gratefully acknowledged
- National Science Foundation (Ecology and Ecosystem Studies Programs)
- The Andrew W. Mellon Foundation.
- USDA NRIGCP Forest/Crop/Range/Aquatic Ecosystems Program
- Joint Fire Sciences Program
- National Geographic Society
- University of Wyoming-National Park Service Research Center's Small Grants Program