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Conifer Cover Increase in the Greater Yellowstone Ecosystem: Frequency, Rates, and Spatial Variation

Abstract

Extensive fires in recent decades in the Greater Yellowstone Ecosystem (GYE) garnered much attention for causing a significant decrease in the extent of conifer forest cover. Meanwhile, conifer forests in unburned parts of the GYE have continued to increase in extent and density. Conifer cover increase has been well documented by repeat historical photography, but the average rate of increase and the spatial variation remain unquantified. We examined changes in conifer cover across biophysical gradients in the GYE based on stratified random samples from aerial photographs. The percent conifer cover for samples in 1971 and 1999 was quantified to determine the frequency and rate of conifer cover change. A slight majority of samples (56%) showed no change, whereas increases (22%) were balanced by decreases (22%). However, among samples that were not recently burned or logged, or already closed-canopy, nearly 40% increased in conifer cover, at an average annual rate of 0.22%. We quantified significant variability in the frequency and rate of conifer cover increase across gradients of elevation, aspect, vegetation type, and proximity to nearby conifer forest. The most dynamic locations were low density conifer woodlands on northerly aspects at lower elevations, with average annual rates of increase up to 0.51%. This study is significant because it demonstrates that rates of conifer cover increase vary across biophysical gradients, an important consideration for management of dynamic forest ecosystems. Improved understanding of this variability helps us to better understand what factors ultimately cause conifer cover increase. It is also a critical step towards accurate quantification of the magnitude of carbon uptake by conifer cover increase.

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Acknowledgments

We thank the NASA Land Cover Land Use Change Program for funding this study. We also thank Jeremy Lougee, Nick Lyman, Lew Stringer, and Jason Bruggeman for aerial photo interpretation in support of this study. Special thanks to Monica Turner and her support from NSF for use of aerial photos for Yellowstone National Park. We also thank Lisa Graumlich, Rick Lawrence, Jay Rotella, and three anonymous reviewers for their thoughtful comments on manuscript drafts.

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Correspondence to Scott L. Powell.

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Powell, S.L., Hansen, A.J. Conifer Cover Increase in the Greater Yellowstone Ecosystem: Frequency, Rates, and Spatial Variation. Ecosystems 10, 204–216 (2007). https://doi.org/10.1007/s10021-007-9023-1

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Keywords

  • Yellowstone
  • conifer expansion
  • biophysical factors
  • forest dynamics
  • conifer cover
  • aerial photos.