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Elk browsing increases aboveground growth of water-stressed willows by modifying plant architecture

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Abstract

In the northern elk wintering range of Yellowstone National Park, USA, wolf (Canis lupus) removal allowed elk (Cervus elaphus) to overbrowse riparian woody plants, leading to the exclusion of beaver (Castor canadensis) and a subsequent water table decline in many small stream valleys. Reduced elk browsing following wolf reintroduction may or may not facilitate willow (Salix sp.) recovery in these areas. To determine if the effect of elk browsing on willow interacts with that of beaver abandonment, we manipulated elk browsing and the water table in a factorial experiment. Under the condition of an ambient (low) water table, elk browsing increased shoot water potential (Ψ s), photosynthesis per unit leaf area (A), stomatal conductance per unit leaf area (g s), and aboveground current annual growth (CAG) by 50%. Elk browsing occurred entirely during dormancy and did not affect total plant leaf area (L). Improved water balance, photosynthetic rate, and annual aboveground productivity in browsed willows appeared to be due to morphological changes, such as increased shoot diameter and decreased branching, which typically increase plant hydraulic conductivity. An elevated water table increased Ψ s, A, g s, CAG, and L, and eliminated or lessened the positive effect of browsing on CAG for most species. Because low water tables create conditions whereby high willow productivity depends on the morphological effects of annual elk browsing, removing elk browsing in areas of water table decline is unlikely to result in vigorous willow stands. As large willow standing crops are required by beaver, a positive feedback between water-stressed willow and beaver absence may preclude the reestablishment of historical conditions. In areas with low water table, willow restoration may depend on actions to promote the re-establishment of beaver in addition to reducing elk browsing.

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Acknowledgments

This work was funded by grants from the Biological Resources Divisions of the U.S. Geological Survey, Yellowstone National Park, and the National Science Foundation (Graduate Research Fellowship Program and Ecosystems Studies Program Award granted to Colorado State University). We thank John S. Sperry and Uwe G. Hacke for generous assistance in developing the vulnerability curves, Michael G. Ryan for help in interpreting the physiological data, and Alan K. Knapp for use of a Li-Cor 6400 portable photosynthesis system. We thank the staff at the Yellowstone Center for Resources for logistical and technical support, particularly Roy A. Renkin, Mary Hektner, Glenn E. Plumb, and Christie Hendrix. We thank the Montana Conservation Corps, the Youth Conservation Corps, Paul Miller, and Evan Wolf for help in constructing exclosures and dams, many field assistants for help in taking willow measurements, and Christopher Williams for helpful suggestions on an early manuscript. This work complies with the current laws of the United States of America.

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Authors and Affiliations

  1. Colorado Division of Wildlife, 711 Independent Avenue, Grand Junction, CO, 81505, USA

    Danielle B. Johnston

  2. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, 80523, USA

    Danielle B. Johnston & N. Thompson Hobbs

  3. Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO, 80523, USA

    David J. Cooper & N. Thompson Hobbs

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  1. Danielle B. Johnston
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  2. David J. Cooper
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Correspondence to Danielle B. Johnston.

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Communicated by Frederick C. Meinzer.

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Johnston, D.B., Cooper, D.J. & Hobbs, N.T. Elk browsing increases aboveground growth of water-stressed willows by modifying plant architecture. Oecologia 154, 467–478 (2007). https://doi.org/10.1007/s00442-007-0854-4

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