Elk browsing increases aboveground growth of water-stressed willows by modifying plant architecture |
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Authors: | Danielle B Johnston David J Cooper N Thompson Hobbs |
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Institution: | (1) Colorado Division of Wildlife, 711 Independent Avenue, Grand Junction, CO 81505, USA;(2) Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523, USA;(3) Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523, USA |
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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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Keywords: | Compensation Herbivory Leaf-specific conductivity Plant hydraulic architecture Salix sp |
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