Facilitative and competitive effects of a N-fixing shrub on white fir saplings |
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| Institution: | 1. College of Forest Resources, Box 352100, University of Washington, Seattle, WA 98195, United States;2. U.S.F.S. Sierra Nevada Research Center, 2121 2nd Ave., Suite A-101, Davis, CA 95616, United States;1. Warnell School of Forestry and Natural Resources, 180 East Green Street, University of Georgia, Athens, GA, 30602, USA;2. College of Forest Resources, Box 9681, Mississippi State University, MS, 39762, USA;3. Southern Regional Extension Forestry, Warnell School of Forestry and Natural Resources, 320 East Green Street, University of Georgia, Athens, GA, 30602, USA;1. Thünen Institute of Rural Studies, Bundesallee 50, D-38116 Brunswick, Germany;2. EDEHN — Equipe d''Economie Le Havre Normandie, Université du Havre, 25 Rue Philippe Lebon, F-76600 Le Havre, France;1. USDA Forest Service, Pacific Northwest Research Station, 1133 North Western Ave., Wenatchee, WA 98801-1229, USA;2. USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR, USA;3. Department of Forest Ecosystems & Society, Oregon State University, Corvallis, OR, USA;4. USDA Forest Service, Pacific Southwest Research Station, Redding, CA, USA;5. Department of Geography & Earth & Environmental Systems Institute, The Pennsylvania State University, PA, USA;6. Rocky Mountain Tree Ring Research, 2901 Moore Lane, Fort Collins, CO 80526, USA;7. Department of Environmental Sciences, Policy & Management, University of California, Berkeley, CA, USA;8. College of Forestry and Conservation, University of Montana, Missoula, MT, USA;9. College of Environment, School of Forest Resources, University of Washington, WA, USA;10. USDA Forest Service, Pacific Northwest Research Station, Wenatchee, WA, USA;11. USDA Forest Service, Pacific Southwest Research Station, Arcata, CA, USA;12. USDA Forest Service, Pacific Southwest Research Station, Davis, CA, USA;13. USDA Forest Service, Pacific Southwest Research Station, Fresno, CA, USA;14. USDA Forest Service, Deschutes National Forest, Bend, OR, USA;1. USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR, United States;2. College of Forestry, Oregon State University, Corvallis, OR, United States;3. US Forest Service, Rocky Mountain Research Station, Corvallis, OR, United States;1. Department of Forest Ecosystems and Society, Oregon State University, 321 Richardson Hall, Corvallis, OR 97331, USA;2. Department of Earth and Environment, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA;3. Pacific Northwest Research Station, USDA Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331, USA;1. PO Box 15018, School of Forestry, Northern Arizona University, Flagstaff, AZ 86011, United States;2. PO Box 15017, Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011, United States;3. PO Box 15018/15017, School of Forestry/Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011, United States |
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| Abstract: | In Sierra Nevada forests, shrubs are considered strong soil moisture competitors with regenerating trees, reducing seedling establishment, and slowing growth. Recent studies, however, suggest that in some circumstances shrubs can facilitate tree establishment and growth by modifying harsh microclimate conditions; increasing acquisition of water, carbon, and/or nutrients via shared mycorrhizal connections; or enhancing soil fertility, particularly under nitrogen-fixing shrubs such as Ceanothus spp. We examined the establishment dates and growth rates and patterns of white fir saplings growing in greenleaf manzanita, whitethorn ceanothus, and bare patches to examine whether establishment was correlated with past wet years, whether saplings growing in ceanothus had nitrogen-enriched foliage or faster growth rates than in the other two patches, and whether saplings in shrub patches experienced competition for light.Sapling establishment was not correlated with high precipitation or heavy snowpack years, suggesting shade-tolerant saplings do not need wet years to become established. Soils under ceanothus were nitrogen enriched, but white fir sapling foliage did not have higher nitrogen concentrations and saplings did not grow faster in ceanothus than in the other two patches. Because growth rates of saplings were comparable in all patch types examined despite significantly different edaphic and abiotic conditions, we inferred that the various competitive and facilitative interactions affecting tree growth are in net balance across the patch types examined. However, competition for light is important—a significant percentage of growth release events occurred after saplings emerged above their host shrubs. Where shrubs are present, shade-tolerant species (i.e., white fir) are favored over drought-tolerant (pine) species. Our results may help interpret changes in understory conditions that are contributing to mixed conifer's compositional shift toward more shade-tolerant species after a century of fire-suppression. |
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