An Ecohydrological Cellular Automata Model Investigation of Juniper Tree Encroachment in a Western North American Landscape
- 326 Downloads
Woody plant encroachment over the past 140 years has substantially changed grasslands in western North American. We studied encroachment of western juniper (Juniperus occidentalis var. occidentalis) into a previously mixed shrub–grassland site in central Oregon (USA) using a modified version of Cellular Automata Tree–Grass–Shrub Simulator (CATGraSS) ecohydrological model. We developed simple algorithms to simulate three encroachment factors (grazing, fire frequency reduction, and seed dispersal by herbivores) in CATGraSS. Local ecohydrological dynamics represented by the model were first evaluated using satellite-derived leaf area index and measured evapotranspiration data. Reconstructed pre-encroachment vegetation cover percentages and the National Land Cover Database (NLCD 2006) vegetation map were used to estimate parameters for encroachment factors to represent juniper encroachment in CATGraSS. Model sensitivity experiments examined the influence of each encroachment factor and their combinations on trajectories of modeled percent cover of each plant functional type and emergent spatial vegetation patterns in the modeled domain. Simulation results identified grazing as the key factor leading to juniper encroachment, by reducing shrub and grass cover and promoting the formation of juniper tree clusters. Reduced fire frequency and increased seed dispersal by grazers further amplified juniper encroachment into grassland patches between clusters of juniper trees. Each encroachment factor showed different consequences on modeled vegetation patterns. Time series of modeled plant cover and spatial patterns of plant functional types were found to be consistent with an existing conceptual model described in the literature. The proposed model provides a tool that can be used to improve our understanding of the drivers and processes of woody plant encroachment and vegetation response to global change.
Keywordsecohydrology fire grazing seed dispersal shrubland grassland woody plants
- Allen CD, Betancourt J, Swetnam T. 2002. Range expansion of woody plants on the Colorado Plateau. Grahame JD, Sisk TD. (Eds.). http://www.cpluhna.nau.ed/.
- Archer S. 1994. Woody plant encroachment into southwestern grasslands and savannas: rates, patterns and proximate causes. Ecological Implications of Livestock Herbivory in the West. Denver: Soc. Range Manage. pp 13–69.Google Scholar
- ASCE-EWRI. 2005. The ASCE Standardized Reference Evapotranspiration Equation. Environmental and Water Resources Institute (EWRI) of the American Society of Civil Engineers Task Committee on Standardization of Reference Evapotranspiration Calculation. Washington DC: ASCE. p 190.Google Scholar
- Ivanov VY, Bras RL, Curtis DC. 2007. A weather generator for hydrological, ecological, and agricultural applications. Water Resour Res; 43(10): W10406. doi: 10.1029/2006WR005364.
- Knapp AK, McCarron JK, Silletti GA, Hoch GI, Heisler MS, Lett JM, Blair JM, Briggs JM, Smith MD. 2008. Ecological consequences of the replacement of the native grassland by Juniper virginiana and other woody plants. van Auken OW, editor. Western North American Juniperus Communities: a Dynamic Vegetation Type. New York: Springer. p156–169Google Scholar
- Miller RF, Rose JA. 1995. Historic expansion of Juniperus occidentalis (western juniper) in southeastern Oregon. Great Basin Naturalist 55:37–45.Google Scholar
- Miller RF, Tausch RJ. 2001. The role of fire in pinyon and juniper woodlands: a descriptive analysis. Tall Timbers Research Station Miscellaneous Publication 11:15–30.Google Scholar
- Miller RF, Bates JD, Svejcar TJ, Pierson FB, Eddleman LE. 2005. Biology, Ecology, and Management of Western Juniper. Technical Bulletin 152, Oregon State University, Agricultural Experiment Station. p 77.Google Scholar
- Miller RF, Tausch RJ, McArthur ED, Johnson DD, Sanderson SC. 2008. Age Structure and Expansion of Piñon-Juniper Woodlands: A Regional Perspective in the Intermountain West. USDA Forest Service RMRS-RP-69. p 21.Google Scholar
- Schaefer MG, Barker BL, Taylor GH, Wallis JR. 2008. Regional precipitation-frequency analysis and spatial mapping of 24-hour precipitation for Oregon final report. SPR 656, January 2008.Google Scholar
- Shuttleworth WJ. 1992. Evaporation. Maidment DR editor. Handbook of Hydrology. New York: McGraw-Hill. pp 4.1–4.53.Google Scholar
- van Wijk MT, Rodriguez-Iturbe I. 2002. Tree-grass competition in space and time: insights from a simple cellular automata model based on ecohydrological dynamics. Water Resour. Res. 38(18):1–15.Google Scholar
- Zhu Z, Yang L, Stehman S, Czaplewski R. 1999. Designing an Accuracy Assessment for USGS Regional Land Cover Mapping Program. Lowell K, Jaton A, editors. Spatial Accuracy Assessment: Land Information Uncertainty in Natural Resources. pp 393–398.Google Scholar