Abstract
In semiarid landscapes, the ratio of herbaceous to woody plant biomass is a major determinant of ecosystem properties. This ratio depends to a large extent on the amount and spatial distribution of soil moisture that is available to plants, and these variables, in turn, are determined primarily by climate and land use. Current conceptual models for determining the ratio of herbaceous to woody plant biomass in semiarid plant communities are based either on differences in soil moisture with depth (vertical heterogeneity) from one site to another (Walter's two-layer model) or on differences in soil moisture between canopy and intercanopy patches at the same site (horizontal heterogeneity) that result from disturbances associated with land use (Schlesinger et al.'s model of desertification). We developed a model that unifies these two perspectives by relaxing two assumptions of Walter's two-layer model. First, our model recognizes that soil moisture varies horizontally between canopy and intercanopy patches, not only due to land-use disturbance, a general assumption of the Schlesinger et al. model, but also due to the physical nature of the canopy itself. Second, while retaining the general assumption of Walter that woody plants obtain moisture from deeper soil layers than do herbaceous plants, our model recognizes the existence of two types of woody plants: those that extract a substantial proportion of their moisture from deeper layers and those that extract mainly from shallower layers. By modifying the two-layer hypothesis to include four soil compartments and distinguishing between shallow- and deeper-rooted woody species, our model integrates three key concepts in semiarid ecology: (1) the proportion of woody cover increases as moisture in the deeper soil layers increases (Walter's two-layer hypothesis for coexistence of herbaceous and woody plants); (2) land use practices that cause a reduction in herbaceous vegetation and compaction of intercanopy soils lead to a long-term increase in the proportion of woody plants (Schlesinger et al.'s concept, or more generally, that at a given site multiple variations in the proportions of herbaceous and woody plant biomass are possible); and (3) changes in the ratios of herbaceous to woody plant biomass exhibit complex behavior (changes can happen quickly and are not directly reversible without intensive management). This integration of concepts results because rather than assuming a simple, one-way dependence of plant functional types on soil moisture heterogeneity, our model assumes an interdependence between the two: soil moisture heterogeneity constrains the composition of the plant community, which in turn modifies soil moisture heterogeneity. The four-compartment model that we propose enables, for the first time, an integrated picture of both dimensions of soil moisture heterogeneity – horizontal and vertical – and of the interdependence between soil moisture heterogeneity and the proportions of the plant functional types that make up a given plant community. This unified conceptual model can be applied to provide insight into the individual and the combined effects of climate and land use on semiarid plant communities within the grassland/forest continuum, which vary in the proportions of canopy and intercanopy patches.
Similar content being viewed by others
References
Allen, C.D. and Breshears, D.D. 1998. Drought-induced shift of a forest-woodland ecotone: rapid landscape response to climate variation. Proceedings of the National Academy of Sciences (USA) 95: 14839–14842.
Aguiar, M.R., Paruelo, J.M, Sala, O.E. and Lauenroth, W.K. 1996. Ecosystem responses to changes in plant functional type composition: an example from the Patagonian steppe. J. Veg. Sci. 7: 381–390.
Ansley, R.J., Jacoby, P.W. and Hicks, R.A. 1991. Leaf and whole plant transpiration in honey mesquite following severing of lateral roots. J. Range Manag. 44: 577–583.
Archer, S., Scifres, C., Bassham, C.R. and Maggio, R. 1988. Autogenic succession in a subtropical savanna: conversion of a grassland to thorn woodland. Ecol. Monog. 58: 111–127.
Archer, S. 1989. Have southern Texas savannas been converted to woodlands in recent history? Am. Nat. 134: 545–561.
Archer, S. 1990. Development and stability of grass/woody mosaics in a subtropical parkland, Texas, U.S.A. J. Biog. 17: 453–462.
Barton, A.M. 1993. Factors controlling plant distributions: drought, competition, and fire in montane pines in Arizona. Ecol. Monog. 63: 367–397.
Belsky, A.J. 1990. Tree/grass ratios in East African savannas: a comparison of existing models. J. Biog. 17: 483–489.
Belsky, A.J. 1994. Influences of trees on savanna productivity: tests of shade, nutrients, and tree-grass competition. Ecology 75: 483–489.
Belsky, A.J., Amundson, R.G., Duxbury, J.M., Riha, S.J., Ali, A.R. and Mwonga, S.M. 1989. The effects of trees on their physical, chemical, and biological environments in a semi-arid savanna in Kenya. J. Appl. Ecol. 26: 1005–1024.
Belsky, A.J. and Canham, C.D. 1994. Forest gaps and isolated savanna trees. BioScience 44: 77–84.
Belsky, A.J., Mwonga, S.W., Amundson, R.G. Duxbury, J.M. and Ali, A.R. 1993. Comparative effects of isolated trees on their undercanopy environments in high-and low-rainfall savannas. J. Appl. Ecol. 30: 143–155.
Breshears, D.D., Meyers, O.B., Johnson, S.R., Meyer, C.W. and Martens, S.N. 1997a. Differential water use of heterogeneous soil moisture by two semiarid woody species: Pinus edulis and Juniperus monosperma. J. Ecol. 85: 289–299.
Breshears, D.D., Nyhan, J.W. Heil, C.E. and Wilcox, B.P. 1998. Effects of woody plants on microclimate in a semiarid woodland: soil temperature and soil evaporation in canopy and intercanopy patches. Int. J. Plant Sci. 159: 1010–1017.
Breshears, D.D., Rich, P.M., Barnes, F.J. and Campbell, K. 1997b. Overstory-imposed heterogeneity in solar radiation and soil moisture in a semiarid woodland. Ecol. Appl. 7: 1201–1215.
Briones, O., Montaña, C. and Ezcurra, E. 1998. Competition intensity as a function of resource availability in a semiarid ecosystem. Oecologia 116: 365–372.
Burgess, T.L. 1996. Desert grassland, mixed shrub savanna, shrub steppe, or semidesert scrub? The Dilemma of coexisting growth forms. In The desert grassland. pp. 31–67. Edited by M.P. McClaran and T.R. Van Devender. University of Arizona Press, Tuscon, Arizona.
Caldwell, M.M., Eissenstat, D.M., Richards, J.H. and Allen, M.F. 1985. Competition for phosphorus: differential uptake from dualisotope-labeled soil interspaces between shrub and grass. Science 229: 384–386.
Canadell, J.R.B., Jackson, R.B., Ehleringer, J.R., Mooney, H.A., Sala, O.E. and Schulze, E.D. 1996. Maximum rooting depth of vegetation types at the global-scale. Oecologia 108: 583–595.
Chong, G.W. 1994. Recommendations to improve revegetation success in a pinon-juniper woodland in New Mexico:a hierarchical approach. Thesis. Department of Biology, University of New Mexico.
Clary, W.P. and Jensen, C.E. 1981. Mathematical hypothesis for herbage production potential on pinyon-juniper areas. U.S. Forest Service Intermountain Forest and Range Experimental Station Research Paper INT-279.
Coffin, D.P. and Lauenroth, W.K. 1990. A gap dynamics simulation model of succession in a semiarid grassland. Ecol. Modelling 49: 229–266.
Coffin, D. P. and D. L. Urban. 1993. Implications of natural history traits to system-level dynamics: comparisons of a grassland and a forest. Ecol. Modelling 67: 147–178.
Cornet, A.F., Montana, C., Delhoume, J.P. and Lopez-Portillo, J. 1992. Water flow and the dynamics of desert vegetation stripes. In Landscape boundaries: consequences for biotic diversity and ecological flows. Ecological Studies 92. pp. 327–345. Edited by A.J. Hansen and F. di Castri. Springer-Verlag, New York.
Davenport, D.W., Breshears, D.D., Wilcox, B.P. and Allen, C.D. 1998. Viewpoint: Sustainability of piñon-juniper ecosystems – a unifying perspective of soil erosion thresholds. J. Range Manag. 51: 231–240.
Dawson, T.E. 1993. Woodland water balance. Trends Ecol. Evol. 8: 120–121.
Eagleson, P.S. 1982. Ecological optimality in water-limited natural soil-vegetation systems. Water Res. Res. 18: 325–340.
Eagleson, P.S. and Segarra, R.I. 1985. Water-limited equilibrium of savanna vegetation systems. Water Res. Res. 21: 1483–1493.
Emerson, F.W. 1932. The tension zone between grama grass and pinon-juniper associations in northeastern New Mexico. Ecology 13: 347–358.
Epstein, H.E., Lauenroth, W.K., Burke, I.C and Coffin, D.P. 1997. Productivity patterns of C3 and C4 functional types in the U.S. Great Plains. Ecology 78: 722–731.
Evans, R.D. and Ehleringer, J.R. 1994. Water and nitrogen dynamics in an arid woodland. Oecologia 99: 233–242.
Flanagan, L.B., Ehleringer, J.R. and Marshall, J.D. 1992. Differential uptake of summer precipitation among co-occuring trees and shrubs in a piñon-juniper woodland. Plant, Cell Env. 18: 831–836.
Golluscio, R.A. and Sala, O.E. 1993. Plant functional types and ecological strategies in Pantagonian forbs. J. Veg. Sci. 4: 839–846.
Gosz, J.R. and Sharpe, P.J.H. 1989. Broad-scale concepts for interactions of climate, topography, and biota at biome transitions. Landscape Ecol. 3: 229–243.
Grover, H.D. and Musick, H.B. 1990. Shrubland encroachment in southern New Mexico, U.S.A.: an analysis of desertification processes in the American southwest. Climatic Change 17: 305–330.
Intergovernmental Panel on Climate Change (IPCC). 1996a. Chapter 2. Rangelands in a changing climate: impacts, adaptations, and mitigation. In Climate change 1995 – Impacts, adaptations and mitigation of climate change: scientific-technical analyses. pp. 131–158. Edited by R.T. Watson, M.C. Zinyowera and R.H. Moss. Cambridge University Press, New York, N.Y.
Intergovernmental Panel on Climate Change (IPCC). 1996b. Chapter 4. Land degredation and desertification. In Climate change 1995 – Impacts, adaptations and mitigation of climate change: scientific-technical analyses. pp. 171–189. Edited by R.T. Watson, M.C. Zinyowera and R.H. Moss. Cambridge University Press, New York, N.Y.
Jackson, R.B., Canadell, J., Ehleringer, J.R., Mooney, H.A., Sala, O.E. and Schulze, E.D. 1996. A global analysis of root distributions for terrestrial biomes. Oecologia 108: 389–411.
Jameson, D.A. 1987. Climax or alternative steady states in woodland ecology. In Proceedings-Pinyon Juniper Conference, USDA Forest Service Intermountain Research Station: General Technical Report INT-215, pp. 9–13.
Jeltsh, F., Milton, S.J., Dean, W.R.J. and Van Rooyen, N. 1996. Tree spacing and coexistence in semiarid savannas. J. Ecol. 84: 583–595.
Joffre, R. and Rambal, S. 1988. Soil water improvement by trees in the rangelands of Southern Spain. Oecolgia Plantarum 9: 405–422.
Joffre, R. and Rambal, S. 1993. How tree cover influences the water balance of Mediterranean rangelands. Ecology 74: 570–582.
Johnsen, T.N., Jr. 1962. One-seed juniper invasion of northern Arizona grasslands. Ecol. Monog. 32: 187–207.
Keeley, S.C. and Mooney, H.A.. 1993. Vegetation in western North America, past and future. In Earth system responses to global change: contrasts between North and South America. pp. 209–237. Edited by H.A. Mooney, E.R. Fuentes & B.I. Kronberg. Academic Press, New York.
Knoop, W.T. and Walker, B.H. 1985. Interactions of woody and herbaceous vegetation in a southern African savanna. J. Ecol. 73: 235–253.
Lauenroth, W.K., Urban, D.L., Coffin, D.P., Parton, W.J., Shugart, H.H., Kirchner, T.B. and Smith, T.M. 1993. Modeling vegetation structure-ecosystem process interactions across sites and ecosystems. Ecol. Modelling 67: 49–80.
Le Roux, X., Bariac, T. and Mariotti, A. 1995. Spatial partitioning of the soil water resource between grass and shrub components in a West African humid savanna. Oecologia 104: 147–155.
Liang, Y.M., Hazlett, D.L. and Lauenroth, W.K. 1989. Biomass dynamics and water use efficiencies of five plant communities in the shortgrass steppe. Oecologia 80: 148–153.
Lockwood, J.A. and Lockwood, D.R. 1993. Catastrophe theory: a unified paradigm for rangeland ecosystem dynamics. J. Range Manag. 46: 282–288.
Ludwig, J.A. and Tongway, D.J. 1995. Spatial organization of landscapes and its function in semi-arid woodlands, Australia. Landscape Ecol. 10: 51–63.
Ludwig, J.A., Tongway, D.J., Freudenberger, D.O., Noble, J.C. and Hodgkinson, K.C. (eds.). 1997. Landscape ecology, function and management: principles from Australia's rangelands. CSIRO Publishing, Collingwood, VIC, Australia.
Martens, S.N., Breshears, D.D., Meyer, C.W. and Barnes, F.J. 1997. Scales of above-ground and below-ground competition in a semiarid woodland as detected from spatial pattern. J. Veg. Sci. 8: 655–664.
McMurtrie, R. and Wolf, L. 1983. A model of competition between trees and grass for radiation, water and nutrients. Ann. Bot. 52: 449–458.
McPherson, G.R. 1997. Ecology and managment of North American savannas. University of Arizona Press, Tucson.
Medina, E. 1987. Nutrient requirements, conservation and cycles in the herbaceous layer. In Determinants of savannas. pp. 39–665. Edited by B. Walker. IRL Press, Oxford.
Medina, E. and Silva, J.F. 1990. Savannas of northern South America: a steady state regulated by water-fire interactions on a background of low nutrient availability. J. Biog. 17: 403–413.
Miller, R.F. and Wigand, P.E. 1994. Holocene changes in semiarid pinyon-juniper woodlands. BioScience 44: 465–474.
Milne, B.T., Johnson, A.R., Keitt, T.H., Hatfield, C.A., David, J. and Hraber, P.T. 1996. Detection of critical densities associated with piñon-juniper woodland ecotones. Ecology 77: 805–821.
Montaña, C., Cavagnaro, B. and Briones, O. 1995. Soil water use by co-existing shrubs and grasses in the Southern Chihuahuan Desert, Mexico. J. Arid Env. 31: 1–13.
Padien, D.J. and Lajtha, K. 1992. Plant spatial pattern and nutrient distribution in pinyon-juniper woodlands along an elevational gradient in northern New Mexico. Int. J. Plant Sci. 153: 425–433.
Peléz, D.V., Distel, R.A., Bóo, R.M., Elia, O.R. and Mayor, M.D. 1994. Water relations between shrubs and grasses in semi-arid Argentina. J. Arid Env. 27: 71–78.
Reynolds, J.F., Virginia, R.A. and Schlesinger, W.H. 1997. Defining functional types for models of desertification. In Plant functional types: their relevance to ecosystem properties and global change. pp. 195–216. Edited by T.M. Smith, H.H. Shugart and F.I. Woodward. Cambridge University Press, New York, N.Y.
Ryel, R.J., Caldwell, M.M. and Manwaring, J.H. 1996. Temporal dynamics of soil spatial heterogeneity in sagebrush-wheatgrass steppe during a growing season. Plant Soil 184: 299–309.
Sala, O.E. and Lauenroth, W.K. 1982. Small rainfall events: an ecological role in semiarid regions. Oecologia 53: 301–304.
Sala, O.E., Golluscio, R.A., Lauenroth, W.K. and Soriano, A. 1989. Resource partitioning between shrubs and grasses in the Patagonian steppe. Oecologia 81: 501–505.
Sala, O.E., Lauenroth, W.K. and Golluscio, R.A. 1997. Plant functional types in temperate semi-arid regions. In Plant functional types: their relevance to ecosystem properties and global change. pp. 217–233. Edited by T.M. Smith, H.H. Shugart and F.I. Woodward. Cambridge University Press, New York, N.Y.
Schlesinger, W.H., Reynolds, J.F., Cunningham, G.L., Huenneke, L.F., Jarrell, W.M., Virginia, R.A. and Whitford, W.G. 1990. Biological feedbacks in global desertification. Science 247: 1043–1048.
Schlesinger, W.H. and Pilmanis, A.M. 1998. Plant-soil interactions in deserts. Biogeochemistry 42: 169–187.
Scholes, R.J. and Archer, S.R. 1997. Tree-grass interactions in savannas. Ann. Rev. Ecol. Syst. 28: 517–544.
Scholes, R.J. and Walker, B.H. 1993. An African Savanna: synthesis of the Nylsvley study. Cambridge University Press, Cambridge.
Schulze, E.D., Mooney, H.A., Sala, O.E., Jobbagy, E., Buchmann, N., Bauer, G., Canadell, J., Jackson, R.B., Loreti, J., Oesterheld, M. and Ehleringer, J.R. 1996. Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia. Oecologia 108: 503–511.
Shugart, H.H. 1998. Terrestrial ecosystems in changing environments. Cambridge University Press, New York.
Smith, T.M., Shugart, H.H. and Woodward, F.I. (eds.). 1997. Plant functional types: their relevance to ecosystem properties and global change. Cambridge University Press, New York, N.Y.
Solbrig, O.T., Medina, E. and Silva, J.F. 1996. Determinants of tropical savannas. In Biodiversity and savanna ecosystem processes: a global perspective. Ecological Studies 121. pp. 31–41. Edited by O.T. Solbrig, E. Medina and J.F. Silva. Springer-Verlag, New York.
Steffen, W.L. 1996. A periodic-table for ecology: a chemist's view of plant functional types. J. Veg. Sci. 7: 425–430.
Stephenson, N.L. 1990. Climatic control of vegetation distribution: the role of water balance. Am. Nat. 135: 649–670.
Tausch, R.J., Wigand, P.E. and Burkhardt, J.W. 1993. Viewpoint: Plant community thresholds, multiple steady states, and multiple successional pathways: legacy of the Quaternary? J. Range Manag. 46: 439–447.
Tilman, D. 1988. Plant strategies and the dynamics and structure of plant communities. Monographs in Population Biology 26. Princeton University Press, Princeton, New Jersey.
Walker, B.H. 1987. A general model of savanna structure and function. In Determinants of savannas. pp. 1–12. Edited by B.H. Walker. IUBS Monograph Series No. 3. IRL Press, Oxford.
Walker, B.H. 1996. Predicting a future terrestrial biosphere: challenges to GCTE. In Global change and terrestrial ecosystems. pp. 595–607. edited by B. Walker and W. Steffen. Cambridge University Press, New York, N.Y.
Walker, B.H., Ludwig, D., Holling, C.S. and Peterman, R.M. 1981. Stability of semi-arid savanna grazing systems. J. Ecol. 69: 473–498.
Walker, B.H. and Noy-Meir, I. 1982. Aspects of the stability and resilience of savanna ecosystems. In Ecology of Tropical Savannas. Ecological Studies 42. pp. 556–590. Edited by B.J. Huntley and B.H. Walker. Springer-Verlag, Berlin, Heidelberg, New York
Walter, H. 1971. Ecology of Tropical and Subtropical Vegetation. Oliver and Boyd, Edinburgh.
Walter, H. 1973. Vegetation of the earth in relation to climate and the eco-physological conditions. Translated from the second German edition by J. Wieser. Springer-Verlag, New York.
Wischmeier, W.H. and Smith, D.D. 1978. Predicting rainfall erosion losses – a guide to conservation planning. USDA Handbook 537. U.S. Government Printing Office, Washington, D.C.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Breshears, D.D., Barnes, F.J. Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: a unified conceptual model. Landscape Ecology 14, 465–478 (1999). https://doi.org/10.1023/A:1008040327508
Issue Date:
DOI: https://doi.org/10.1023/A:1008040327508