Effects of Competition on Resource Availability and Growth of a California Bunchgrass

In California, little is known about the sensitivity of native bunchgrasses to competition or to changes in resource availability. We investigated the effect of nonnative annual vegetation on resource availability and growth of a native bunchgrass, Nassella pulchra, in a pair of factorial field experiments that incorporated effects of both interspecific and intraspecific competition as well as variation in soil depth. Plots of differing target densities and neighborhoods were used to assess changes in aboveground (light) and be- lowground (water) resource availability over multiple seasons in two sites with differing soil depth. N. pulchra grown without interspecific competitors grew larger and produced more culms at all planting densities compared to plants in plots with interspecific com- petitors. Intraspecific competition significantly influenced growth only in the absence of interspecific competition. Reproductive effort, as measured by flowering culm production, was more sensitive than vegetative growth to both forms of competition. Light availability and variability at the soil surface was greatly reduced by the nonnative annual neighborhood. As expected, soil moisture was rapidly depleted by annuals to 30 cm in all plots. In deep- soil plots, soil moisture was reduced at 60-150 cm depths only when annual vegetation was removed, and depletion was correlated with N. pulchra basal area. This result suggests that the interspecific neighborhood reduced root growth in N. pulchra and its subsequent ability to use deep moisture. Within California's inland grasslands, nonnative annual veg- etation has changed seasonal patterns of resource availability. We conclude that (1) increased competition for light during the spring, when growth of annuals is most rapid, suppresses growth and reproduction of N. pulchra; (2) by suppressing bunchgrass growth, annual grasses reduce access to belowground resources by competitive interference; and (3) the loss of perennial grasses in California grasslands and the general dominance by nonnative annual species results in the relative underutilization of deep soil resources. These con- clusions suggest that the dominance of California grasslands by nonnative annual vegetation has shifted the primary limiting resource from soil moisture to light and the timing of resource limitation from summer to winter and spring.

[1]  Ariel Novoplansky,et al.  On the relative importance of competition in unproductive environments , 1997 .

[2]  L. Jackson.  Ecological origins of California's Mediterranean grasses , 1985 .

[3]  Robert Ornduff,et al.  Terrestrial Vegetation of California. , 1977 .

[4]  S. McNaughton,et al.  Structure and Function in California Grasslands , 1968 .

[5]  S. Turitzin.  Canopy Structure and Potential Light Competition in Two Adjacent Annual Plant Communities , 1978 .

[6]  James B. Grace,et al.  Components of resource competition in plant communities. , 1990 .

[7]  W. C. Krueger,et al.  Soil Moisture Extraction by Vegetation in a Mediterranean/Maritime Climatic Regime , 1992 .

[8]  D. Goldberg.  Influence of Competition at the Community Level: An Experimental Version of the Null Models Approach , 1994 .

[9]  G. Harris.  Some competitive relationships between Agropyron spicatum and Bromus tectorum. , 1967 .

[10]  Peter M. Vitousek,et al.  Biological invasions and ecosystem processes : towards an integration of population biology and ecosystem studies , 1990 .

[11]  P. Vitousek,et al.  Biological invasions by exotic grasses, the grass/fire cycle, and global change , 1992 .

[12]  J. P. Grime,et al.  Plant Strategies and Vegetation Processes. , 1980 .

[13]  K. Rice.  Competitive Interactions in California Annual Grasslands , 1989 .

[14]  J. Gurevitch.  Competition and the Local Distribution of the Grass Stipa Neomexicana , 1986 .

[15]  Deborah E. Goldberg,et al.  NEIGHBORHOOD COMPETITION IN AN OLD-FIELD PLANT COMMUNITY' , 1987 .

[16]  J. R. Griffin,et al.  Long‐Term Patterns in Coastal California Grasslands in Relation to Cultivation, Gophers, and Grazing , 1996 .

[17]  D. Tilman.  Plant Strategies and the Dynamics and Structure of Plant Communities. (MPB-26), Volume 26 , 1988 .

[18]  T. Miller,et al.  Competitive Effects and Responses Between Plant Species in a First‐Year Old‐Field Community , 1987 .

[19]  K. Rice,et al.  Patterns of Growth and Soil-water Utilization in some Exotic Annuals and Native Perennial Bunchgrasses of California , 1996 .

[20]  J. Bartolome.  Germination and Seedling Establishment in California Annual Grassland , 1979 .

[21]  J. Bartolome,et al.  THE ECOLOGICAL STATUS OF STIPA PULCHRA (POACEAE) IN CALIFORNIA , 1981 .

[22]  L. Aarssen.  Ecological Combining Ability and Competitive Combining Ability in Plants: Toward a General Evolutionary Theory of Coexistence in Systems of Competition , 1983, The American Naturalist.

[23]  H. Biswell.  Ecology of California Grasslands. , 1956 .

[24]  J. Roy,et al.  Growth patterns of Mediterranean annual and perennial grasses under simulated rainfall regimes of southern France and California , 1986 .

[25]  K. Landa,et al.  Competitive effect and response: hierarchies and correlated traits in the early stages of competition , 1991 .

[26]  R. Kadmon,et al.  COMPETITION IN A VARIABLE ENVIRONMENT: AN EXPERIMENTAL STUDY IN A DESERT ANNUAL PLANT POPULATION , 1990 .

[27]  A. R. Dyer,et al.  EMERGENCE AND SURVIVAL OF NASSELLA PULCHRA IN A CALIFORNIA GRASSLAND , 1996 .

[28]  Norma L. Fowler,et al.  The Role of Competition in Plant Communities in Arid and Semiarid Regions , 1986 .

[29]  Peter P. Pratt,et al.  Archaeological Survey of the Flint Creek Project for the United States Department of Agriculture Soil Conservation Service , 1974 .

[30]  K. Rice,et al.  INTRASPECIFIC AND DIFFUSE COMPETITION: THE RESPONSE OF NASSELLA PULCHRA IN A CALIFORNIA GRASSLAND , 1997 .

[31]  T. Seastedt,et al.  Detritus Accumulation Limits Productivity of Tallgrass PrairieThe effects of its plant litter on ecosystem function make the tallgrass prairie unique among North American biomes , 1986 .

[32]  K. L. White.  Native Bunchgrass (Stipa Pulchra) on Hastings Reservation, California , 1967 .

[33]  F. Bazzaz,et al.  Underground Niche Separation in Successional Plants , 1976 .

[34]  E. Newman,et al.  Competition and Diversity in Herbaceous Vegetation , 1973, Nature.

[35]  R. A. Evans,et al.  Chapter 16 – Competition within the Grass Community , 1972 .

[36]  M. W. Talbot,et al.  Fluctuations in the Annual Vegetation of California , 1939 .

[37]  F. Bazzaz,et al.  IS PHYSICAL SPACE A SOIL RESOURCE , 1991 .

[38]  J. E. Weaver,et al.  Effects of Excessive Natural Mulch on Development, Yield, and Structure of Native Grassland , 1952, Botanical Gazette.

[39]  J. C. Hickman,et al.  The Jepson Manual: Higher Plants of California , 1993 .

[40]  P. Keddy,et al.  Root and shoot competition intensity along a soil depth gradient , 1995 .

[41]  James C. Hickman,et al.  The Jepson Manual: Higher Plants of California , 1993 .

[42]  B. Kay,et al.  Paraquat for Range Seeding in Cismontane California , 1970, Weed Science.

[43]  D. Diamond,et al.  CHANGES IN GROWTH OF TWO C4 GRASSES (SCHIZACHYRIUM SCOPARIUM AND PASPALUM PLICATULUM) IN MONOCULTURE AND MIXTURE: INFLUENCE OF SOIL DEPTH' , 1994 .