Indirect effects in a desert plant community: is competition among annuals more intense under shrub canopies?

An unresolved discussion in contemporary ecology deals with the relative importance of competition along environmental gradients. In deserts, local-scale differences in environmental productivity may be caused by the presence of shrubs, which represent a favorable habitat for annual populations within a nutrient-poor matrix. In this study, we attempted to test the hypothesis that facilitation of desert annuals by shrubs increase the intensity of competition among the annual plants. Such negative indirect effects have so far been ignored in studies about plant-plant interactions. We tested our hypothesis by measuring seedling survival and fecundity of four abundant annual plant species with and without neighbors in open areas and under shrub canopies in a sandy desert area. Our findings did not indicate indirect negative effects of shrubs on their understory annuals. Sensitivity to the presence of neighbors varied between species and surprisingly, the species with the smallest seeds was the only one which was not negatively affected by the presence of neighbors. In contrast to our hypothesis, there was no difference between the habitat types shrubs and openings in absolute and relative competition intensity. Our overall results suggest that negative indirect effects of shrubs are unimportant in determining demographic response of understory annual plants.

[1]  R. Kadmon,et al.  Relationships between shrubs and annual communities in a sandy desert ecosystem: a three-year study , 1997, Plant Ecology.

[2]  Osvaldo E. Sala,et al.  Competition, facilitation, seed distribution and the origin of patches in a Patagonian steppe , 1994 .

[3]  R. Whittaker,et al.  PATTERN AND BIOLOGICAL MICROSITE EFFECTS IN TWO SHRUB COMMUNITIES, SOUTHERN CALIFORNIA' , 1981 .

[4]  R. Kadmon,et al.  Testing for source-sink population dynamics : an experimental approach exemplified with desert annuals , 1999 .

[5]  James B. Grace,et al.  On the Measurement of Plant Competition Intensity , 1995 .

[6]  James B. Grace,et al.  Perspectives on Plant Competition , 1991 .

[7]  Professor Dr. Avinoam Danin.  Plants of Desert Dunes , 1996, Adaptations of Desert Organisms.

[8]  C. Mckell,et al.  Contribution of Shrubs to the Nitrogen Economy of a Desert‐Wash Plant Community , 1970 .

[9]  J. P. Grime,et al.  Competitive Exclusion in Herbaceous Vegetation , 1973, Nature.

[10]  C. Klausmeier,et al.  Regular and irregular patterns in semiarid vegetation , 1999, Science.

[11]  S. Pennings,et al.  Salt Marsh Plant Zonation: The Relative Importance of Competition and Physical Factors , 1992 .

[12]  Variation in the effects of vegetation and litter on recruitment across productivity gradients , 1999 .

[13]  M. Huston.  A General Hypothesis of Species Diversity , 1979, The American Naturalist.

[14]  R. Kadmon.  Neighbor competition modifies demographic responses of desert dune annuals to gradients in sand stability , 1997 .

[15]  Fabian M Jaksic,et al.  Spatial distribution of soil nutrients and ephemeral plants underneath and outside the canopy of Porlieria chilensis shrubs (Zygophyllaceae) in arid coastal Chile , 1993, Oecologia.

[16]  R. Prasse,et al.  Effect of microbiotic soil surface crusts on emergence of vascular plants , 2000, Plant Ecology.

[17]  S. Pennings,et al.  Impact of a parasitic plant on the structure and dynamics of salt marsh vegetation , 1996 .

[18]  John F. Mull.  Factors determining the spatial variability of seed densities in a shrub-steppe ecosystem: the role of harvester ants , 1996 .

[19]  D. Patten,et al.  Productivity and flowering of winter ephemerals in relation to Sonoran Desert shrubs , 1975 .

[20]  A. Winn.  Effects of seed size and microsite on seedling emergence of Prunella vulgaris in four habitats , 1985 .

[21]  P. Nobel,et al.  TEMPERATURE, WATER AVAILABILITY, AND NUTRIENT LEVELS AT VARIOUS SOIL DEPTHS‐CONSEQUENCES FOR SHALLOW‐ROOTED DESERT SUCCULENTS, INCLUDING NURSE PLANT EFFECTS , 1989 .

[22]  J. P. Grime,et al.  An Experimental Test of Plant Strategy Theory , 1992 .

[23]  B. Foster.  Establishment, competition and the distribution of native grasses among Michigan old‐fields , 1999 .

[24]  R. Kadmon.  Plant Competition along Soil Moisture Gradients: A Field Experiment with the Desert Annual Stipa Capensis , 1995 .

[25]  H. D. Valle,et al.  The influence of shrubs on some chemical and physical properties of an aridic soil in north-eastern Patagonia, Argentina , 1991 .

[26]  Jessica Gurevitch,et al.  EMPIRICAL APPROACHES TO QUANTIFYING INTERACTION INTENSITY: COMPETITION AND FACILITATION ALONG PRODUCTIVITY GRADIENTS , 1999 .

[27]  F. Pugnaire,et al.  Facilitation and succession under the canopy of a leguminous shrub, Retama sphaerocarpa, in a semi-arid environment in south-east Spain , 1996 .

[28]  F. Went.  The Dependence of Certain Annual Plants on Shrubs in Southern Californi Deserts , 1942 .

[29]  L. Walker,et al.  Competition and facilitation: a synthetic approach to interactions in plant communities , 1997 .

[30]  K. Tielbörger.  The vegetation of linear desert dunes in the north-western Negev, Israel , 1997 .

[31]  J. P. Grime.  Competition and diversity in herbaceous vegetation , 1973 .

[32]  William M. Schaffer,et al.  Plant strategies and the dynamics and structure of plant communities , 1989 .

[33]  O. Sala,et al.  Patch structure, dynamics and implications for the functioning of arid ecosystems. , 1999, Trends in ecology & evolution.

[34]  R. Kadmon,et al.  TEMPORAL ENVIRONMENTAL VARIATION TIPS THE BALANCE BETWEEN FACILITATION AND INTERFERENCE IN DESERT PLANTS , 2000 .

[35]  C. Chanway,et al.  INTERACTIVE EFFECTS OF NUTRIENTS AND DISTURBANCE: AN EXPERIMENTAL TEST OF PLANT STRATEGY THEORY' , 1993 .

[36]  J. Nelson,et al.  Factors Affecting Seed Reserves in the Soil of a Mojave Desert Ecosystem, Rock Valley, Nye County, Nevada , 1977 .

[37]  R. Kadmon,et al.  Effect of shrubs on emergence, survival and fecundity of four coexisting annual species in a sandy desert ecosystem , 1995 .

[38]  T. Miller,et al.  Direct and Indirect Species Interactions in an Early Old-Field Plant Community , 1994, The American Naturalist.

[39]  Yosef Steinberger,et al.  A proposed mechanism for the formation of ‘Fertile Islands’ in the desert ecosystem , 1989 .

[40]  D. Tilman.  Resource competition and community structure. , 1983, Monographs in population biology.

[41]  William H. Schlesinger,et al.  ON THE SPATIAL PATTERN OF SOIL NUTRIENTS IN DESERT ECOSYSTEMS , 1995 .

[42]  O. Reichman.  Spatial and temporal variation of seed distributions in Sonoran Desert soils , 1984 .

[43]  O. Reichman,et al.  Distribution of Seeds in Sonoran Desert Soils: Implications for Heteromyid Rodent Foraging. , 1987, Ecology.

[44]  J. Grace.  The effects of habitat productivity on competition intensity. , 1993, Trends in ecology & evolution.

[45]  M. Rees.  Community Structure in Sand Dune Annuals: Is Seed Weight a Key Quantity? , 1995 .