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Variation in competitive ability due to variation in soil characteristics is one possible mechanism allowing the local coexistence of plant species. We measured soil water, depth, and nitrogen pools and fluxes in distinct patches of three serpentine grassland species to determine whether soil heterogeneity existed and was correlated with plant species abundance. Through experimental manipulation of species' abundances, we also examined the relative importance of inherent site characteristics vs. plant species' effects in generating heterogeneity in the measured soil characteristics; and measured species' competitive abilities in different patch types. The three common grassland annuals, Calycadenia multi-glandulosum, Plantago erecta, and Lasthenia californica, were segregated with respect to the measured soil characteristics. Differences in soil water, soil depth, soil microbial nitrogen, and soil carbon to nitrogen ratio were due to inherent site characteristics, while differences in nitrate availability were strongly affected by the identity of the species currently growing in a soil patch. Furthermore, all species performed significantly better against one other species in the patch type where they are normally most abundant. These results demonstrate that species diversity within this grassland contributes to soil heterogeneity and suggest that soil heterogeneity could contribute to the coexistence of these species. Key words: California grassland; Calycadenia multiglandulosum; Lasthenia californica; plant coexistence and competition; Plantago erecta; soil characteristics; soil heterogeneity; species diversity., INTRODUCTION Field experiments have established that moderate to strong competition between plants exists in a wide variety of natural communities (Connell 1983, Schoener 1983, Fowler 1986, Goldberg and Barton 1992, [...]

A model of nutrient uptake was used to determine the factors that control the uptake of ammonium, glycine, and nitrate for an important arctic sedge, Eriophorunt vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well understood and a growing number of studies suggest that organic forms of N, including glycine, are an important source of N for E. vaginatum and other plants. E. vaginatum was selected as an exemplary system to explore nitrogen (N) uptake of a native species in situ, because it appears to be strongly N limited in the field, its N nutrition has been extensively studied, and its root growth habits make it well suited to modeling nutrient uptake. The model accounts for N supply from microbial mineralization and other sources, flux of N through the soil to the root surface, and uptake by the plant root. We included glycine in our simulations of N uptake because laboratory experiments have shown that E. vaginatum can use glycine and other low-molecular-mass amino acids as a source of N. However, the extent to which glycine contributes to the N nutrition of E. vaginatum in the field is unknown. Based on these simulations we find that: (1) Ammonium, glycine, and nitrate could all potentially make significant contributions to the N nutrition of E. vaginatum. The relative contribution of glycine is difficult to assess, because its behavior in the soil has not been characterized. However, glycine and ammonium contributed in roughly equal proportions for most model parameterizations. (2) The importance of factors that regulated modeled ammonium and glycine uptake can be ranked as follows: supply rate [is greater than] [soil factors (buffer capacity and diffusion coefficient) = root density] [is greater than] root uptake kinetics. Supply rate was the only factor that regulated nitrate uptake. These simulation results and other considerations suggest that N uptake kinetics of roots and soil-solution concentrations may not be reliable predictors of whole-plant N uptake in tundra or other natural ecosystems., Keywords: amino-acid uptake by plants; arctic sedge, nitrogen economy of; Eagle Creek, Alaska, USA; Eriophorum vaginatum; modeling nutrient uptake; nitrogen availability cf. uptake; nutrient uptake, physiological factors in; nutrient uptake, [...]

Delayed inducible resistance (DIR) is triggered by artificial or herbivore-caused foliar damage and is manifested as decreased performance of herbivore generation(s) feeding on the trees subsequent to the generation during which the damage took place. DIR is associated with increase in concentrations of foliage phenolics and decrease in nitrogen. The growth-differentiation balance hypothesis, and the carbon-nutrient balance (CNB) hypothesis contained in it, claim that DIR is caused by nutritional stress after defoliation of trees growing on nutrient-poor soils. In these environments, nutrient uptake limits plant growth more strongly than does photosynthesis; that is, carbon-based secondary metabolites (e.g., phenolics) are prone to accumulate. According to the CNB hypothesis, an excess of limiting nutrient(s) or reduced photosynthetic rate should lead to elimination of DIR. We tested this using same-aged Betula pubescens ssp. tortuosa trees of five open-pollinated families growing in a common arboretum in northernmost Finland. In addition to unmanipulated control trees, we had three nutritional treatments during three successive growth seasons: N-fertilization, PK-fertilization adding all nutrients except N, and shading to decrease carbon assimilation. Half of the trees in each treatment were artificially defoliated (50% leaf area) in the second study year, one year before the growth trial with geometrid (Epirrita autumnata) larvae. Tree growth measurements showed that N is the growth-limiting nutrient in our study area. N-fertilization and shading of the trees affected foliage chemistry generally as proposed by the CNB hypothesis. For example, they reduced foliar concentrations of total phenolics and condensed tannins. The birch families differed significantly in foliage chemistry, suggesting genetic differences, but the differences were not associated with fertilizations, shading, or defoliation of the trees. Contrary to fertilizing-shade treatments, changes in leaf chemistry and E. autumnata performance caused by defoliation were not consistent with the CNB hypothesis. For example, defoliation caused significant DIR irrespective of N-fertilization or shading. There were no significant differences among the birch families in performance of the moth larvae or in effects of fertilization, shade, or defoliation on larval performance. These responses to defoliation contrast with those of some other studies, especially those on the Betula resinifera-Rheumaptera hastata system in Alaska, which provide clear support for the CNB as an explanation of DIR. We find methodological differences to be an unlikely explanation for the different results but are unable to propose any single mechanism that will explain the diverse plant responses. Key words: Betula pubescens ssp. tortuosa; carbon nutrient balance hypothesis; condensed tannins; delayed inducible resistance; Epirrita autumnata; fertilization; larval performance; manual defoliation; nitrogen; shading; total phenolics., INTRODUCTION Plant secondary metabolites could have evolved as an active defense against herbivores, or their contribution to resistance against herbivores could represent a secondary phenomenon that fortuitously benefits the plant [...]