Summary 0Responses to spatial heterogeneity of soil nutrients were tested in 09 plant species that di}er in life form and successional status\ but which co!occur in the South Carolina coastal plain[ The morphological responses of the root system were tested by assessing scale "represented by root mass and root length densities#\ precision "preferential proliferation of roots in nutrient!rich patches compared with less fertile patches# and discrimination "ability to detect and proliferate within the richest patches when patches vary in nutrient concentration#[ We also investigated sensitivity "growth bene_ts gained as spatial heterogeneity of nutrients increases\ measured as total biomass#[ 1 Ten individuals of each species were grown in pots under four treatments that had di}ering nutrient distribution but the same overall nutrient addition[ Plants were harvested when roots reached pot edge[ 2 We observed high variation between species in scale\ precision and sensitivity[ No signi_cant discrimination responses were observed\ although greatest root mass density occurred at intermediate fertility levels for all species[ 3 We rejected the hypothesis that scale and precision are negatively correlated[ Indeed\ in herbaceous species alone\ scale and precision were positively correlated[ 4 Sensitivity was not closely related to precision\ indicating that proliferation of roots in fertile patches does not always yield growth bene_ts in heterogeneous soils[ Further\ some sensitive species had very low precision\ suggesting that a positive growth response in heterogeneous environments may be related to plasticity in physiology or root life span\ rather than morphology[ 5 Plant life form was not correlated with precision or sensitivity[ However\ scale of response was greater in herbs than in woody plants\ possibly because the two life forms develop root systems at di}erent rates[ Keywords] morphological plasticity\ nutrient heterogeneity\ root distribution\ root proliferation\ South Carolina Journal of Ecology "0888# 76\ 598Ð508

Between 1987 and 1990, we estimated seedfall and recorded age, growth, and survival of 10 933 tree and vine seedlings growing in the understories of four bottomland hardwood forests in South Carolina. The forests differed in flood frequency, soils, and vegetation structure but had a number of woody plant species in common.Several demographic processes were consistent for all four forests as well as for floodplain forests described in other published studies. Smaller seeded species had larger numbers of seeds dispersed, germinants, and established seedlings. Seed size, however, was not clearly related to seedling survival. Published rankings of shade and flood tolerances were also unrelated to survival, at least during the first growing season after germination. Seedling survival rates were least during the first growing season and greater in subsequent years. Within a growing season, early germinants had greater survival. For some species, survival was negatively related to basal area of neighboring conspecific adult trees.Some aspects of the regeneration process were more site specific. Within species, seedfall densities relative to adult tree abundance differed across forests by an order of magnitude.First-year seedling mortality rates were significantly affected by site and site x species interactions. Mortality in subsequent years was also significantly affected by site. Although the role of flooding in site-specific mortality was not clear, small elevation changes within flooded sites were correlated with changes in germination and survival for some species.

Summary• Nutrient heterogeneity, root foraging and competitive interactions were investigated for six species native to south-eastern USA.• Monocultures, two-and six-species garden plots were fertilized to create spatially homogeneous or heterogeneous nutrient conditions. After 3.5 months, root proliferation in rich patches (precision), mean above-ground biomass per plant (scale) and influence of nutrient treatment on total plot biomass (sensitivity) in monocultures were measured. Competition (above-ground biomass) was assessed in two-and six-species plots.• In monoculture plots, two species were relatively precise foragers, but no species showed significant sensitivity to nutrient treatment. Correlations between precision, scale and sensitivity were weak ( − 0.40 < r < 0.17), which contrasts with previous work showing a scale-precision trade-off. In two-species plots, competition was influenced by soil heterogeneity in two of six cases tested ( ANOVA , P < 0.05), and precise foragers grew larger in heterogeneous than in homogeneous conditions. In six-species plots, nutrient treatment had no influence on growth or competition.• In our study system, heterogeneity effects on competition are context specific, generally weak and potentially mediated by the degree of root foraging precision.

Abstract. Research examining the relationship between community diversity and invasions by nonnative species has raised new questions about the theory and management of biological invasions. Ecological theory predicts, and small-scale experiments confirm, lower levels of nonnative species invasion into species-rich compared to species-poor communities, but observational studies across a wider range of scales often report positive relationships between native and nonnative species richness. This paradox has been attributed to the scale dependency of diversity-invasibility relationships and to differences between experimental and observational studies. Disturbance is widely recognized as an important factor determining invasibility of communities, but few studies have investigated the relative and interactive roles of diversity and disturbance on nonnative species invasion. Here, we report how the relationship between native and nonnative plant species richness responded to an experimentally applied disturbance gradient (from no disturbance up to clearcut) in oakdominated forests. We consider whether results are consistent with various explanations of diversity-invasibility relationships including biotic resistance, resource availability, and the potential effects of scale (1 m 2 to 2 ha). We found no correlation between native and nonnative species richness before disturbance except at the largest spatial scale, but a positive relationship after disturbance across scales and levels of disturbance. Post-disturbance richness of both native and nonnative species was positively correlated with disturbance intensity and with variability of residual basal area of trees. These results suggest that more nonnative plants may invade species-rich communities compared to species-poor communities following disturbance.

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