1 Understanding the relative distributions of ecological generalists vs. specialists requires precise characterization of the environmental ranges of closely related taxa. The ecological breadth of four annual species in the genus Polygonum was determined from field measurements taken from five natural populations per species in a common geographical range.
2 Significant early and late-season differences among the species were found for available light (photosynthetically active radiation) at canopy and mid-canopy levels, and for soil temperature, moisture availability, macronutrient content, pH, cation exchange capacity (CEC) and structure, at two depths. Field sites within each species also differed significantly for these variables.
3 The field distribution of P. persicaria covers the broadest range of habitats, from moderate shade with very dark microsites to full insolation, cool to very warm soils, flooded to dry moisture conditions, and organic, high-nutrient to nutrient-poor soils.
4Polygonum lapathifolium is comparatively intolerant of shade, particularly early in the growth season. Although this species occurs in flooded to moderately dry conditions and in poor as well as rich soils, its moisture and nutrient ranges do not include such low extremes as those of P. persicaria, and its range of soil temperatures is also narrower.
5Polygonum cespitosum is restricted to low-light habitats and to consistently moist soils that do not flood. The species is, however, found in a moderately broad range of soil types and macronutrient availabilities. Polygonum cespitosum occurs in extremely low-light habitats that are evidently beyond the shade tolerance of the other species.
6Polygonum hydropiper is restricted to high-light sites with highly organic, consistently very moist or flooded soils. Unlike its congeners, this species can tolerate flooded soils during seedling establishment. The species is limited to soils with high early nitrate and calcium content, moderate CEC, and pH close to 6.0, but tolerates a broad range of soil temperatures.
7 The occurrence of spatial and temporal environmental variability within as well as among field populations of Polygonum species suggests that tolerance of such variability may result from individual phenotypic plasticity rather than from ecotypic adaptation of entire populations.