CONTRASTING ECOLOGICAL BREADTH OF COOCCURRING ANNUAL POLYGONUM SPECIES

1 Understanding the relative distributions of ecological generalists v s. 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 geographic al range. 2 Significant early and late-season differences among the sp ecies were found for available light (photosynthetically active radiat ion) at canopy and mid-canopy levels, and for soil temperature, moistu re availability, macronutrient content, pH, cation exchange capacity ( CEC) and structure, at two depths. Field sites within each species als o 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. 4 Polygonum lapathifolium is comparatively in tolerant of shade, particularly early in the growth season. Although t his species occurs in flooded to moderately dry conditions and in poor as well as rich soils, its moisture and nutrient ranges do not includ e such low extremes as those of P. persicaria, and its range of soil t emperatures is also narrower. 5 Polygonum 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 typ es and macronutrient availabilities. Polygonum cespitosum occurs in ex tremely low-light habitats that are evidently beyond the shade toleran ce of the other species. 6 Polygonum hydropiper is restricted to high- light sites with highly organic, consistently very moist or flooded so ils. Unlike its congeners, this species can tolerate flooded soils dur ing 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 suc h variability may result from individual phenotypic plasticity rather than from ecotypic adaptation of entire populations.