We examined relationships of vascular plant species richness with mean July temperature and components of landscape heterogeneity to determine the relative influence of temperature and the physical landscape on plant richness along the north-flowing Hood River in the Northwest Territories of Canada. We also examined variations in the composition of the flora to better understand the relationship between riparian gradients, environmental controls, environmental heterogeneity, and species richness. The vascular flora for the area studied includes 210 species. Richness at 17 sites along the river ranged from 69 to 109 species within 2400-m2 sample areas. Sites with the lowest richness were those in the upper reaches of the river, with richness generally increasing downstream. Variation in richness along the river is correlated with increasing environmental heterogeneity (r2 = 0.598, P = 0.0003), calculated as an index summarizing the range of site-level variation in a set of components including substrate type and texture, topographic variation (slope and aspect), relative surface area, substrate moisture, and soil pH. The most significant component of the index is an increase in the range of soil pH. Soil pH tends to increase downstream, and average site soil pH is the single best predictor of species richness (r2 = 0.857, P < 0.0001). The primary cause of higher soil pH is the presence of uplifted marine sediments, and tills derived from nonacidic Precambrian rock common along the lower river. Key words: species richness, arctic, riparian, pH, mean July temperature, environmental heterogeneity.
