Species diversity in the Arctic varies dramatically across abiotic gradients, including topography, moisture, pH and nutrient availability. We hypothesized that vascular plant species density, richness and diversity in Alaskan tundra would be correlated with: (i) site age, (ii) soil pH, (iii) above-ground productivity and biomass, and (iv) spatial heterogeneity. We sampled moist tussock, snowbed and water track communities in three sites that differed in substrate age (11,000-4.5 million years since deglaciation) for a variety of environmental and vegetation data over one growing season. Productivity, biomass and heterogeneity were not consistently correlated with species density. However, variation in canopy height was correlated with species density and richness in a unimodal fashion, suggesting that heterogeneity of the light regime may be important for maintaining higher species numbers. The 11,000-year-old site supported more vascular plant species than the two older sites, primarily due to greater number of forb species on the youngest site. Soil pH was significantly positively correlated with species richness (R2 = 0.82) and species density (R2 = 0.61). In general the species found on acidic substrates (pH < 5.5) also occurred on non-acidic substrates (pH > 5.5). This pattern of higher richness with higher pH occurs across other tundra types throughout the Arctic, suggesting that soil pH in an important filter of the regional species pool within northern regions, although other factors may become more important at local scales.