Vascular plant species richness in Alaskan arctic tundra: the importance of soil pH

1 Species diversity in the Arctic varies dramatically across abiotic gradie nts, including topography, moisture, pH and nutrient availability. We hypot hesized that vascular plant species density, richness and diversity in Alas kan tundra would be correlated with: (i) site age, (ii) soil pH, (iii) abov e-ground productivity and biomass, and (iv) spatial heterogeneity. We sampl ed moist tussock, snowbed and watertrack communities in three sites that di ffered in substrate age (11 000-4.5 million years since deglaciation) for a variety of environmental and vegetation data over one growing season. 2 Productivity, biomass and heterogeneity were not consistently correlated with species density. However, variation in canopy height was correlated wi th species density and richness in a unimodal fashion, suggesting that hete rogeneity of the light regime may be important for maintaining higher speci es numbers. 3 The 11 000-year-old site supported more vascular plant species than the t wo older sites, primarily due to greater numbers of forb species on the you ngest site. 4 Soil pH was significantly positively correlated with species richness (R- 2 = 0.82) and species density (R-2 = 0.61). In general the species found on acidic substrates (pH < 5.5) also occurred on non-acidic substrates (pH > 5.5). 5 This pattern of higher richness with higher pH occurs across other tundra types throughout the Arctic, suggesting that soil pH is an important filte r of the regional species pool within northern regions, although other fact ors may become more important at local scales.