Hill Slope Variations in Chlorophyll Fluorescence Indices and Leaf Traits in a Small Arctic Watershed

Physiological processes responsible for ecosystem carbon and nitrogen cycling may vary across hill slopes and be controlled by watershed hydrology and the associated nutrient transport. Mass transport of nutrients down slope and into water tracks may increase nutrient delivery to plant roots, nutrient uptake, and perhaps photosynthetic activity. Small arctic watersheds are commonly characterized by increased biomass, particularly of woody deciduous shrubs, both down slope and in water tracks. We ask if photosynthetic physiology varies with hill slope position and if it is correlated to observed changes in above ground biomass. Chlorophyll fluorescence surveys from six common species reveal that maximum photosynthetic electron transport decreased significantly (by as much as 85%) down slope in 4 species. Leaf nitrogen concentrations varied from 1 to 2.5% across all leaves sampled, but show little trend with hill slope position, and as a result are not well correlated with photosynthetic electron transport. We hypothesize that trace metal concentrations may have increased in the leaves of plants growing in down slope positions and that this may be responsible for the reduction in electron transport. The relationship between the measured maximum energy conversion by photosystem II and maximum electron transport rate is species specific and indicative of light adaptation in these arctic species. Increased plant growth down slope and in water tracks does not appear to be correlated to the physiological parameters measured and instead are more likely a product of increased canopy nitrogen concentrations and leaf area accumulation.