Effect of defoliation upon root growth, phosphate absorption and respiration in nutrient-limited tundra graminoids.

Moderate experimental defoliation stimulated root respiration and phosphate absorption in two tundra graminoids, Eriophorum vaginatum and Carex aquatilis, growing under nutrient-limited field conditions in northern Alaska. The increase in phosphate absorption rate following defoliation of Eriophorum was associated with a decrease in root phosphate and available carbohydrate contents per unit root length but a constant root nitrogen content. Only after four repeated defoliations did phosphate absorption rate decrease below control levels. We suggest that the stimulation of root respiration and phosphate absorption immediately following defoliation resulted from lowered root phosphorus status as nutrient reserves were reallocated to support shoot regrowth. Root growth was affected more severely by defoliation than was root activity. Two or more defoliations reduced root elongation, initiation and weight per unit length, but root mortality increased only after four defoliations. Carex aquatilis, a species with large belowground biomass, was less sensitive to defoliation than Eriophorum. Phosphate absorption rate increased only after four defoliations in this species, and root elongation, initiation and mortality were affected only by the most severe clipping regimes. Responses of plants to repeated defoliation over two growing seasons were consistent with results of short-term studies.