COS 275-5 - CANCELLED - Increased phosphorus availability intensifies tree-grass competition

Abstract: Essential macronutrients like nitrogen and phosphorus have been known to limit savanna tree growth and are important determinants of savanna vegetation dynamics. However, the mechanisms that facilitate tree-grass coexistence with regards to nitrogen and phosphorus remain unclear, hindering our ability to predict how savannas may respond to global change and altered nutrient cycling and dynamics in the future. Here, we examine how 6 tree species and 1 grass species from Kruger National Park, South Africa, respond to competition under different soil nutrient conditions in a greenhouse study in constant and non-limiting field water conditions. We grew saplings for 16 weeks under fully factorial nutrient and grass competition treatments (with/without grasses, low/high rate of N supply, and low/high rate of P supply) to evaluate if (1) trees and grasses compete for different resources (N vs. P) to minimize competition or (2) if grasses are more competitive in low nutrient environments while trees are more competitive in high nutrient environments. Trees and grasses compete for the same nutrients, specifically nitrogen, and allocate biomass according to nitrogen availability, though tree biomass allocation responded to competition as well. While trees did grow more with increased nutrients, trees were not better competitors in high nutrient environments. Instead, grasses were better competitors in both low and high nutrient environments as competition intensity increased for trees in high phosphorus treatments because grasses utilize aggressive nutrient foraging strategies, which include high phosphatase activity, large root biomass allocation, and high relative growth rates. In low nutrient environments, trees may grow less due to nutrient limitation but in higher nutrient environments, nutrients may facilitate increased tree growth, providing a chance for saplings to escape the fire trap with the caveat that grasses will also increase in size and create a more substantial fuel layer. Soil nutrients likely interact dynamically with soil texture and fire in savannas, and future changes in nutrient cycling and N deposition may have important effects on savanna vegetation dynamics.