COS 106-6 - Plant strategies of nitrogen acquisition in a frequently-disturbed, pyrogenic system

Wednesday, August 9, 2017: 3:20 PM
C122, Oregon Convention Center
Cari D. Ficken, University Program in Ecology, Duke University and Justin P. Wright, Biology, Duke University, Durham, NC

Understory plant regrowth following fire is hypothesized to be a dominant mechanism controlling post-fire nitrogen (N) leaching losses. Consequently, assessing if and why resprouting species differ in their capacity to acquire N will have important implications for predicting N biogeochemistry and forest productivity after prescribed burns. At a more fundamental level, pulses of high soil N availability are ubiquitous and occur following many environmental drivers, but the role of temporal resource heterogeneity is rarely tested as an ecological mechanism structuring terrestrial plant communities. In this study, we combined an isotopic 15N tracer experiment with a paired-species competition experiment to examine how regrowth following clipping is mediated by interspecific differences in N uptake. Our study focuses on co-occurring understory species from southeastern pine forests because this ecosystem experiences frequent N pulses associated with prescribed burns, and the plant community begins resprouting within a few days. We address the following questions: Do species differ in N uptake rate under low and high N supply? If so, does the capacity to capture a N pulse affect biomass accumulation under pulsed (single high fertilization event) versus pressed (multiple low fertilization events) N supply?


Four co-occuring understory species (Ilex glabra, Lyonia lucida, Pinus palustris, and Vaccinnium formosum) exhibited substantial differences in average N uptake under low and high supply. Under low N supply, Vaccinnium had the highest maximum uptake rate (Vmax), while Ilex had the highest Vmax under high supply (p=0.003). Under low N, higher root:shoot ratios were correlated with N uptake of each species (R2=0.74, p<0.001); specific root length explained the most variation in uptake rates under high N (p<0.001, R2=0.60). This suggests that N uptake strategies may be affected by tradeoffs in root growth and architecture. In the next experiment, we planted the press-competitor (Vaccinnium) and the pulse-competitor (Ilex) in all pairwise combinations. Under pulsed N supply, Ilex accumulated ~3x more biomass when paired with Vaccinnium, the poor competitor for pulsed N, than when paired with itself (p=0.005). Under pressed N supply, Ilex accumulated ~55% less biomass when paired with Vaccinnium, the superior competitor for pressed N, than when paired with itself (p=0.025). This suggests that strategies for N uptake under different supply regimes may influence competitive interactions and biomass regeneration following disturbance. An experiment scaling these responses to field-patterns of N retention following disturbance is ongoing.