Submitted to: Ecological Society of America Bulletin
Publication Type: Abstract Only
Publication Acceptance Date: 4/19/2005
Publication Date: 8/7/2005
Citation: James, J.J. 2005. Seasonal timing of n pulses alters competitive interactions in a desert shrub community [abstract]. Ecological Society of America Bulletin. Paper No. 303. Interpretive Summary:
Technical Abstract: The major processes that structure plant communities in nutrient-poor systems are poorly understood and much debated. Abiotic factors associated with low soil resource supply rates are classically thought to be the major forces shaping these communities. Resource supply rates in these systems, however, are not constant but instead fluctuate between relatively resource-rich pulse phases and resource-poor inter-pulse phases. For example, in arid systems available soil N can increase up to 10-fold following water inputs but then quickly decline due to plant and microbial uptake. Because plant growth is tightly linked to the ability of a species to rapidly exploit an N pulse, competition may be intense in pulse-driven systems and an important factor structuring these communities. In addition, the timing of N pulses may alter the relative competitive abilities of coexisting species if species differ in seasonal patterns of growth rate and N demand. To evaluate the effects of pulsed resource supply on competitive interactions, seedlings of the shrubs Atriplex confertifolia, Sarcobatus vermiculatus, and Atriplex parryi were planted into experimental plots in the Great Basin alone and with intra- and interspecific neighbors. 15N-labeled pulses were applied in either winter or spring or as a continuous low supply through the season. After two years, roots and shoots of target plants were harvested to quantify target plant 15N capture. Across competition treatments competitive intensity was greater when N was supplied as a pulse compared to when N was supplied continuously (P < 0.01). Sarcobatus and A. confertifolia had higher growth rates than A. parryi early in the growing season and these species had a greater effect on A. parryi N capture when N pulses occurred in winter than when pulses occurred in spring (P < 0.05). Likewise, A. parryi had a greater effect on A. confertifolia N capture when pulses occurred in spring compared to when pulses occurred in winter (P < 0.05). This study suggests that competitive interactions can be intense in when N is supply is pulsed and that the competitive interactions between species depend on when the pulses occur in the season. By altering competitive advantages over time, variation in seasonal timing of N pulses also may facilitate species coexistence in these pulse-driven systems.