InXuence of temporal heterogeneity in nitrogen supply on competitive interactions in a desert shrub community

Soil nutrients in arid systems are supplied to plants in brief pulses following precipitation inputs. While these resource dynamics have been well documented, little is known about how this temporal heterogeneity inXuences competitive interactions. We examined the impacts of the temporal pattern of N supply on competitive intensity and ability in an N-limited desert shrub community. At our Weld site, the three codominant shrubs, Atriplex confertifolia, A. parryi, and Sarcobatus vermiculatus, diVer in seasonal growth patterns, with A. confertifolia and S. vermiculatus achieving higher growth rates earlier in the growing season than A. parryi. We predicted that these timing diVerences in maximum growth rate may interact with temporal variation in N supply to alter competitive abilities over time. Seedlings of the two Atriplex species were planted either individually in Weld plots or as target plants surrounded by neighbor seedlings. After one year of establishment, the same amount of N was applied to plots either as early spring pulses, mid spring pulses or continuously through the second growing season. Competitive eVects were observed under continuous and pulsed N supply. Averaged across all target–neighbor treatments, competitive intensity was »1.8-fold greater when N was pulsed compared to when N was supplied continuously, but overall, the outcome of competitive interactions was not inXuenced by N pulse timing. While the timing of resource supply did not diVerentially inXuence the competitive abilities of coexisting species in this system, the temporal pattern of resource supply did alter the intensity of competitive interactions among species. While additional studies in other systems are needed to evaluate the generality of these results, this study suggests that competitive intensity may not necessarily be a direct function of productivity or resource availability as traditionally assumed. Instead, the intensity of competitive interactions in resource-poor systems may depend upon the temporal pattern of resource supply.