Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2007
Publication Date: 3/10/2007
Citation: James, J.J., Richards, J. 2007. Influence of temporal heterogeneity in nitrogen supply on competitive interactions in a desert shrub community. Oecologia 152:721-727. Interpretive Summary: Although temporal variation in soil resource availability has been well-documented in rangeland systems little is known about how this variation influences plant competitive interactions. We applied the same amount of nitrogen to shrubs surrounded by neighbor plants either at a single point in time during the growing season or continuously through the growing season. Results show that the ability of a species to compete for nitrogen was not influenced by the pattern of nitrogen supply but the intensity of competition was greater when nitrogen was supplied at a single point in time. While competitive interactions are traditionally assumed to be weak in rangeland systems our results suggest the intensity of competition in rangeland systems can be strongly influenced by the temporal pattern of resource supply.
Technical Abstract: Soil resources in arid systems are supplied to plants in brief pulses following precipitation inputs. While these resource dynamics have been well-documented, little is know about how this temporal heterogeneity influences competitive interactions. We examined the impacts of the temporal pattern of N supply on competitive intensity and competitive ability in an N-limited desert shrub community. At our field site, the three co-dominant shrubs, Atriplex confertifolia, A. parryi, and Sarcobatus vermiculatus, differ in seasonal growth patterns, with A. confertifolia and S. vermiculatus achieving higher growth rates earlier than A. parryi. We predicted that these timing differences in maximum growth rate may interact with temporal variation in N supply to alter competitive abilities overtime. Seedlings of the two Atriplex species were planted either individually in plots or as target plants surrounded by neighbor seedlings. The same amount of 15N was applied to plots either as early spring pulses, mid spring pulses or continuously through the growing season, and competitive effects were observed. 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, competitive interactions were not influenced by N pulse timing. Therefore, while the timing of resource supply did not differentially influence the competitive abilities of coexisting species in this system, the temporal pattern of resource supply did alter the intensity of competitive interactions among species. Instead of competitive intensity being a direct function of productivity or resource availability as traditionally assumed, theses results suggest the intensity of competitive interactions in resource-poor systems can depend on the temporal pattern of resource supply.