Species-specific influences on resource island nitrogen may limit drought recovery in the Colorado Desert

Background/Question/Methods

Shrub resource islands promote diversity by facilitating establishment in arid plant communities.  Diversity is enhanced when facilitators are abundant, but abundance alone does not promote facilitation.    At our study site in the Colorado Desert,  long-term study shows that perennial seedlings are strongly and positively associated with Ambrosia dumosa, but not with Tetracoccus hallii or Larrea tridentata, three dominant species defined by their canopy cover and spatial distribution.  Adults of these dominants suffered skewed drought mortality with the A. dumosa population reduced by 66%, T. hallii by 58% and L. tridentataby 2%, potentially compromising future recruitment if these species differ greatly in canopy resources.  To assess species variation in canopy resources, we measured nitrogen levels using ion exchange resin bags placed in 6 plots at least 35x35m.  In each plot, 7 plants of each species were identified and sampled under the canopy and at 45 from the canopy edge.  In each plot, resin bags were also placed at 5 locations 150 cm from the edge of any adult canopy.  For three years, all bags were collected and replaced at the end of spring and summer to account for seasonal variation in nitrogen.

Results/Conclusions

We found significant variation in canopy nitrogen among species.  Nitrogen mineralization (P=0.006) and nitrification (P<0.0001) rates differed among species.  A. dumosa typically showed significantly higher mineralization rates than T. hallii and L. tridentata (α=0.05) while nitrification rates were significantly greater under T. hallii canopies than A. dumosa or L. tridentata (α=0.05).  Location relative to plant canopies also significantly influenced nitrogen mineralization rates (P<0.0001) but not nitrification rates.  Growing season NH₄⁺ concentrations were significantly greater under plant canopies and decreased with distance from plant canopies ( P<0.001).  This gradient was absent during the summer, which is likely due to reduced microbial activity when water is limited.  These results demonstrate species-specific controls over nitrogen availability and suggest that observed positive associations of perennial seedlings with A. dumosa are a direct consequence of the quality of resource island associated with this species.  These results further suggest that the severe mortality of A. dumosa adults may reduce local diversity if the co-dominants are unable to facilitate common perennials.