The relative importance of abiotic and biotic factors for seedling establishment in the Colorado Desert

Background/Question/Methods

The US Southwest has experienced widespread plant mortality that is largely attributed to extensive drought. In the Colorado Desert, CA this drought caused unprecedented mortality among common adult shrubs and almost complete mortality of juveniles in a long-term study plot that has been monitored for over 25 years. To date, no significant regeneration of these species has been observed.  Ongoing research at this site demonstrates that plant facilitators, species that improve seed germination and seedling survival, are important for seedling establishment of many common species.  Ambrosia dumosa, Larrea tridentata and Tetracoccus hallii are the dominant shrubs at this study site.  Juvenile perennials show distinct spatial associations with these dominants.  The extent to which spatial heterogeneity is driven by abiotic or biotic factors is unclear.  The objective of this study was to determine the influence of dominant adult shrubs on resource distribution.  During August of 2009 soil samples of 2.5 cm diameter were taken at a depth of 7.5 cm from the canopy center, canopy periphery, and 1.5 m from the canopy periphery of randomly selected adult A. dumosa, L. tridentata and T.  hallii.  Soils were analyzed for ammonium and nitrate.   

Results/Conclusions

Ambrosia dumosa has a significant canopy effect for ammonium (P=0.008) and nitrate (ANOVA P= 0.005).  Larrea tridentata has a significant canopy effect for ammonium (P=0.004) but not for nitrate (P=0.40). Tetracoccus hallii has a marginally significant canopy effect for ammonium (P=0.061) but not for nitrate (P=0.32).  These results suggest that dominant shrub species drive local nitrogen distributions.  Additionally, shrub species identity appears important for the distribution of nitrogen. The magnitude at which nitrogen changes spatially and temporally at this long-term study site is the focus of current research.   These results suggest that facilitation of seedlings is species specific.  Germination and seedling survival in response to the resource profiles of dominant species may be an important component of community recovery.