Spring annual plants play an important role in arid and semi-arid systems because of their rapid response to resource pulses, which affects species composition and primary production with cascading effects on herbivores and pollinators. Understanding the factors that control production and diversity of annual plant communities is key to predicting how arid and semi-arid ecosystems will respond to changes in climate and resource availability from anthropogenic activities. Increased nitrogen (N) deposition alters production and threatens biodiversity in herbaceous plant communities, but their effects in arid urban ecosystems and their interactions with interannual variability in climate are not well studied. In the Sonoran Desert, annual plant communities make up most of the plant diversity, support wildlife, and contribute to nutrient cycling and ecosystem processes. Functional tradeoffs allowing coexistence of a diversity of annual plant species are well established, but maintenance of diversity in urban conditions and with increased availability of limiting nutrients has not been explored. We conducted a 13-year N and phosphorus (P) addition experiment in Sonoran Desert preserves in and around Phoenix, AZ to test how nutrient availability interacts with growing season precipitation, urban location, and microhabitat to affect winter annual plant productivity and diversity.
Results/Conclusions
Aboveground net primary production (ANPP) of annual plants had a positive, non-linear relationship to amount of precipitation received from October through March in the current plus previous year. We found evidence for sequential nutrient limitation of ANPP from water to nitrogen and phosphorus, although this relationship varied with soil properties and under versus between shrubs. Using structural equation modeling and generalized linear mixed models, we found that annual plant taxonomic diversity was significantly reduced in N-enriched and urban plots. Water availability of both current and previous growing seasons also strongly influenced annual plant diversity, with significant interaction effects resulting in increased diversity in wetter years and greater responsiveness of the community to N in sequential wet years. However, there were no significant interactions between N enrichment and water availability, urban location, or microhabitat on annual plant diversity. Our study emphasizes how microhabitat interacts with seasonal precipitation to affect inter-annual variation in spring annual plant production. Our results also demonstrated that N enrichment reduced taxonomic richness. Changes in biodiversity in urban preserves can have important implications for connections between urban residents and nature, and reduced diversity and community restructuring with N enrichment represents a challenge for future preservation of aridland biodiversity.