Seasonal Rainfall, Shrub Cover and Soil Properties Drive Production of Winter Annuals in the Northern Sonoran Desert

Winter annual plants play an important role in arid and semiarid ecosystems because of their rapid response to resource pulses, which drive primary production that provides resources for herbivores and pollinators. Understanding the factors that control annual plant growth is key to predicting how arid and semiarid ecosystems will respond to changes in climate and resource availability from anthropogenic activities. We used a long-term nutrient enrichment experiment that spanned precipitation and urbanization gradients in central Arizona, USA, to examine the effects of climate, surface soil properties, soil nutrient availability and shrub cover on winter annual plant growth. At a landscape scale, aboveground net primary production (ANPP) of winter annual plants had a positive, nonlinear relationship to the amount of precipitation received from October through March of the current growing season. We found evidence for sequential resource limitation of ANPP initially from water then nitrogen and phosphorus. The resource limitation cascade was modified by surface soil properties and location relative to shrubs (under or between shrubs), highlighting the effect of small-scale factors on large-scale processes. Specifically, gravel cover had a negative effect on ANPP, and the effect of shrub cover on ANPP depended on nitrogen and current season rainfall. Our study emphasizes how small-scale factors, such as gravel cover, nutrient availability and presence of shrubs, can interact with large-scale drivers, such as seasonal precipitation, to affect interannual variation in winter annual plant production in the northern Sonoran Desert.