Patterns of surface rock cover and implications for plant, water, and nutrient dynamics at long-term ecological research sites in Phoenix, AZ
Water and nutrient availability are the primary and secondary drivers of net primary productivity (NPP) in arid ecosystems. Although precipitation regulates water inputs, soil properties influence water availability for plant growth. Aridland soils are often covered with surface rocks, which can increase or decrease water availability by modifying evaporation, infiltration, light levels, and temperature. Due to the complexity of these direct and indirect mechanisms, the relationship between rock cover and NPP is not well understood. In this research we explore the relationship between rock cover, soil nutrient availability, and aboveground growth of desert annual plants over four years across a long-term nutrient enrichment experiment in the Sonoran Desert. We surveyed surface rock cover at fifteen sites in central Arizona that have been fertilized with nitrogen (N) and phosphorus (P), alone and in combination, for seven years. Using ANCOVA, we then explored the relative importance of rock cover, precipitation, and nutrient treatment on peak aboveground biomass of spring herbaceous annual plants that were collected in 2008, 2009, 2010, and 2013. We expected surface rocks to strengthen the positive relationship between precipitation, nutrient additions, and annual plant growth.
Precipitation, nutrient additions, and surface rock cover together significantly influence growth of Sonoran Desert annual plants. As expected, nutrients and precipitation were the strongest drivers of annual plant biomass. Plant growth was positively related to N additions across all four years (ANCOVA, p <0.01); P in 2008 and 2010 years (p = 0.01 and 0.005, respectively);and precipitation in three of the four years (p < 0.05). Precipitation was the primary driver of plant biomass in the two driest years, 2009 and 2013 (partial eta2 = 0.51 and 0.52, respectively). Gravel (2-64 mm diameter) was only rock size class that was significantly related to annual plant biomass. Contrary to our expectations, gravel cover was negatively related to biomass in two of four years (2008 and 2009) with a relatively low effect size compared to rainfall and N (partial eta2=0.136 and 0.1, respectively), but comparable in effect size to P additions . The mechanism underlying the negative relationship between gravel cover and annual plant productivity is unclear. Although rocks can reduce evaporation, which would increase water availability, they may also reduce light availability and water infiltration, or increase soil temperature.