Does drought control emergence and survival of grass seedlings in semi-arid rangelands?. An example with a Patagonian species

Current climate change models for arid ecosystems challenge our ability to understand the effects of droughts on plant population dynamics. In a sandy soil plant community from the Patagonian steppe, we studied soil water dynamics and its effects on grass regeneration. Our hypothesis was that a decrease in precipitation would not change soil water dynamics of the top soil layer where most of the seeds and seedling roots are located. First, we simulated soil water dynamics with a balance model (DINAQUA) for different years and soil layers, in two different periods related to the emergence and survival of plants. Then, we performed a manipulative field experiment in which we generated a drought gradient by intercepting 0%, 25%, 50%, and 80% of the precipitation for two different microsites (vegetated and bare soil). We quantified drought effects on emergence, survival, and biomass of seedlings using a native grass (Bromus pictus Hook). Soil water content in the top soil layer at the end of growing season was a 61% less than mid and bottom soil layers, independently of the climatic condition of the year. The simulations suggested that drought reduces the maximum depth of water infiltration, but available water for plants in the top layer where seedlings have their roots, was not modified, in agreement with our field experiment measurements. Consistently, the emergence (51%), survival (65%), and biomass (15 g/ind) of seedlings were not affected by drought levels. However, there is a positive microsite effect on biomass (36%), indicating a facilitative role of adult plants on grass seedlings. We interpreted these results as indicative that seed germination and seedling recruitment will not be heavily impacted by increasing water deficit in the Patagonian steppe.