The Ecohydrological Impacts of Secondary Precipitation Processes

Mean annual precipitation is a major driver of terrestrial primary productivity and has exhibited long-term stationary behavior over land areas. On the contrary, rainfall variability has been impacted by climate change. and in-situ studies have shown its importance on ecosystem productivity. However, existing studies have rarely been performed on continental to global scales due to the difficulty in accounting for external factors varying across biomes such as climatologies, soil characteristic or plant functional traits that influence ecosystem sensitivity to precipitation variance. The ecohydrological role of secondary precipitation processes is therefore investigated in this thesis on continental scales through three distinct projects. The first project quantifies the frequency of dew formation across the US using a novel network of in-situ infrared radiometers. While xeric grasslands receives 1-2 dew events per week independently of air moisture levels, the dew frequency in mesic and tropical grasslands varies from 15% to 95% of dewfall per night depending on relative humidity. Forests show a consistent frequency of dew formation of 25% of the nights due to a denser canopy cover. The second project investigates the impact of rainfall variability on the productivity of five major biomes using a combination of satellite data and a network of rain gauge measurements. Mesic forests and grasslands show decreased productivity of 28% and 7% (respectively) with higher intra-annual rainfall variability, while this sensitivity is halved in response to higher inter-annual variability. Xeric grasslands respond positively to an increase of rainfall variability on an inter-annual scale, but their productivity decrease with more irregular rain within the year. Conversely, boreal forests are the only biome that will be more productive with higher variability on both timescales. The last project is dedicated to the development of a daily rain generator that better captures rainfall variability compared to the most popular generator used by ecohydrologists. The ecological importance of this difference is demonstrated with the use of a soil moisture model. This thesis brings a significant contribution to the ecohydrology field through the development of novel procedures to isolate the impact of secondary precipitation processes on ecosystems.