More persistent rainfall regimes : consequences and solutions for natural and managed grassland systems
Authors:   , Reynaert Simon 
Published: June 2024

Global warming is changing the intra-annual variability of weather patterns around the world. One of the more recent discoveries for the mid-latitudes indicates that summer weather persistence is increasing, resulting in both longer dry and wet spells compared to historic averages. However, the consequences of these newly emerging climate regimes for terrestrial ecosystems have barely been studied. This thesis investigates how increasing weather persistence influences mesic grassland ecosystem functioning in a series of manipulation experiments with model grassland systems under different levels of management intensity. In addition, we explore several solutions for the observed consequences in terms of plant and soil remediation. In unfertilized, species-rich systems, we found evidence that precipitation regimes with longer dry and wet spells reduce aboveground productivity and native plant diversity in the short-term (one to two years) (Chapters II, III & IV). These changes related to differences in species-specific responses to drought and more pronounced fluctuations in nutrient availability over time as well as shifting species interactions under increasing weather persistence. In particular, the length of the longest drought period throughout the growing season was the strongest determinant of short-term diversity and productivity responses. Moreover, the disproportionate loss of legumes and other forbs compared to graminoids, likely negatively affected forage quality. Despite these adverse effects, ecosystems subjected to longer dry and wet spells also showed acclimation in the second year through community reorganization and adapted biochemistry, allowing them to be more productive under recurrent and contrasting extreme soil conditions. In fertilized monoculture systems, longer dry and wet spells reduced yield yet slightly improved forage digestibility (Chapters V & VI). Moreover, soils from permanent grasslands with elevated organic carbon (OC) showed declines in soil water availability under increased weather persistence. However, not all species and soil types were affected equally. Growing Festulolium, Festuca arundinacea or Dactylis glomerata led to significantly smaller yield losses compared to Lolium perenne cultivars. Regarding soil remediation, high OC soils with a permanent grassland history limited negative impacts on plant quality and yield in sandy-loam soils. Moreover, mixing basalt powder with the top soil layer improved yield in unfertilized monoculture systems under prolonged dry and wet spells. To conclude, these findings indicate that increasing weather persistence will likely negatively affect grassland ecosystem service provisioning (i.e., plant diversity, productivity and soil water availability) by altering soil water and nutrient dynamics. However, choosing more drought resistant species and management practices that improve soil carbon sequestration shows potential to limit adverse effects on plant performance, ultimately making natural and managed grassland systems more climate resilient.