FROM LEAVES TO CANOPY: THE EFFECTS OF EXPERIMENTAL DROUGHT ON SPECTRA

Abstract

Grasslands are water limited ecosystems increasingly impacted by drought, yet the mechanisms linking precipitation change to ecosystem structure and function remain poorly resolved. This dissertation integrates leaf level and canopy-level responses to both experimentally manipulated drought and temporal (seasonal and interannual) variation in rainfall while also addressing recovery to understand what shapes the structure and function of a mixed grass prairie. Our results showed that variation in precipitation strongly influences grassland canopy reflectance, which can be detected using near-infrared (NIR) remote sensing. Drier years lead to lower greenness (NDVI) and higher leaf area index (LAI), reflecting reduced vegetation and increased litter. We found that plant species composition, specifically the abundance of drought-adapted species (C3 legume, Lespedeza cuneata) significantly shaped canopy responses. Further, we found that plant species exhibit different drought strategies. Cool season species (C3 forbs and grasses) avoided drought through early growth, while warm season species (C4 grasses) adopted a conservative, stress-tolerant strategy. Taken together, seasonal shifts in traits occurred, with resource-acquisitive traits early in the season and more conservative traits later; with severe drought reducing key leaf traits (specific leaf area and equivalent water content) and overall canopy function, while moderate drought can promote competitive or stress-tolerant strategies. At the community level, community-weighted traits are closely linked to canopy reflectance, especially under extreme drought, explaining much of the variation in vegetation indices. Drought reduces greenness, increases senescence, and alters canopy structure and pigments. Importantly, overall annual precipitation—not just experimental treatments—drives year-to-year variability. Finally, we find that experimental drought can have legacy effects with cool season species (C3) shifting towards competitive or ruderal strategies that persist after drought, while warm season grasses (C4 species) remain stable. Community structure changes, including reduced plant dominance and altered leaf trait distributions, continue into drought recovery. Overall, drought causes long-term reorganization of grassland communities, and recovery is slow, shaped by legacy effects. Together, these findings advance trait-based interpretation of remotely sensed signals and provide mechanistic insight into grassland responses to increasing summer drought events.