Drought has inconsistent effects on seed trait composition despite their strong association with ecosystem drought sensitivity

1. Seeds provide the basis of genetic diversity in perennial grassland communities and their traits may influence ecosystem resistance to extreme drought. However, we know little about how drought effects the community functional composition of seed traits and the corresponding implications for ecosystem resistance to drought. 2. We experimentally removed 66% of growing season precipitation for four years across five arid and semiarid grasslands in northern China and assessed how this multi-year drought impacted community-weighted means (CWM) of seed traits, seed trait functional diversity, and aboveground net primary productivity (ANPP). 3. Experimental drought had limited effects on CWM traits and the few effects that did occur varied by site and year. For three separate sites, and in different years, drought reduced seed length and phosphorus content but increased both seed and seed-coat thickness. Additionally, drought led to increased seed functional evenness, divergence, dispersion, and richness, but only in some sites, and mostly in later years following cumulative effects of water limitation. However, we observed a strong negative relationship between drought-induced reductions in ANPP and CWMs of seed-coat thickness, indicating that a high abundance of dominant species with thick seeds may increase ecosystem resistance to drought. Seed trait functional diversity was not significantly predictive of ANPP, providing little evidence for a diversity effect. 4. Our results suggest that monitoring community composition with a focus on seed traits may provide a valuable indicator of ecosystem resistance to future droughts despite inconsistent responses of seed trait composition overall. This highlights the importance of developing a comprehensive seed and reproductive traits database for arid and semiarid grassland biomes.