Nearby woody patches and microtopography reduce grass dieback during extreme drought

Extreme droughts related to climate change can be a driver of habitat transition by affecting the survival and reproduction of dominant plant species, while different microrefugia can buffer against this. The extreme drought of 2022 caused a significant dieback of the dominant perennial grasses in open sandy grasslands, in Hungary. We used this event to test our hypothesis: nearby woody cover and certain microtopography support the survival and recovery of dominant grasses after drought. We surveyed grass dieback in 200 plots within an unmanaged grassland site. A fineresolution digital terrain model and aerial photos were used to determine topographic variables (slope, aspect, topographic position) and woody cover. We tested the effect of these factors on the ratio of dead grass and the amount of grass seedlings. Nearby woody cover had a significant sheltering effect: there was almost no grass dieback at high woody cover, modified by aspect and topographic position. High woody cover had the strongest effect on the northern aspect and on hilltops. While at low woody cover, aspect had no influence and valley position had a positive impact. The dead grass ratio was also lower on steeper slopes. Seedlings behaved differently: there were fewer seedlings on the more northern aspects and on steeper slopes, likely due to the lack of microhabitats available in the absence of grass dieback. We conclude that both woody cover and microtopography are important for survival and recovery of open sand grasslands, as they provide favourable microhabitats for grasses to survive extreme weather events.