Recurrent summer drought affects biomass production and community composition independently of snowmelt manipulation in alpine grassland

Earlier snowmelt and more frequent summer drought due to climate warming are considered particularly influential for extratropical alpine plants, which are adapted to a short growing season and high water availability. Here, we explored the combined effects of the two drivers with a field experiment in late-successional alpine grassland in the Swiss Alps (2500 m a.s.l.) over 6-7 years. We advanced and delayed snowmelt by removing and adding snow to experimental plots prior to natural snowmelt for 7 years and combined this treatment with 5 and 10 weeks of summer drought for 6 years. We measured plant biomass formation, community composition and ecosystem respiration, and monitored soil moisture as well as soil temperature. Natural snowmelt dates varied by 42 days across years. Snow manipulations advanced and delayed snowmelt by 4.6 and 8.0 days on average but did not affect annual growth (peak biomass) above- nor below-ground. Interactions between snowmelt and drought were nonsignificant, implying that drought effects were independent of snowmelt. Drought reduced total annual above-ground biomass in the 10-week treatment by 16 & PLUSMN; 7% across years, while the 5-week treatment lowered biomass in the last year only. This decline in biomass was accountable to high drought sensitivity of biomass production in a few forb and graminoid species. In contrast, drought did not affect the biomass production of the dominant sedge Carex curvula, whose proportion of total plant cover increased from 36% in controls to 48% in 10-week drought. Below-ground biomass slightly increased under drought (5-week treatment only), resulting in a higher root mass fraction (both treatments). Despite continued root formation, drought reduced ecosystem respiration by 13%-23% per season, assessed nine times during three growing seasons. Since more than 85% of ecosystem respiration stemmed from below-ground activities and roots continued growing under drought, we assume that soil microorganisms were heavily constrained by the drought treatments. Synthesis. We conclude that snowmelt timing is unrelated to productivity, while recurrent drought will shift biomass allocation from shoots to roots in this typical alpine grassland, with potential implications for grazers but also for nutrient and carbon cycling. Species-specific drought-sensitivity will considerably alter community composition under more frequent drought.