Prepared by Andreas Stampfli & Michaela Zeiter
During severe droughts grassland herbs dehydrate, they either persist or die. The composition of vegetation after drought is determined by species which re-sprout from protected tissue or recruit from seed. Potential drivers of change in vegetation composition and function, specifically the impacts of drought seasonal timing and seed availability, are not fully understood and were therefore experimentally investigated in a temperate semi-natural grassland.
Seed availability in grassland is locally controlled by management and modified by regional-scale changes in land use which has fragmented habitats, altered pastoral systems and impoverished dispersal agents. Management practices which tend to advance harvesting dates and accelerate harvesting speed are thus expected to exacerbate the seed deficits due to regional-scale dispersal limitations.
This experimental study unravels how three crossed factors – drought in late summer, drought in spring and enhanced seed rain – affect established plants and recruitment and how the effects on recruits modify community composition and function over five years post drought. The overarching hypothesis is that drought timing and seed availability alter community composition and functioning via effects on recruitment.
The experiment was performed in species-rich grassland which has been managed by smallholder families respecting late cutting dates in summer and autumn in compliance with current federal regulations for dry meadows and pastures of Swiss national importance. Under automated rainout shelters, extreme seasonal drought events were simulated in late summer and the following spring. Rainwater was added to simulate normal conditions in no-drought controls, while haymaking was maintained at times of normal practice. Towards the end of the summer drought, ten locally collected species, representing major plant functional groups, were added at a density of normal seed rain in a well-replicated three-factor split-plot arrangement. Biomass of graminoids and forbs was assessed at regular harvesting dates. The frequency of species was assessed for established plants and recruits before treatment start. These assessments were repeated for three years thereafter. New seedlings were marked with colored ringlets to distinguish recruits of three intervals between yearly censuses. Biomass of annual recruit cohorts was measured in year 4 after drought.
One-time seed addition augmented recruits, modified species composition and enhanced species diversity. These effects propagated to increased reproductive shoots of recruits and community biomass four years later. Single and repeated seasonal droughts only caused low adult plant mortality but clearly reduced recruitment from seed, while post-drought establishment was slightly enhanced. Seed augmentation compensated the negative spring drought effect on forb recruits and in turn mitigated the negative drought impact on species diversity after drought. This experimental study shows that seed deficiency results in a longer-term yield decline. Compensation of seed deficiency can support rejuvenation in a critical phase during extreme drought, moderate negative drought impacts and contribute to stabilizing grassland species diversity and functioning in the long run. Releasing the current constraints of management on seed supply in grassland would therefore also assist in mitigating negative drought impacts linked to climate warming in semi-natural grasslands.
This is a plain language summary for the paper of Stampfli & Zeiter published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12889).