Extreme press and pulse droughts diversify rooting depth in a semi-natural grassland

In Central Europe extreme summer droughts are becoming more intense while extremely long periods with intermittent drought stress are likely getting extended as sequential precipitation deficits follow one another at shorter intervals with climate warming. Since the effects of long and mild ‘press’ vs. short and intense ‘pulse’ droughts have rarely been directly compared at the same site so far, the interplay between drought duration and intensity is largely unknown. Using precipitation-reduction and precipitation-exclusion shelters consecutively in a Swiss semi-natural grassland we investigated the impacts of a mild but extremely long (4 years) press drought and an extremely intense but short (5 months) pulse drought on species composition and biomass from 2015-2021.
The press drought showed a consistently lower intensity than the pulse drought based on meteorological and soil moisture metrics: treatments resulted in a tenfold smaller average daily water deficit between gains from precipitation and losses from evapotranspiration and a tenfold smaller average daily difference compared to controls in topsoil water potential for the press than for the pulse drought. Contrary, tenfold longer reduction than exclusion of precipitation resulted in higher drought severity based on meteorological, but lower drought severity based on soil moisture metrics: treatments showed a threefold larger cumulative reduction in precipitation, but an eightfold shorter duration of limiting conditions for plant growth (topsoil moisture <−100 kPa) for the press drought than for the pulse drought.
Plant responses to both extreme droughts were not related to precipitation-based, but mirrored soil moisture-based drought severity metrics. Productivity and composition showed higher resistance but lower resilience to the press drought than to the pulse drought. Secondly, the interactions between press and the pulse drought were only faint and overshadowed by additive main effects expressed by species groups that differ in maximal rooting depth. Specifically, press drought had a strong persistent impact on forb coverage and persistently changed the coverage of species that maximally root at medium (50-100 cm) depth, while pulse drought had a strong persistent impact on graminoid coverage and the coverage of species without deep roots.
Thus, the impact of intermittent and intense stress induced by sequential very extreme precipitation-reduction and precipitation-exclusion treatments in a temperate-humid climate was moderated by different plant functional groups supporting the idea that extreme droughts of various intensity and duration contribute to the coexistence of functional groups in semi-natural grassland.