Root plasticity maintains growth of temperate grassland species under pulsed water supply

The frequency of rain is predicted to change in high latitude areas with more precipitation in heavy, intense events interspersed by longer dry periods. These changes will modify soil drying cycles with unknown consequences for plant performance of temperate species. We studied plant growth and root traits of juveniles of four grasses and four dicots growing in a greenhouse, when supplying the same total amount of water given either regular every other day or pulsed once a week. Pulsed water supply replenished soil moisture immediately after watering, but caused substantial drought stress at the end of the watering cycle, whereas regular watering caused more moderate but consistent drought. Grasses had lower water use efficiency in the pulsed watering compared to regular watering, whereas dicots showed no difference. Both grasses and dicots developed thinner roots, thus higher specific root length, and greater root length in the pulsed watering. Growth of dicots was slightly increased under pulsed watering. Temperate species coped with pulsed water supply by eliciting two responses: i) persistent shoot growth, most likely by maximizing growth at peaks of soil moisture, thus compensating for slower growth during drought periods; ii) plasticity of root traits related to increased resource uptake. Both responses likely account for subtle improvement of growth under changed water supply conditions.