Transpiration of four common understorey plant species according to drought intensity in temperate forests

Water use patterns of understorey vegetation are species-dependent. Calluna vulgaris showed little or no regulation of transpiration in response to soil water depletion or air vapour pressure deficit, unlike Pteridium aquilinum, Rubus sp. and Molinia caerulea. Evapotranspiration at forest stand scale is the sum of three components: overstorey and understorey transpiration, and evaporation from soil. During periods of soil water shortage, evapotranspiration of trees declines significantly, but the response of understorey vegetation is less well known. Some reports suggest that understorey vegetation can sometimes be the main source of water depletion in a forest stand during drought episodes. We assessed transpiration in response to decreased soil water content (SWC) and increased vapour pressure deficit (VPD) in the atmosphere for four understorey species with contrasting patterns of resource capture. Potted plants of Pteridium aquilinum, Molinia caerulea, Calluna vulgaris and Rubus sect. Fruticosi were grown under two radiation levels combined with three levels of SWC. Temperature, radiation, VPD and transpiration were monitored. Calluna vulgaris displayed a water spender behaviour with little or no regulation of transpiration during soil water depletion and increased VPD, whereas Pteridium aquilinum showed a low transpiration rate whatever the conditions. Rubus sect. Fruticosi gradually decreased transpiration during soil water depletion and increased VPD, whereas Molinia caerulea responded strongly to soil water depletion but only moderately to VPD. This study highlights the importance of adding identity and water use strategy of understorey species to the tree canopy component to establish a reliable forest water balance.