Drivers of nocturnal water flux in a tallgrass prairie

1. Nocturnal transpiration can impact water balance from the local community to earth-atmosphere fluxes. However, the dynamics and drivers of nocturnal transpiration among coexisting plant functional groups in herbaceous ecosystems are unknown. 2. Here, we addressed the following questions: (1) How do nocturnal (E-night) and diurnal (E-day) transpiration vary among coexisting grasses, forbs, and shrubs in a tallgrass prairie? (2) What environmental variables drive E-night and do these differ from the drivers of E-day? (3) Is E-night associated with daytime physiological processes? 3. We measured diurnal and nocturnal leaf gas exchange on perennial grass, forb and woody species in a North American tallgrass prairie. Measurements were made periodically across two growing seasons (May-August 2014-2015) on three C-4 grasses (Andropogon gerardii, Sorghastrum nutans and Panicum virgatum), two C-3 forbs (Vernonia baldwinii and Solidago canadensis), one C-3 sub-shrub (Amorpha canescens) and two C-3 shrubs (Cornus drummondii and Rhus glabra). 4. By extending our study to multiple functional groups, we were able to make several key observations: (1) E-night was variable among co-occurring plant functional groups, with the highest rates occurring in C-4 grasses, (2) E-night and E-day exhibited different responses to vapour pressure deficit and other environmental drivers, and (3) rates of E-night were strongly related to predawn leaf water potential for grasses and woody species, and were likely modulated by small-scale changes in soil moisture availability. 5. Our results provide novel insight into an often-overlooked portion of ecosystem water balance. Considering the high rates of E-night observed in C-4 grasses, as well as the widespread global occurrence of C-4 grasses, nocturnal water loss might constitute a greater proportion of global evapotranspiration than previously estimated. Additionally, future predictions of nocturnal water loss may be complicated by stomatal behaviour that differs between the day and at night. Finally, these data suggest a water-use strategy by C-4 grasses wherein the high rates of E-night occurring during wet periods may confer a competitive advantage to maximize resource consumption during periods of greater availability.