Nocturnal transpiration is highly variable within a tallgrass prairie community
The impact of nocturnal transpiration on ecohydrology has largely been ignored due to the assumption that plants close their stomata at night to preserve water loss during periods with no carbon gain. Thus, little is known about this phenomenon despite recent observations that nocturnal transpiration occurs often and can contribute significantly to total plant water use. Here, we assessed rates and variability in nocturnal transpiration for multiple grassland species within a tallgrass prairie. Our objectives were to: (1) identify which species transpire at night, (2) quantify changes in nocturnal transpiration over time, and (3) link nocturnal water loss with daytime plant physiology. Field work was conducted at the Konza Prairie Biological Station located in the Flint Hills region of northeastern Kansas, USA. We measured leaf-level nighttime transpiration and daytime photosynthetic rates, as well as whole-plant sap flow rates on eight grass, forb and shrub species. Measurements were made periodically across a single growing season (May-September 2014) on three C4 grasses (Andropogon gerardii, Sorghastrum nutans and Panicum virgatum), two C3 forbs (Vernonia baldwinii and Solidago canidensis), and three C3 woody species (Cornus drummondii, Rhus glabra and Amorpha canescens).
We found that substantial nocturnal transpiration occurs in multiple grassland species. At the leaf level, nighttime transpiration rates varied among species and across the growing season. Nighttime transpiration was greatest in the grasses early in the growing season and then decreased in all species as the year progressed. These results are consistent with patterns of decreasing daytime stomatal conductance and photosynthetic rates as the growing season became hotter and drier. Nighttime sap flow rates also varied among species and typically accounted for 5-15% of total daily water flux at the whole-plant level. Our results suggest that patterns of nocturnal transpiration are variable at a small spatial scale, are dynamic through time, and can be a considerable portion of a plant water budget. This research highlights the importance of including nocturnal water loss when describing earth-atmosphere fluxes in grassland ecosystems.