Drivers of nocturnal water flux in a tallgrass prairie

Abstract

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. Here, we addressed the following questions: (1) How do nocturnal (Enight) and diurnal (Eday) transpiration vary among coexisting grasses, forbs, and shrubs in a tallgrass prairie? (2) What environmental variables drive Enight and do these differ from the drivers of Eday? (3) Is Enight associated with daytime physiological processes? 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 C4 grasses (Andropogon gerardii, Sorghastrum nutans and Panicum virgatum), two C3 forbs (Vernonia baldwinii and Solidago canadensis), one C3 sub‐shrub (Amorpha canescens) and two C3 shrubs (Cornus drummondii and Rhus glabra). By extending our study to multiple functional groups, we were able to make several key observations: (1) Enight was variable among co‐occurring plant functional groups, with the highest rates occurring in C4 grasses, (2) Enight and Eday exhibited different responses to vapour pressure deficit and other environmental drivers, and (3) rates of Enight 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. Our results provide novel insight into an often‐overlooked portion of ecosystem water balance. Considering the high rates of Enight observed in C4 grasses, as well as the widespread global occurrence of C4 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 C4 grasses wherein the high rates of Enight occurring during wet periods may confer a competitive advantage to maximize resource consumption during periods of greater availability. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13072/suppinfo is available for this article.

Bridging the Flux Gap: Sap Flow Measurements Reveal Species‐Specific Patterns of Water Use in a Tallgrass Prairie

“…Consequently, nocturnal E C was a nonnegligible portion of the total water used by these species over the course of the growing season (Table ). Our observation that grasses exhibited higher proportional nocturnal E C than co‐occurring forbs and shrubs validates previously reported patterns of leaf‐level nocturnal transpiration at this site and provides further evidence that nocturnal water loss is likely a substantial component of grassland water budgets (Muench et al, ; O'Keefe & Nippert, ). O'Keefe and Nippert () measured diel leaf gas exchange on these same species and found that nocturnal transpiration was greatest in the C 4 grasses, reaching as much as 35.5% of daytime transpiration rates during wet periods.…”

“…Our observation that grasses exhibited higher proportional nocturnal E C than co‐occurring forbs and shrubs validates previously reported patterns of leaf‐level nocturnal transpiration at this site and provides further evidence that nocturnal water loss is likely a substantial component of grassland water budgets (Muench et al, ; O'Keefe & Nippert, ). O'Keefe and Nippert () measured diel leaf gas exchange on these same species and found that nocturnal transpiration was greatest in the C 4 grasses, reaching as much as 35.5% of daytime transpiration rates during wet periods. While several mechanisms have been proposed to explain nocturnal transpiration including enhanced nutrient acquisition (Scholz et al, ), increased carbohydrate export (Marks & Lechowicz, ), and circadian rhythm (Resco de Dios et al, ), O'Keefe and Nippert () suggested that nocturnal water loss may actually function as a competitive strategy in grasslands.…”

“…We show that these grasses also maintain consistently high G C regardless of seasonal changes in soil moisture (Figures , , and S8), which result in high nocturnal E C , particularly during periods of higher nocturnal VPD (Figures ). Thus, our ability to quantify whole‐plant water use and link modeled nighttime fluxes with multiple environmental drivers not only validated the leaf‐level measurements made by O'Keefe and Nippert () but has also provided additional insight regarding the competitive role of nocturnal transpiration in grassland communities.…”

“…O'Keefe and Nippert () measured diel leaf gas exchange on these same species and found that nocturnal transpiration was greatest in the C 4 grasses, reaching as much as 35.5% of daytime transpiration rates during wet periods. While several mechanisms have been proposed to explain nocturnal transpiration including enhanced nutrient acquisition (Scholz et al, ), increased carbohydrate export (Marks & Lechowicz, ), and circadian rhythm (Resco de Dios et al, ), O'Keefe and Nippert () suggested that nocturnal water loss may actually function as a competitive strategy in grasslands. They proposed that C 4 grasses, which are typically more physiologically tolerant to drought than co‐occurring forbs and shrubs (Tucker et al, ), transpire at high rates during the day and at night to rapidly deplete surface soil water and negatively impact neighboring plants (O'Keefe & Nippert, ).…”

“…Nocturnal transpiration has been shown to contribute substantially to the water budgets of forests (Dawson et al, ; Resco de Dios et al, ; Zeppel et al, ) and other ecosystems (Snyder et al, ; Bucci et al, ; Domec et al, ; Caird et al, ; Ogle et al, ;). Recent work has shown that nocturnal leaf transpiration rates can be high in grassland plants as well (O'Keefe & Nippert, ), but no work has yet been able to quantify total daytime and nighttime water use for multiple species in a native grassland ecosystem.…”