PARENT SESSION
Organized Oral Session 52: Casting light on nocturnal stomatal and canopy conductance
Organizer(s): NG Phillips and M Barbour
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 510a, Level 5, Palais des congrès de Montréal

Nighttime transpiration in contrasting ecosystems: Implications for linking plant function with environment.

Dawson, Todd^*,1, Burgess, Stephen^*,2, Oliveira, Rafael^*,1, Tu, Kevin^*,1, Ambrose, Anthony^*,1, Simonin, Kevin^*,1, ¹ University of California - Berkeley, Berkeley, CA, USA² University of Western Australia, Crawley, WA, Australia

ABSTRACT- A common assumption among plant biologists and hydrologists is that plants do not transpire at night. We present data from a variety of woody plants inhabiting temperate and tropical forests, savannas, and woodlands showing that this assumption may often be false. Furthermore, our comparative data suggest species from moist environments transpire at night more readily than species from arid environments. Using the heat-ratio sap flow method, porometery and other supporting data, we show that nighttime water loss (En) can be rapid (up to 40% of the summer maximum transpiration rate) and can constitute a significant fraction of the total daily or seasonal water use. Across species and biome types we have found that the chief driver of En is nighttime vapor pressure deficit, but wind speed and soil water status were also important. The widespread existence of En raises concerns for a host of reasons. I. From a methodological perspective these are: a) Sap flow methods that are accurate at slow flows should be used (e.g. the heat ratio method is preferred over the compensation heat pulse method); b) Users of Granier-style sap flow methods should not assume zero flux at night without supporting evidence; and c) Xylem refilling and other flow dynamics in stems or roots must be carefully distinguished from En. II. From an ecophysiological perspective these are: a) En prevents overnight recovery of plant water potential and impinges on the next days plant water status which reduces the capacity to store water and hinders embolism repair; b) En reduces hydraulic redistribution (lift), affecting root-zone processes; and c) En may increase nutrient uptake during the night but at the expense of plant water status. III. Finally, from a theoretical perspective these are: a) If En occurs predawn plant water potential cannot be assumed to equal soil water potential that requiring either direct measurements of soil water potential or bagging plants overnight to prevent uptake or loss; b) Because En may result in plant/soil water potential disequilibrium, predawn plant water potential should not be used for calculating whole plant conductances; and c) If En occurs then new methods are needed for parameterizing En in climate models for accurate representation of site water balance and water relations along the soil-plant-atmosphere continuum.

Key words: transpiration, nighttime