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

Interspecific variation in nighttime transpiration and stomatal conductance in a mixed New England deciduous forest.

Daley, Michael^*,1, Phillips, Nathan¹, ¹ Boston University, Boston, MA, USA

ABSTRACT- Transpiration is generally assumed to be insignificant at night when stomata close in response to the lack of photosynthetically active radiation. However, there is increasing evidence that the stomata of some species remain open at night, which would allow for nighttime transpiration if there were a sufficient environmental driving force. The purpose of this research was to examine nighttime water use in co-occurring species in a mixed deciduous stand at Harvard Forest, MA, using whole-tree and leaf-level water use measurements. Diurnal whole-tree water use was monitored continuously using Granier-style sap flux sensors in paper birch (Betula papyrifera Marsh.), red oak (Quercus rubra L.), and red maple (Acer rubrum L.). An analysis was conducted in which nighttime water flux could be partitioned between refilling of internal water stores and transpiration. Substantial nighttime sap flux was observed in all species and much of this flux can be attributed to the refilling of depleted water stores. However, in paper birch nighttime sap flux frequently exceeded recharge estimates. Over 10% of the total daily sap flux during the growing season was due to transpiration at night in paper birch. Nighttime sap flux was over 9% of the total daily flux in red oak and 2% in red maple, however, this was mainly due to recharge. On nights with elevated vapor pressure deficit, sap flux continued in paper birch through the night while sap flux stopped in red oak and red maple. Measurements of leaf-level gas exchange on a night with elevated vapor pressure deficit show conductance rates dropping only 25% in paper birch while approaching zero in red oak and red maple. The study highlights differences in ecophysiological controls on sap flux exerted by co-occurring species. Paper birch is a fast-growing, shade intolerant species with an earlier successional status than red oak and red maple. Risking water loss through nighttime transpiration may be an ecological strategy utilized by paper birch to maximize photosynthesis and support rapid growth. Nighttime transpiration may also be a mechanism to deliver oxygen to respiring cells in the deep sapwood of paper birch.

Key words: leaf gas exchange, sap flux