Seasonal photosynthetic gas exchange and leaf reflectance characteristics of male and female cottonwoods in a riparian woodland
Matthew G. Letts (1, 2), Colleen A. Phelan (3), Davin R. E. Johnson (1) and Stewart B. Rood (3)
1. Department of Geography, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada / 2. Corresponding author () / 3. Department of Biological Sciences, University of Lethbridge, Lethbridge, AB T1K 3M4, Canada / Received September 16, 2007; accepted January 10, 2008; published online May 1, 2008
Summary
Cottonwoods (Populus spp.) are dioecious phreatophytes of hydrological and ecological importance in riparian woodlands throughout the Northern
Hemisphere. In streamside zones of southern Alberta, groundwater and soil water typically decline between May and September.
To understand how narrowleaf cottonwoods (Populus angustifolia James) are adapted to this seasonal decrease in water availability, we measured photosynthetic gas exchange, leaf reflectance,
chlorophyll fluorescence and stable carbon isotope composition (δ13C) in trees growing in the Oldman River valley of southern Alberta during the 2006 growth season. Accompanying the seasonal
recession in river flow, groundwater table depth (Zgw) declined by 1.6 m, but neither mean daily light-saturated net photosynthetic rate (Amax) nor stomatal conductance (gs) was correlated with this change. Both Amax and gs followed a parabolic seasonal pattern, with July 24 maxima of 15.8 µmol m–2 s–1 and 559 mmol m–2 s–1, respectively. The early summer rise in Amax was related to an increase in the chlorophyll pool during leaf development. Peak Amax coincided with the maximum quantum efficiency of Photosystem II (Fv/Fm), chlorophyll index (CI) and scaled photochemical reflectance index (sPRI), but occurred one month after maximum volumetric
soil water (θv) and minimum Zgw. In late summer, Amax decreased by 30–40% from maximum values, in weak correlation with θv (r2 = 0.50). Groundwater availability limited late-season water stress, so that there was little variation in mean daily transpiration
(E). Decreasing leaf nitrogen (% dry mass), CI, Fv/Fm and normalized difference vegetation index (NDVI) were also consistent with leaf aging effects. There was a strong correlation
between Amax and gs (r2 = 0.89), so that photosynthetic water-use efficiency (WUE; Amax/E) decreased logarithmically with increasing vapor pressure deficit in both males (r2 = 0.75) and females (r2 = 0.95). The male:female ratio was unequal (2:1, χ2 = 16.5, P < 0.001) at the study site, but we found no significant between-sex differences in photosynthetic gas exchange, leaf reflectance
or chlorophyll fluorescence that might explain the unequal ratio. Females tended to display lower NDVI than males (P = 0.07), but mean WUE did not differ significantly between males and females (2.1 ± 0.2 versus 2.5 ± 0.2 mmol mol–1), and δ13C remained in the –28.8 to –29.3‰ range throughout the growth season, in both sexes. These results demonstrate changes in
photosynthetic and water-use characteristics that collectively enable vigorous growth throughout the season, despite seasonal
changes in water supply and demand.
Keywords:
chlorophyll fluorescence, photochemical reflectance index, Populus angustifolia, sexual dimorphism, stable carbon isotope.
