HR: 1340h
AN: B33D-1588    [Abstracts]
TI: Seasonal patterns of photosynthetic gas-exchange and leaf reflectance characteristics in male and female riparian cottonwoods of southern Alberta
AU: * Letts, M G
EM: matthew.letts@uleth.ca
AF: Department of Geography, University of Lethbridge, 4401 University Dr., Lethbridge, AB T1K 3M4, Canada
AU: Phelan, C A
EM: colleen.phelan@uleth.ca
AF: Department of Biological Sciences, University of Lethbridge, 4401 University Dr., Lethbridge, AB T1K 3M4, Canada
AU: Johnson, D R
EM: davin.johnson@uleth.ca
AF: Department of Geography, University of Lethbridge, 4401 University Dr., Lethbridge, AB T1K 3M4, Canada
AU: Pearce, D W
EM: pearce@uleth.ca
AF: Department of Biological Sciences, University of Lethbridge, 4401 University Dr., Lethbridge, AB T1K 3M4, Canada
AU: Rood, S B
EM: rood@uleth.ca
AF: Department of Biological Sciences, University of Lethbridge, 4401 University Dr., Lethbridge, AB T1K 3M4, Canada
AB: Riparian, or streamside, cottonwood trees ( Populus spp.) are dioecious phreatophytes of hydrological and ecological importance in arid and semi-arid ecosystems throughout the northern hemisphere. In southern Alberta, groundwater and soil moisture levels typically decline during the May to September growth season. To understand how narrowleaf cottonwoods ( Populus angustifolia James) respond to this seasonal decrease in moisture availability, we repeatedly measured photosynthetic gas exchange, leaf reflectance, chlorophyll fluorescence and stable carbon isotope composition (δ13C) in four male and four female trees of the Oldman River valley, throughout the 2006 growth season. Maximum light-saturated net photosynthesis rates (A_max), near 16 μmol m^-2 s^-1, occurred on day of year (DOY) 205, one month after peak soil moisture, but coincident with the maximum quantum efficiency of Photosystem II (F_v/F_m), chlorophyll index (CI) and scaled photochemical reflectance index (sPRI). CI data suggest that the early-season rise in A_max and F_v/F_m was partly due to growth in the chlorophyll pool. Thereafter, A_max fell to near 10 μmol m^-2 s^-1, largely due to its positive logarithmic relationship with stomatal conductance (g_s; r²=0.89), which decreased from 559 to 246 mmol m^-2 s^-1 from DOY 205 to 237. The normalized difference vegetation index (NDVI), CI, sPRI and quantum yield of electron transfer at Photosystem II (ΦPSII) also declined in response to lower volumetric soil moisture content (θ_v) and increasing groundwater depth (Z_gw). Little change in transpiration rate (E) was observed in response to changing environmental conditions, except on DOY 237, when a combination of unseasonably low vapour pressure deficit (D) and low θ_v above the deepening capillary fringe caused E to decrease. No significant difference was observed between the mean WUE (A_max/E) of males (2.1 ± 0.2 mmol mol^-1) and females (2.5 ± 0.2 mmol mol^-1; repeated measures ANOVA, df=6, F=2.39, p=0.16). Leaves of females showed a tendency for lower NDVI than those of males (repeated measures ANOVA, F=4.82, p=0.07). Otherwise, no significant differences were observed between males and females in any gas-exchange, leaf reflectance or fluorescence characteristic (repeated measures ANOVA, α=0.05) and δ13C remained in the -28.8 to -29.3 ‰ range throughout the season, in both sexes.
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0476 Plant ecology (1851)
DE: 0480 Remote sensing
DE: 0483 Riparian systems (0744, 1856)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting