© 1991 Heron Publishing—Victoria, Canada
Photosynthesis, water relations and mortality in Abies lasiocarpa seedlings during natural establishment
Muyi Cui and William K. Smith
Department of Botany, University of Wyoming, Laramie, WY 892071, USA / Received June 7, 1990
Summary
Few studies have investigated the ecophysiology of natural seedling establishment in forest trees not associated with anthropogenic
disturbance. Photosynthesis and water relations measurements were made on one- through four-year-old seedlings of Abies lasiocarpa Nutt. (subalpine fir) establishing naturally in an understory environment. First (current)-year seedlings generally had only
cotyledons, whereas most second-year seedlings had both cotyledons and primary leaves. Mortality was high (> 60%) in first-year
seedlings with the greatest mortality (> 90%) measured at the more open, sun-exposed sites within the understory. Seedling
mortality was negligible after the first year of growth at shaded microsites and after the second year of growth at sunny
microsites.
Photosynthetic CO2 uptake at light saturation was considerably lower in first-year than in fourth-year seedlings (0.6 μmol m–2 s–1 versus 1.7 μmol m–2 s–1) and occurred at lower solar irradiance (240 μmol m–2 s–1 versus 600 μmol m–2 s–1 of photosynthetically active radiation). Differences in photosynthetic capacity were due to differences in both stomatal
and non-stomatal limitations to CO2 uptake. Carbon dioxide assimilation in first- and second-year seedlings was 28 and 29%, respectively, of the mean value measured
for fourth-year seedlings. Although first-year seedlings had low transpiration rates, their water use efficiency (photosynthesis/transpiration)
was less than half that of fourth-year seedlings and their water potentials were lower than those of all other age classes
(0.46 mol CO2 mmol–1 H2O and –3.0 MPa, respectively). The stomatal limitation to CO2 uptake was approximately 21% in first- and second-year seedlings, and increased to 39% in fourth-year seedlings. Intercellular
CO2 concentrations were greater in first- and second-year seedlings (255 and 250 μl l–1, respectively) than in third- and fourth-year seedlings (203 and 186 μl l–1, respectively). Thus, abrupt increases in water status and photosynthetic capacity after the first or second year of growth
appear crucial for survival to maturity. Moreover, differences in temperature and water relations according to microsite may
be major factors determining seedling establishment and, thus, the distributional and successional patterns observed for adult
trees of Abies lasiocarpa.
