Photosynthesis, water relations and mortality in Abies lasiocarpa seedlings during natural establishment

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 CO(2) uptake at light saturation was considerably lower in first-year than in fourth-year seedlings (0.6 micromol m(-2) s(-1) versus 1.7 micromol m(-2) s(-1)) and occurred at lower solar irradiance (240 micromol m(-2) s(-1) versus 600 micromol m(-2) s(-1) of photosynthetically active radiation). Differences in photosynthetic capacity were due to differences in both stomatal and non-stomatal limitations to CO(2) 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 CO(2) mmol(-1) H(2)O and -3.0 MPa, respectively). The stomatal limitation to CO(2) uptake was approximately 21% in first- and second-year seedlings, and increased to 39% in fourth-year seedlings. Intercellular CO(2) concentrations were greater in first- and second-year seedlings (255 and 250 microl l(-1), respectively) than in third- and fourth-year seedlings (203 and 186 microl 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.