Comparative Ozone Dose Response of Gas Exchange in a Ponderosa Pine Stand Exposed to Long-Term Fumigations

Abstract

Photosynthesis (by ¹⁴CO₂ uptake) and stomatal conductance of attached ponderosa pine (Pinus ponderosa Laws.) needles were studied in relation to cumulative incident ozone dose in a stand of sapling trees in the San Bernardino National Forest. These trees have been exposed to long-term O₃ fumigations throughout their history. A steady-state, ventilated, minicuvette system was used to measure photosynthesis and conductance at light saturation (photon flux density > 1000 μeinsteins m⁻²s⁻¹), 20°C, and a vapor pressure gradient (needle to air) of 14 mb. Nine trees were stratified into three chronic injury classes (I, slight; II, moderate; III, severe) of three trees each, based on morphological O₃ injury symptoms, and the trees were sampled monthly from May to October of 1977. Although these three injury classes grew in a similar environment, they had differential photosynthetic and stomatal responses, suggesting ecotypic variation in O₃ sensitivity. The decline in photosynthesis and stomatal function normally associated with aging was accelerated as O₃ injury symptoms increased. Photosynthesis in class I, II, and III trees was reduced to about 10% of the maximum rate of class I current needles after integrating 800, 700, and 450 ppm-h, respectively, at which point needle abscission occurred. At the October sampling, class I, II, and III trees retained 3, 2, and 1 annual needle whorls, respectively. Stomatal conductances tended to be somewhat larger in the more O₃–sensitive trees during July and August, when the needles were rapidly expanding and possibly most vulnerable to the high O₃ concentrations occurring during this period of the year. It cannot be stated conclusively that this trend in conductances is a determining factor that explains differential O₃ sensitivity among the study trees. The relationship in conductances among injury classes reversed after the initial season of needle growth, primarily because stomatal function recovery during the relatively O₃-free winter months was inversely related to O₃ injury, in all trees and needle ages, losses in photosynthetic capacity exceeded reductions in stomatal conductance, suggesting that injury to the mesophyll, carboxylation, or excitation components of the CO₂ diffusion pathway was greater than injury to the stomata.