LEAF DELTA-C-13 IN PINUS-RESINOSA TREES AND UNDERSTORY PLANTS - VARIATION ASSOCIATED WITH LIGHT AND CO2 GRADIENTS

Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO2 for va riation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon i sotope ratios on four understory species (Dryopteris carthusiana, Epip actus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light m icroclimate for carbon gain in these plants. During midday, CO2 concen tration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO2 concentrati on significantly elevated above that measured at 15 m. Based on the st rong linear relationship between chan in CO2 concentration and delta(1 3)C values for air samples collected during a diurnal cycle, we calcul ated the expected vertical profile for the carbon isotope ratio of atm ospheric CO2 within the forest. These calculations indicated that leav es at 3 m height and above were exposed to CO2 of approximately the sa me isotopic composition during daylight periods. There was no signific ant difference between the daily mean delta(13)C values at 15 m (-7.77 parts per thousand) and 3 m (-7.89 parts per thousand), but atmospher ic CO2 was significantly depleted in C-13 closer to the ground surface , with daily average delta(13)C values of -8.85 parts per thousand at 5 cm above ground. The light intensity gradient in the forest was subs tantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the cano py. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was rela tively constant from the top of the canopy to the forest floor. For re d pine and elm tree samples, there was a significant correlation betwe en leaf delta(13)C value and the height at which the leaf sample was c ollected. Leaf tissue sampled near the forest floor, on average, had l ower delta(13)C values than samples collected near the top of the cano py. We suggest that the average light intensity gradient through the c anopy was the major factor influencing vertical changes in tree leaf d elta(13)C values. In addition, there was a wide range of variation (gr eater than 4 parts per thousand) among the four understory plant speci es for average leaf delta(13)C values. Measurements of leaf gas exchan ge, under natural light conditions and with supplemental light, were u sed to estimate the influence of the light microclimate on the observe d variation in leaf carbon isotope ratios in the understory plants. Ou r data suggest that one species, Epipactus helleborine, gained a subst antial fraction of carbon during sunflecks.