Seasonal trends of light‐saturated net photosynthesis and stomatal conductance of loblolly pine trees grown in contrasting environments of nutrition, water and carbon dioxide

Repeated measures analysis was used to evaluate the effect of long‐term CO 2 enhancement on seasonal trends of light‐saturated rates of net photosynthesis ( A sat ) and stomatal conductance to water vapour ( g sat ) of 9‐year‐old loblolly pine ( Pinus taeda L.) trees grown in a 2 × 2 factorial experimental design of nutrition and water. A significant interaction effect of CO 2 and nutrition on mean A sat was observed for juvenile foliage. Also, juvenile foliage exposed to +350 μmol mol −1 CO 2 had a higher rate of increase of A sat between late summer and early autumn. This would lead to a greater potential for recharging carbohydrate reserves for winter. Mature foliage was affected by CO sat , water and nutrient treatments in two ways. First, A sat was significantly increased as a result of elevated CO 2 in January, a period when stomatal conductance was only 47% of the maximum observed rate. Secondly, the rate of increase of A sat from winter to early spring was accelerated as a result of both nutrient + water and + 350 μmol mol −1 CO 2 treatments. This accelerated response resulted in a greater potential for photosynthate production during the period when growth initiation occurred. Nutrient, water or carbon dioxide treatments did not significantly alter trends in g sat for mature or juvenile foliage. A significant nutrition × CO 2 interaction was observed for the mature foliage, suggesting that g sat increased with increasing CO 2 and nutrition. These results may have important consequences for the determination of the water use efficiency of loblolly pine. In spite of low g sat in the winter to early spring period, there was a substantial gain in A sat attributable to elevated CO 2 concentrations.