Integrating within-crown variation in net photosynthesis in loblolly and slash pine families

We examined photosynthetic characteristics of two fast- and two slow-growing half-sib families of both loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii var. elliottii Engelm.) on two sites in northern Florida to: (1) quantify variation in light-saturated net photosynthesis (A(max)) associated with vertical crown position and foliage age; (2) quantify the amount and distribution of leaf area by foliage age class; and (3) determine whether photosynthetic indices, ranging from leaf-level through whole-crown A(max), were related to growth differences among species and families. In both species, leaf-level A(max) was higher in more recently formed foliage both within the same year (where A(max) in the third flush averaged 10 to 30% higher than A(max) in the first flush) and between years (where A(max) in current-year foliage averaged 20 to 40% higher than A(max) in 1-year-old foliage). When expressed on a leaf area basis, A(max), of current-year foliage was higher in slash pine than in loblolly pine, but A(max) expressed on a mass basis did not differ between species. Loblolly pine had higher whole-tree leaf area than slash pine, whereas whole-tree A(max) did not differ between species. When the mean values for fastgrowing families were compared with the mean values for slow-growing families, there were no differences in leaf-level characteristics, whereas at the whole-tree level, fast-growing families had higher leaf area and whole-tree A(max) than slow-growing families in both species. When comparisons were made among the individual fast- and slow-growing families, however, results were more variable. In both species, stem volume growth was strongly correlated with whole-tree A(max), with most of the strength of the correlation deriving from the relationship between volume growth and tree leaf area.