Spatial and seasonal variability of photosynthetic parameters and their relationship to leaf nitrogen in a deciduous forest

We used gas exchange techniques to estimate maximum rate of carboxylation ( V-cmax), a measure of photosynthetic capacity, in the understory and upper crown of a closed deciduous forest over two seasons. There was extensive va riability in photosynthetic capacity as a result of vertical canopy positio n, species type, leaf age and drought. Photosynthetic capacity was greater in oaks than in maples and greater in the overstory than in the understory. Parameter V-cmax was maximal early in the season but declined slowly throu ghout most of the summer, and then more rapidly during senescence. There wa s also an apparent decline during drought in some trees. Variability in V-c max as a result of species or vertical canopy gradients was described well by changes in leaf nitrogen per unit area (N-a). However, temporal changes in V-cmax were often poorly correlated with leaf nitrogen, especially in sp ring and summer and during drought. This poor correlation may be the result of a seasonally dependent fractional allocation of leaf nitrogen to Rubisc o; however, we could not discount Rubisco inactivation, patchy stomatal clo sure or changes in mesophyll resistance. Consequently, when a single annual regression equation of V-cmax versus N-a was used for this site, there wer e substantial errors in the temporal patterns in V-cmax that will inevitabl y result in modeling errors.