In situ field measurements of photosynthetic rates of tropical tree species: a test of the functional group hypothesis

We examined photosynthetic characteristics of 21 tree species from a Panama nian forest differing in successional status. We hypothesized that function al guilds of species, grouped by successional status, would differ in photo synthetic performance and that pioneers would be more sensitive to seasonal ity and more variable in response to light than intermediate or shade toler ants. Steady-state leaf-level photosynthesis (A) was measured in situ on ei ght trees per species. Light response curves were generated by fitting a hy perbolic model to these data. Average light saturated photosynthetic rates (A(max)) were then calculated for each species. Variability of light, photo synthesis, and leaf characteristics were quantified using coefficients of v ariation (CV). Significant differences were detected among species and func tional groups for A, A(max), and leaf N concentration. Functional group exp lained 46% of the observed variation in A. Pioneers exhibited higher light- saturated photosynthetic rates than intermediates; both were higher than sh ade tolerants. Intermediates were the most seasonally plastic group and had the highest leaf N concentration. Shade tolerants were found in lower, mor e variable light environments than pioneers. A strong positive correlation between diameter growth rate and photosynthetic rate (r(2) = 0.55, p = 0.00 4) was observed across species. Our results tend to confirm the hypothesis that physiological traits can be used to differentiate among functional gro ups of plants. However, no evidence was found for higher plasticity of pion eer compared with shade-tolerant species.