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

Abstract

We examined photosynthetic characteristics of 21 tree species from a Panamanian forest differing in successional status. We hypothesized that functional guilds of species, grouped by successional status, would differ in photosynthetic performance and that pioneers would be more sensitive to seasonality and more variable in response to light than intermediate or shade tolerants. Steady-state leaf-level photosynthesis (A) was measured in situ on eight trees per species. Light response curves were generated by fitting a hyperbolic model to these data. Average light saturated photosynthetic rates (Amax) were then calculated for each species. Variability of light, photosynthesis, and leaf characteristics were quantified using coefficients of variation (CV). Significant differences were detected among species and functional groups for A, Amax, and leaf N concentration. Functional group explained 46% of the observed variation in A. Pioneers exhibited higher light-saturated photosynthetic rates than intermediates; both were higher than shade tolerants. Intermediates were the most seasonally plastic group and had the highest leaf N concentration. Shade tolerants were found in lower, more variable light environments than pioneers. A strong positive correlation between diameter growth rate and photosynthetic rate (r2= 0.55, p = 0.004) was observed across species. Our results tend to confirm the hypothesis that physiological traits can be used to differentiate among functional groups of plants. However, no evidence was found for higher plasticity of pioneer compared with shade-tolerant species.Key words: tropical trees, physiological plasticity, photosynthesis, nitrogen, growth, tropical succession.