Functional linkages between leaf traits and net photosynthetic rate: reconciling empirical and mechanistic models

1. We had two objectives: (i) to determine the generality of, and extend the applicability of, a previously reported empirical relationship between leaf-level net photosynthetic rate (A(M), nmol g(-1) s(-1)), specific leaf area (SLA, m(2) kg(-1)) and leaf nitrogen mass fraction (N-M, mmol g(-1)); and (ii) to compare these empirical results with a mechanistic model of photosynthesis in order to provide a mechanistic justification for the empirical pattern. 2. Our results were based on both literature and original data. There were a total of 160 and 87 data points for the leaf-level and whole-plant data, respectively. 3. Our multiple regression for single leaves was ln(A(M)) = 0.66 + 0.71 ln(SLA) + 0.79 ln(N-M), r(2) = -0.80; only the intercept (0.11) differed for the whole-plant data. These results are not significantly different from previously published relationships. 4. We then converted the mechanistic model of Evans and Poorter, and a modified version which includes leaf lamina thickness (T) and leaf dry matter (tissue) concentration (C-M), into directed acyclic graphs. We then derived reduced graphs that involved only T, C-M, SLA, N-M and A(M). These were tested using structural equation modelling, with measured lamina thickness (T') and leaf dry matter ratio (LDMR, g dry mass g(-1) fresh mass) as indicators of T and C-M. The original Evans-Poorter model was rejected, but the modified version fitted the structural relationships well. The same qualitative models also applied to the whole-plant data, although the path coefficients sometimes differed. 5. Using simulations, we show that the original Evans-Poorter model predicts a positive correlation between SLA and N-M that maximizes A(M). The data closely follow this predicted relationship. The correlation between the actual values of A(M) (standardized units) and the predicted values obtained from the modified Evans-Poorter model was 0.74 and increased to 0.82 once three outlier points were removed. 6. These results provide a mechanistic explanation for the empirical trends relating leaf form and carbon fixation, and predict that SLA and leaf N must be quantitatively co-ordinated to maximize C fixation.