Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species

The relationship between photosynthetic capacity (A_max) and leaf nitrogen concentration (N) among all C₃ species can be described roughly with one general equation, yet within that overall pattern species groups or individual species may have markedly different A_max-N relationships. To determine whether one or several predictive, fundamental A_max-N relationships exist for temperate trees we measured A_max, specific leaf area (SLA) and N in 22 broad-leaved deciduous and 9 needle-leaved evergreen tree species in Wisconsin, United States. For broad-leaved deciduous trees, mass-based A_max was highly correlated with leaf N (r²=0.75, P<0.001). For evergreen conifers, mass-based A_max was also correlated with leaf N (r²=0.59, P<0.001) and the slope of the regression (rate of increase of A_max per unit increase in N) was lower (P<0.001) by two-thirds than in the broad-leaved species (1.9 vs. 6.4 μmol CO₂ g^-1 N s^-1), consistent with predictions based on tropical rain forest trees of short vs. long leaf life-span. On an area basis, there was a strong A_max-N correlation among deciduous species (r²=0.78, P<0.001) and no correlation (r²=0.03, P>0.25) in the evergreen conifers. Compared to deciduous trees at a common leaf N (mass or area basis), evergreen trees had lower A_max and SLA. For all data pooled, both leaf N and A_max on a mass basis were correlated (r²=0.6) with SLA; in contrast, area-based leaf N scaled tightly with SLA (r²=0.81), but area-based A_max did not (r²=0.06) because of low A_max per unit N in the evergreen conifers. Multiple regression analysis of all data pooled showed that both N (mass or area basis) and SLA were significantly (P<0.001) related to A_max on mass (r²=0.80) and area (r²=0.55) bases, respectively. These results provide further evidence that A_max-N relationships are fundamentally different for ecologically distinct species groups with differing suites of foliage characteristics: species with long leaf life-spans and low SLA, whether broad-leaved or needle-leaved, tend to have lower A_max per unit leaf N and a lower slope and higher intercept of the A_max-N relation than do species with shorter leaf life-span and higher SLA. A single global A_max-N equation overestimates and underestimates A_max for temperate trees at the upper and lower end of their leaf N range, respectively. Users of A_max-N relationships in modeling photosynthesis in different ecosystems should appreciate the strengths and limitations of regression equations based on different species groupings.