title: Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species.
abstract: The relationship between photosynthetic capacity (Amax) and leaf nitrogen concentration (N) among all C3species can be described roughly with one general equation, yet within that overall pattern species groups or individual species may have markedly different Amax-N relationships. To determine whether one or several predictive, fundamental Amax-N relationships exist for temperate trees we measured Amax, 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 Amaxwas highly correlated with leaf N (r2=0.75, P<0.001). For evergreen conifers, mass-based Amaxwas also correlated with leaf N (r2=0.59, P<0.001) and the slope of the regression (rate of increase of Amaxper unit increase in N) was lower (P<0.001) by two-thirds than in the broad-leaved species (1.9 vs. 6.4 μmol CO2g-1N 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 Amax-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 Amaxand SLA. For all data pooled, both leaf N and Amaxon 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 Amaxdid not (r2=0.06) because of low Amaxper 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 Amaxon mass (r2=0.80) and area (r2=0.55) bases, respectively. These results provide further evidence that Amax-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 Amaxper unit leaf N and a lower slope and higher intercept of the Amax-N relation than do species with shorter leaf life-span and higher SLA. A single global Amax-N equation overestimates and underestimates Amaxfor temperate trees at the upper and lower end of their leaf N range, respectively. Users of Amax-N relationships in modeling photosynthesis in different ecosystems should appreciate the strengths and limitations of regression equations based on different species groupings.
authors.affiliation: Department of Forest Resources, University of Minnesota, 55108, St. Paul, MN, USA.
authors.forename: P B
authors.initials: PB
authors.lastname: Reich
authors.affiliation: Department of Forest Resources, University of Minnesota, 55108, St. Paul, MN, USA.
authors.forename: M B
authors.initials: MB
authors.lastname: Walters
authors.affiliation: Department of Forestry, University of Wisconsin, 53706, Madison, WI, USA.
authors.forename: B D
authors.initials: BD
authors.lastname: Kloeppel
authors.affiliation: Biosystems and Process Sciences Division, Brookhaven National Laboratory, 11973, Upton, NY, USA.
authors.forename: D S
authors.initials: DS
authors.lastname: Ellsworth
keywords: Deciduous; Evergreen; Leaf life-span; Nitrogen; Photosynthesis