title: Photosynthesis-nitrogen relations in Amazonian tree species : I. Patterns among species and communities.
abstract: Among species, photosynthetic capacity (Amax) is usually related to leaf nitrogen content (N), but variation in the species-specific relationship is not well understood. To address this issue, we studied Amax-N relationships in 23 species in adjacent Amazonian communities differentially limited by nitrogen (N), phosphorus (P), and/or other mineral nutrients. Five species were studied in each of three late successional forest types (Tierra Firme, Caatinga and Bana) and eight species were studied on disturbed sites (cultivated and early secondary successional Tierra Firme plots). Amaxexpressed on a mass basis (Amass) was correlated (p<0.05) with Nmassin 17 of 23 species, and Amaxon an area basis (Aarea) was correlated (p<0.05) with Nareain 21 of 23 species. The slopes of Amax-N relationships were greater and intercepts lower for disturbance adapted early successional species than for late successional species. On a mass basis, the Amax-N slope averaged ≈15 μmol CO2[g N]-1s-1for 7 early secondary successional species and ≈4 μmol CO2[g N]-1s-1for 15 late successional species, respectively. Species from disturbed sites had shorter leaf life-span and greater specific leaf area (SLA) than late successional species. Across all 23 species, the slope of the Amass-Nmassrelationship was related (p<0.001) positively to SLA (r2=0.70) and negatively to leaf life-span (r2=0.78) and temporal niche during secondary succession (years since cutting-and-burning, r2=0.90). Thus, disturbance adapted early successional species display a set of traits (short leaf life-span, high SLA and Amaxand a steep slope of Amax-N) conducive to resource acquisition and rapid growth in their high resource regeneration niches. The significance and form of the Amax-N relationship were associated with the relative nutrient limitations in the three late successional communities. At species and community levels, Amaxwas more highly dependent on N in the N-limited Caatinga than in the P-and N-limited Bana and least in the P-and Ca-limited Tierra Firme on oxisol-and differences among these three communities in their massbased Amax-N slope reflects this pattern (6.0, 2.4, and 0.7 μmol CO2[g N]-1s-1, respectively). Among all 23 species, the estimated leaf Nmassneeded to reach compensation (net photosynthesis ≈ zero) was positively related to the Amass-Nmassslope and to dark respiration rates and negatively related to leaf life-span. Variation among species in the Amax-N slope was well correlated with potential photosynthetic N use efficiency, Amaxper unit leaf N. The dependence of Amaxon N and the form of the relationship vary among Amazonian species and communities, consistent with both relative availabilities of N, P, and other mineral nutrients, and with intrinsic ecophysiological characteristics of species adapted to habitats of varying resource availability.
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 Botany, University of Vermont, 05405, Burlington, VT, USA.
authors.forename: D S
authors.initials: DS
authors.lastname: Ellsworth
authors.affiliation: Department of Biology, Pennsylvania State University, 16802, University Park, PA, USA.
authors.forename: C
authors.initials: C
authors.lastname: Uhl
keywords: Amazon; Leaf life-span; Nitrogen; Photosynthesis; Rain forests