Photosynthesis-nitrogen relations in Amazonian tree species. I. Patterns among species and communities

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 ($\text{A}_{\text{mass}}$) was correlated (p < 0.05) with Nmassin 17 of 23 species, and Amaxon an area basis ($\text{A}_{\text{area}}$) was correlated (p < 0.05) with$\text{N}_{\text{area}}$in 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$[\text{g}\ \text{N}]^{-1}\text{s}^{-1}$for 7 early secondary successional species and ≈4 μmol CO₂$[\text{g}\ \text{N}]^{-1}\text{s}^{-1}$for 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$\text{A}_{\text{mass}}-\text{N}_{\text{mass}}$relationship 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) conductive 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 mass-based Amax-N slope reflects this pattern (6.0, 2.4, and 0.7 μmol CO₂$[\text{g}\ \text{N}]^{-1}\text{s}^{-1}$, respectively). Among all 23 species, the estimated leaf Nmassneeded to reach compensation (net photosynthesis ≈