PHOTOSYNTHESIS-NITROGEN RELATIONS IN AMAZONIAN TREE SPECIES .1. PATTERNS AMONG SPECIES AND COMMUNITIES

Among species, photosynthetic capacity (A(max)) 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 A(max)-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). A(max) expressed on a mass basis (A(mass)) was correlated (p < 0.05) with N(mass) in 17 of 23 species, and A(max) on an area basis (A(area)) was correlated (p<0.05) with Nar,a in 21 of 23 species. The slopes of A(max)-N relationships were greater and intercepts lower for disturbance adapted early successional species than for late successional species. On a mass basis, the A(max)-N slope averaged almost-equal-to 15 mumol CO2 [g N]-1 s-1 for 7 early secondary successional species and almost-equal-to 4 mumol CO2 [g N]-1 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 A(mass)-N(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 A(max) and a steep slope of A(max)-N) conducive to resource acquisition and rapid growth in their high resource regeneration niches. The significance and form of the A(max)-N relationship were associated with the relative nutrient limitations in the three late successional communities. At species and community levels, A(max) was 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 A(max)-N slope reflects this pattern (6.0, 2.4, and 0.7 mumol CO2 [g N]-1 s-1, respectively). Among all 23 species, the estimated leaf N(mass) needed to reach compensation (net photosynthesis almost-equal-to zero) was positively related to the A(mass)-N(mass) slope and to dark respiration rates and negatively related to leaf life-span. Variation among species in the A(max)-N slope was well correlated with potential photosynthetic N use efficiency, A(max) per unit leaf N. The dependence of A(max) on 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.