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 relat ionship is not well understood. To address this issue, we studied A(ma x)-N relationships in 23 species in adjacent Amazonian communities dif ferentially limited by nitrogen (N), phosphorus (P), and/or other mine ral nutrients. Five species were studied in each of three late success ional forest types (Tierra Firme, Caatinga and Bana) and eight species were studied on disturbed sites (cultivated and early secondary succe ssional 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(ma x) 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 th an 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 secondar y successional species and almost-equal-to 4 mumol CO2 [g N]-1 s-1 for 15 late successional species, respectively. Species from disturbed si tes had shorter leaf life-span and greater specific leaf area (SLA) th an 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, r 2 = 0.90). Thus, disturbance adapted early successional species displa y a set of traits (short leaf life-span, high SLA and A(max) and a ste ep slope of A(max)-N) conducive to resource acquisition and rapid grow th in their high resource regeneration niches. The significance and fo rm of the A(max)-N relationship were associated with the relative nutr ient limitations in the three late successional communities. At specie s 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 t hree communities in their mass-based A(max)-N slope reflects this patt ern (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 re lated 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 t he relationship vary among Amazonian species and communities, consiste nt with both relative availabilities of N, P, and other mineral nutrie nts, and with intrinsic ecophysiological characteristics of species ad apted to habitats of varying resource availability.