PHOTOSYNTHESIS-NITROGEN RELATIONS IN AMAZONIAN TREE SPECIES .2. VARIATION IN NITROGEN VIS-A-VIS SPECIFIC LEAF-AREA INFLUENCES MASS-BASED AND AREA-BASED EXPRESSIONS

The relationships between leaf nitrogen (N), specific leaf area (SLA) (an inverse index of leaf ''thickness'' or ''density''), and photosynt hetic capacity (A(max)) were studied in 23 Amazonian tree species to c haracterize scaling in these properties among natural populations of l eaves of different ages and light microenvironments, and to examine ho w variation within species in N and SLA can influence the expression o f the A(max)-to-N relationship on mass versus area bases. The slope of the A(max)-N relationship, change in A per change in N (mumol CO2 gN- 1 s-1), was consistently greater, by as much as 300%, when both measur es were expressed on mass rather than area bases. The x-intercept of t his relationship (N-compensation point) was generally positive on a ma ss but not an area basis. In this paper we address the causes and impl ications of such differences. Significant linear relationships (p < 0. 05) between mass-based leaf N (N(mass)) and SLA were observed in 12 sp ecies and all 23 regressions had positive slopes. In 13 species, mass- based A(max) (A(mass)) was positively related (p < 0.05) with SLA. The se patterns reflect the concurrent decline in N(mass) and SLA with inc reasing leaf age. Significant (p < 0.05) relationships between area-ba sed leaf N (N(area)) and SLA were observed in 18 species. In this case , all relationships had negative slopes. Taken collectively, and consi stent in all species, as SLA decreased (leaves become ''thicker'') acr oss increasing leaf age and light gradients, N(mass) also decreased, b ut proportionally more slowly, such that N(area) increased. Due to the linear dependence of A(mass) on N(mass) and a negative 4-intercept, ' 'thicker'' leaves (low SLA) therefore tend, on average, to have lower N(mass) and A(mass) but higher N(area) than ''thinner'' leaves. This t endency towards decreasing A(mass) with increasing N(area) decreases t he rate at which A(area) increases with Na(area) resulting in a lower slope of the A(max)-N relationship on an area than mass basis in 16 of 17 species where both were significant. For the sole species exceptio n (higher area than mass-based slope) variation in N(area) was related to variation in N(mass) and not in SLA, and thus, these data are also consistent with this explanation. The relations between N, SLA and A( max) explain how the rate of change in A(max) per change in N can vary three-fold depending on whether a mass or area mode of expression is used.