Photosynthesis-nitrogen relations in Amazonian tree species : II. Variation in nitrogen vis-a-vis specific leaf area influences mass- and area-based expressions.

The relationships between leaf nitrogen (N), specific leaf area (SLA) (an inverse index of leaf "thickness" or "density"), and photosynthetic capacity (A) were studied in 23 Amazonian tree species to characterize scaling in these properties among natural populations of leaves of different ages and light microenvironments, and to examine how variation within species in N and SLA can influence the expression of the A-to-N relationship on mass versus area bases. The slope of the A-N relationship, change in A per change in N (μmol CO gN s), was consistently greater, by as much as 300%, when both measures were expressed on mass rather than area bases. The x-intercept of this relationship (N-compensation point) was generally positive on a mass but not an area basis. In this paper we address the causes and implications of such differences. Significant linear relationships (p<0.05) between mass-based leaf N (N) and SLA were observed in 12 species and all 23 regressions had positive slopes. In 13 species, mass-based A (A) was positively related (p<0.05) with SLA. These patterns reflect the concurrent decline in N and SLA with increasing leaf age. Significant (p<0.05) relationships between area-based leaf N (N) and SLA were observed in 18 species. In this case, all relationships had negative slopes. Taken collectively, and consistent in all species, as SLA decreased (leaves become "thicker") across increasing leaf age and light gradients, N also decreased, but proportionally more slowly, such that N increased. Due to the linear dependence of A on N and a negative 4-intercept, "thicker" leaves (low SLA) therefore tend, on average, to have lower N and A but higher N than "thinner" leaves. This tendency towards decreasing A with increasing N, resulting in a lower slope of the A-N relationship on an area than mass basis in 16 of 17 species where both were significant. For the sole species exception (higher area than mass-based slope) variation in N was related to variation in N and not in SLA, and thus, these data are also consistent with this explanation. The relations between N, SLA and A explain how the rate of change in A per change in N can vary three-fold depending on whether a mass or area mode of expression is used.