LEAF ANATOMY PLASTICITY OF ALCHORNEA-TRIPLINERVIA (EUPHORBIACEAE) UNDER DISTINCT LIGHT REGIMES IN A BRAZILIAN MONTANE ATLANTIC RAIN-FOREST

Alchornea triplinervia trees occur in a montane Atlantic rainforest at the Macae de Cima State Ecological Reserve (Nova Friburgo, RJ, Brazil ). They are found in two adjacent secondary forests at distinct succes sional stages: a late-secondary (shaded and humid) and an early-second ary forest (drier, higher light intensities reaching the understory). Leaf samples collected in these environments were compared in regard t o various anatomic parameters. Histochemical tests, anatomic measureme nts and counting indicated no significant variation for the basic leaf anatomy. Nevertheless, as compared to the late-secondary forest, the leaves of the early-secondary forest individuals showed an increased b ulk of sclerenchyma at the main nervation and petiole, gelatinous fibe rs with hygroscopic walls all along the central vascular system, thick er cuticle, a higher abaxial stomatal density, abundance of tannin in the mesophyll, a significantly thicker palisade and spongy parenchyma, and compactation of the spongy parenchyma. This anatomical variation indicates an increased xeromorphism of leaves under the drier conditio ns of the early-secondary forest. Leaf anatomy plasticity may contribu te to the high ecological plasticity of this widespread neotropical sp ecies.