FLORISTIC-STRUCTURAL DESCRIPTION OF THE PHYSIONOMIES DOMINATED BY TREES IN THE lNTEGRAL RESERVE OF PUNTA LARA (PCIA. DE BUENOS AIRES, REPÃ?BLICA.ARGENTINA).

Abstract

This paper deals with a structural description of forest units present in the Reserva Integral de Punta Lara (Punta Lara Integral Reserve),located at the Río de la Plata shore (34°47' lat S, 58°1' long W). Thoseunits are; marginal or alluvial forest, old and young "ligustro" forest with totalbasal areas of 30.78 m2/ha, 28.71 m2/ha and 20.13 m2/ha respectively.The marginal forest is a subelimax community, with subtropical species,invade<! in some areas by the exotic Ligustrum lucidum that gives place toyoung and old "ligustro" forest.This marginal forest dwells the narrow and discontinuous band of naturallevee and is ecologically dependent on the river "albardon" interr lationship dueto pulsatil floods with its export-import of water, inorganic and organic dissolvedand suspended matter that constitutes part of the energy signature of the riveTineecosystem.The road parallel to tbe beach between forest and river, and artificial drainageobviously modified the frequency, intensity and duration of flood events,affecting hydrologic, chemical and physical variables of the natural forest environment.Tbe introduction of Ligustrum ltwidum some decades ago (Cabrera,1944) added a biological element of disturbance that is now naturalized andsuccesfully competes with native tree species mainly in those places were theterrain is less subject to flooding because of man-made structures.The importance of the trees (0.1 m DBH) in the alluvial fore t is sometimesgiven by size and density (Pouteria salidfolia: relative basal area 30 %, density2111ha), rnainIy by size (Ocotea acutífolia: relative basal area 12 %, density29/ha), or only by density (ex. Sebastiania brasiliensis: relative basal area4.8 %, density 71/ha). The "ligustro" seedlings and saplings (0.1 m DBH) density and mortality decreases from young ligustro forest to marginal forest. Density of native gallel)' forest seedlings and plantings is low in the three forest types, probably as indication of environmental disturbance. In the young ligustro forest native speeies seedlings and saplin.gs are greater probably due to wetter soil conditions because of a swamp in the vicinity. Diversity index is greater for the alluvial forest and lower in the ligustro old forest; Holdridge "omplexity index is simiL1r in both. Both forest units show structural differenees related to species composition and architectural chao racteristics of trees presento The marginal forest is deher in speeies, with different types of trunk and branches arrangement and crown size and shape. The ligustro forest is more homogeneous physiognomically with a dominant tree of dght trunks and spherieal crowns.