Chasmophytic vegetation and mesoclimates of rock outcrops in Ventania (Buenos Aires, Argentina)

Abstract

Estudiamos las relaciones entre las variaciones mesoclimaticas de roquedales y la composicion de las comunidades casmofiticas sobre un gradiente altitudinal (500-1243 m s.n.m.) W-E en las Sierras Australes (Ventania). En 5 estaciones meteorologicas transitorias se registraron variables climaticas horaria y simultaneamente durante 3-4 dias cada estacion del ano durante 2 anos. Se utilizaron datos de censos fitosociologicos (20 releves) realizados por Frangi y Bottino (1995) en sitios con topografia similar a las estaciones meteorologicas. El analisis mostro la influencia de la exposicion sobre los mesoclimas, ya que las propiedades fisicas de las rocas aumentan los contrastes termicos causados por la radiacion solar directa, luego evidencio gradientes altitudinales. Los sitios S (550 m s.n.m.) y SW (850 m s.n.m.) fueron sombrios, humedos, poco evaporantes, y compartieron especies; a mayor altitud las bajas temperaturas favorecieron las especies microtermicas: endemicas, subantarticas y andinas. El sitio basal al N fue el biotopo mas caliente y seco; incluyo endemismos y especies xerotermofilas austrobrasilenas y del NW argentino. Los faldeos NE fueron floristica y mesoclimaticamente transicionales entre los roquedales basales; sin especies subantarticas y xerotermofilas. Las cumbres, evaporantes, soleadas y frescas, poseyeron menor riqueza. Las sierras albergan mesoclimas, especies y comunidades exclusivas para las Pampas, cuya disposicion en el paisaje es coherente con su linaje.

We studied the relationships between mesoclimate variations of rock outcrops and the composition of the chasmophytic communities over a W-E altitudinal gradient (500-1243 m a.s.l.) in the Sierras Australes (Ventania). In transitory weather stations located on 5 sites we recorded climatic variables hourly and simultaneously during 3-4 days at each season for 2 years. We used phytosociological data obtained with the relevé method by Frangi y Bottino (1995) in sites with same topography that weather stations. The analysis showed the influence of aspect on mesoclimates, since physical properties of rocks enhance thermal contrast originated in direct sun radiation, then, it showed altitudinal gradients. Sites to S (550 m a.s.l.) and SW (850 m a.s.l.) was shadowy, humid with low evaporation, and have a common species assemblage; at higher altitude lower temperatures favour microthermic species: endemic, Subantarctic or Andean. North-facing lower site was the hottest and driest biotope; includes endemisms, xerothermophilous southern-Brazilian, and northwestern Argentina species connected through the Andean-pampean mountainous arch. Northeast-facing mid-elevation outcrops were floristically and mesoclimatically transitional between both lower sites, lacking xerothermophilous and Subantarctic species. Summit outcrops, evaporative, sunny and relatively cold, had lower richness. These mountains enclose mesoclimates, species and communities exclusive to the Pampas, whose arrangement in the landscape are coherent with their lineage.