Estructura de la biomasa de pastizales en sierra de la Ventana. (Buenos Aires, Argentina)

This paper deals with the distribution of different plant compartments (aerial green, standing dead, litter and roots) during winter and spring (peak biomass) in six grasslands. The biomass structure relations with the altitudinal gradient, aspect and other environmental conditions, as well as grasslands biomass structure are discussed. In sunny conditions aerial dryweigh t d1minishedexponentially from 400 to 1,100 m a.s.!: 1,625.3 - 2,798.8 g/m in winter and 345.5 - 174.6 g/m in spring. Belowground biomass increased lineallywith altitude: 247.3 - 852.6 g/m and 311.6 - 1,349.3 g/m, along the same gradient and seasons. Tussock-type communities (Sttpa caudata, Paspalum quadrifartum, and Festuca pampeana grasslands) had a 10wer percentage oí underground related to total biomass, than do short grasslands from greater altitudes (Brlza subarlstata, B. brlzotdes and Aira caryophyllea, Sorghastrum pellttum - Sttpafiliculmis, and Piptochaetium hackelli + P. napostaense and Brlza subaristata). However, tussock grasslands, typically of the mountain slopes (F. pampeana type) had a greater percentage of roots and a lower green/root ratio than those from lower altttudes (S.caudata and P. quadrifartum). Grasslands in the less sunny situations (F.pampeana), from the sou thern steeper slopes, had greater total biomass and lower roots/total biomass ratio, than those exposed habitats at similar altitude. These relations are showing different adaptative community tactics according to the energy signature of the diverse mountain habitats. Vegetation alIocatesdífferent proportions ofphotosyntates to underground structures according to the varying combinations of altitude, slope, aspect and soil types, affecting availabllity and balance of water in plants. The more unfavourable the conditions are, the greater root/total or green biomass ratios are. Grasslands adjust biomass structure biomass structure independently to the dominant grass Ufe form (tussock or not). The characteristic proportions root/ total or root/ green biomass of a particular grassland are related to its dominant life form.Se analiza la distribución de compartimientos vegetales aéreo y subterráneo, y la hojarasca en el invierno y la primavera (biomasa pico) en seis pastizales. Se discuten las relaciones entre la estructura de la biomasa, el gradien te altitudinal, la exposición y pendiente y otras características físicas del ambiente. En condiciones de solana el peso seco aéreo disminuyó exponencialmente desde 400 msm a 1.100 msm: 1.625,3 a 345,5 g/m en invierno y 2.798,8 g/m a 174,6 g/m en prtmavera. La biomasa subterránea se incrementó linea1mente con la altitud: 247,3a852,6g/m en inviernoy3 11,6a 1.349,3 g/m enprtmavera en el mismo gradiente. Las comunidades de tipo "tussock" (Stipa caudata, Paspalum quadrifarium, y Festuca pampeana), tuvieron menor porcentaje de biomasa subterránea con relación a la biomasa total que los pastizales bajos serranos de mayor altitud (Briza subaristata, B. brizoides yAira caryophyilea, Sorghastrumpellitum -Stipaflliculmis, y Piptochaetium hackelli + P. napostaense - Briza subarlstata). Sin embargo los pastizales "tussock" característicos de pendientes abruptas de los cerros (F.pampeana), tuvieron mayor porcentaje de raíces, respecto de la biomasa total, y cociente raíces/verde que aquellos de altitudes más bajas (S. caudata y P. quadrifarium). Los pastizales en los sitios menos soleados (F.pampeana), de fuertes pendientes y exposición sur, presentaron mayor biomasa total y menor cociente raíces/biomasa total que aquellos en sitios de solana a similar altitud