Seasonal leaf dynamics across a tree density gradient in a Brazilian savanna

Abstract

"Seasonal leaf dynamics across a tree density gradient in a Brazilian savanna" (2005 Abstract Interactions between trees and grasses that influence leaf area index (LAI) have important consequences for savanna ecosystem processes through their controls on water, carbon, and energy fluxes as well as fire regimes. We measured LAI, of the groundlayer (herbaceous and woody plants <1-m tall) and shrub and tree layer (woody plants >1-m tall), in the Brazilian cerrado over a range of tree densities from open shrub savanna to closed woodland through the annual cycle. During the dry season, soil water potential was strongly and positively correlated with grass LAI, and less strongly with tree and shrub LAI. By the end of the dry season, LAI of grasses, groundlayer dicots and trees declined to 28, 60, and 68% of mean wet-season values, respectively. We compared the data to remotely sensed vegetation indices, finding that field measurements were more strongly correlated to the enhanced vegetation index (EVI, r 2 =0.71) than to the normalized difference vegetation index (NDVI, r 2 =0.49). Although the latter has been more widely used in quantifying leaf dynamics of tropical savannas, EVI appears better suited for this purpose. Our ground-based measurements demonstrate that groundlayer LAI declines with increasing tree density across sites, with savanna grasses being excluded at a tree LAI of approximately 3.3. LAI averaged 4.2 in nearby gallery (riparian) forest, so savanna grasses were absent, thereby greatly reducing fire risk and permitting survival of fire-sensitive forest tree species. Although edaphic conditions may partly explain the larger tree LAI of forests, relative to savanna, biological differences between savanna and forest tree species play an important role. Overall, forest tree species had 48% greater LAI than congeneric savanna trees under similar growing conditions. Savanna and forest species play distinct roles in the structure and dynamics of savanna-forest boundaries, contributing to the differences in fire regimes, microclimate, and nutrient cycling between savanna and forest ecosystems.

Spatial and temporal variability of canopy cover and understory light in a Cerrado of Southern Brazil

“…Even when subtracting the highest stem area index (SAI) value of 0.7 for Cerrado trees (Asner, 1998), obtained values remained higher than those described by Miranda et al (1997). In another study in five Cerrado sites, Hoffmann et al (2005) found that LAI values ranged from 2.48 in the "cerradão" to 0.28 in the "campo sujo". They also estimated that the stem area index (SAI) corresponded to 13.9% of the vegetation area index (VAI).…”

Plant- and stand-level variation in biophysical and physiological traits along tree density gradients in the Cerrado

“…Vegetation structure varies from open savannas with relatively few trees in the lower portions of the topographic gradient (campo sujo) to woodlands with a relatively high density of trees in the upper portions of the gradient (cerradão). Herbaceous plant abundance follows the opposite trend, with higher coverage in open savannas to very low coverage in closed savannas (Hoffmann et al, 2005a). In the present study we characterize plant-and stand-level variations in tree morpho-physiological traits related to water economy along gradients of Cerrado ecosystem structure.…”

Modelagem de espectros temporais NDVI-MODIS, no período de 2000 a 2008, na bacia do rio Paracatu, Brasil

“…Os espectros NDVI-MODIS correlacionados com a abertura de dossel obtida em campo demonstraram um valor de correlação significativo (R 2 >0,84), que corrobora com os dados obtidos por Hoffmann et al (2005). A reta apresenta uma relação inversamente proporcional entre NDVI e os dados de abertura de dossel (Fig.…”

“…7), sendo que especificamente, no Cerrado teve um aumento da abertura de dossel de passou de 74,85% para 86,08%, na Floresta de Galeria de 27,03% para 31,05%, e na Floresta Estacional de 44,14% para 55,12%, corroborando estes dados com as mudanças no uso da terra e redução dos ambientes naturais (Machado et al, 2004;Hoffmann et al, 2005;Klink & Machado, 2005 …”