HR: 10:20h
AN: B52B-01 INVITED     [Abstracts]
TI: Site- and Patch-Scale Patterns of Near-Ground Solar Radiation Along the Grassland / Forest Continuum: Are There General Trends?
AU: * Breshears, D D
EM: daveb@email.arizona.edu
AF: University of Arizona, 325 Bioscience East, Tucson, AZ 85721-0043 United States
AU: Ludwig, J A
EM: John.Ludwig@csiro.au
AF: Tropical Savannas Cooperative Research Centre and CSIRO Sustainable Ecosystems, P.O. Box 780, Atherton, QLD 4883 Australia
AB: The characteristic tree cover at a site is a fundamental determinant of near-ground, incoming solar radiation at a site, which itself is a key driver for numerous ecosystem processes. Tree canopy cover can create mosaic patterns of shading relative to locations directly beneath tree canopies versus intercanopy patches between trees. Trends in how solar radiation varies along a gradient of increasing tree cover, referred to as the grassland/forest continuum, have been hypothesized as a function of tree cover and height. However, these studies did not considered a common element of canopy structure: the lower level of foliage on taller trees may be much higher above the ground than foliage on shorter trees. We evaluated patterns of near-ground solar radiation for a set of transects with tall trees along a grassland/forest continuum in Queensland, Australia. We used hemispherical photographs to estimate the Direct Site Factor (DSF): the proportion of incoming solar radiation that could reach near the ground (1 m above the surface). Mean site DSF exceeded 70% for all transects, dropping off most dramatically between 0 and 9% canopy cover; associated site variance in DSF was greater for the two intermediate levels of cover (9% and 50%) than for either of ends of the continuum (0 and 71% tree cover), consistent with previous predictions. Mean DSF for canopy patches was not significantly less than that of intercanopy patches for any of Queensland sites, in contrast to previous studies in New Mexico where trees where shorter. Simulations placing the tall trees in other locations indicated that more than 75% of the differences between the tall and short tree mean DSFs were due to canopy structure, with the remainder due to location, which was split nearly equally among a N versus S hemisphere effect and a location (mostly latitude) effect. We build on these results to hypothesize how four key properties of near ground solar radiation (site mean, variance, CV, and canopy/intercanopy ratio) vary differently along the grassland forest continuum for tall trees with their lower foliage many m above the ground versus for short trees with their lower foliage near the ground. Our findings provide a basis for moving towards a more general understanding of how shading effects vary along the grassland/forest continuum. These shading effects, in turn, drive many key dynamics of water, energy, and nutrients along the grassland/forest continuum.
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0476 Plant ecology (1851)
DE: 0495 Water/energy interactions (1878)
DE: 1813 Eco-hydrology
SC: Biogeosciences [B]
MN: Fall Meeting 2005