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Abstract: The local-scale spatial distribution of photosynthetically active radiation (PAR), absorbed PAR (APAR) and net all-wave radiation (Q ⁎) across the top of a forest canopy was investigated as a function of topography, sky conditions and forest heterogeneity for a forested hilly study site located in south-central Indiana, USA that is part of the FLUXNET and SpecNet networks. The method to estimate spatial variability of radiation components utilized theoretical radiation modeling applied to a topographic model combined with spatial distribution of leaf area index derived from IKONOS imagery and empirical models derived from data collected on a single flux tower. Modeled PAR and Q ⁎ compared consistently well with observations from a single tower with differences typically less than 10%, although clear-sky conditions were simulated more accurately than cloudy conditions. Spatial variability of radiation was found to be very sensitive to topographic relief and could be scaled linearly by mean slope angle. Decreases in optical transmissivity and increases in cloudiness had a strong effect of reducing both the spatial average and standard deviation of radiation components. Spatial variability of APAR was 53% greater than PAR and the characteristic scale of variance was reduced due to finer scale and magnitude of variance of LAI. Clear seasonal patterns existed in both spatial average and standard deviation values with summer producing the largest mean values and weakest spatial variability due to smaller solar zenith angles and seasonality in both optical transmissivity (scaled linearly by specific humidity) and cloudiness. These findings of spatial variability illustrate the need to characterize the complex landscape patterns at flux tower sites, particularly where the goal is to relate flux tower data to satellite imagery. [Copyright &y& Elsevier]

Leaf area index (LAI) and its seasonal dynamics are key determinants of terrestrial productivity and, therefore, of the response of ecosystems to a rising atmospheric CO2 concentration. Despite the central importance of LAI, there is very little evidence from which to assess how forest LAI will respond to increasing [CO2]. We assessed LAI and related leaf indices of a closed-canopy deciduous forest for 4 years in 25-m-diameter plots that were exposed to ambient or elevated CO2 (542 ppm) in a free-air CO2 enrichment (FACE) experiment. LAI of this Liquidambar styraciflua (sweetgum) stand was about 6 and was relatively constant year-to-year, including the 2 years prior to the onset of CO2 treatment. LAI throughout the 1999–2002 growing seasons was assessed through a combination of data on photosynthetically active radiation (PAR) transmittance, mass of litter collected in traps, and leaf mass per unit area (LMA). There was no effect of [CO2] on any expression of leaf area, including peak LAI, average LAI, or leaf area duration. Canopy mass and LMA, however, were significantly increased by CO2 enrichment. The hypothesized connection between light compensation point (LCP) and LAI was rejected because LCP was reduced by [CO2] enrichment only in leaves under full sun, but not in shaded leaves. Data on PAR interception also permitted calculation of absorbed PAR (APAR) and light use efficiency (LUE), which are key parameters connecting satellite assessments of terrestrial productivity with ecosystem models of future productivity. There was no effect of [CO2] on APAR, and the observed increase in net primary productivity in elevated [CO2] was ascribed to an increase in LUE, which ranged from 1.4 to 2.4 g MJ-1. The current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO2-enriched atmosphere and that increases in LUE and productivity in elevated [CO2] are driven primarily by functional responses rather than by structural changes. Ecosystem or regional models that incorporate feedbacks on resource use through LAI should not assume that LAI will increase with CO2 enrichment of the atmosphere. [ABSTRACT FROM AUTHOR]