Jörg Szarzynski

The role of canopy structure in the spectral variation of transmission and absorption of solar radiation in vegetation canopies

This paper presents empirical and theoretical analyses of spectral hemispherical reflectances and transmittances of individual leaves and the entire canopy sampled at two sites representative of equatorial rainforests and temperate coniferous forests. The empirical analysis indicates that some simple algebraic combinations of leaf and canopy spectral transmittances and reflectances eliminate their dependencies on wavelength through the specification of two canopy-specific wavelength-independent variables. These variables and leaf optical properties govern the energy conservation in vegetation canopies at any given wavelength of the solar spectrum. The presented theoretical development indicates these canopy-specific wavelength-independent variables characterize the capacity of the canopy to intercept and transmit solar radiation under two extreme situations, namely, when individual leaves 1) are completely absorptive and 2) totally reflect and/or transmit the incident radiation. The interactions of photons with the canopy at red and near-infrared (IR) spectral bands approximate these extreme situations well. One can treat the vegetation canopy as a dynamical system and the canopy spectral interception and transmission as dynamical variables. The system has two independent states: canopies with totally absorbing and totally scattering leaves. Intermediate states are a superposition of these pure states. Such an interpretation provides powerful means to accurately specify changes in canopy structure both from ground-based measurements and remotely sensed data. This concept underlies the operational algorithm of global leaf area index (LAI), and the fraction of photosynthetically active radiation absorbed by vegetation developed for the moderate resolution imaging spectroradiometer (MODIS) and multiangle imaging spectroradiometer (MISR) instruments of the Earth Observing System (EOS) Terra mission.

Micrometeorological conditions and canopy energy exchanges of a neotropical rain forest (Surumoni-Crane Project, Venezuela)

Vertical profiles of air temperature, humidity, wind speed and photosynthetically active radiation were examined systematically within and above a primary Amazonian rain forest in Southern Venezuela. During daylight hours the observed gradients of temperature and humidity suggest that turbulent mixing between vegetation and the atmosphere is reasonably efficient in the top two-thirds of the forest, whereas the understorey remains partially isolated. At night, however, the canopy exhibits a significant decoupling from the overlying atmospheric layer due to substantial radiative cooling, causing a stable density stratification above. It is fairly clear that these variations in microclimate produce a spectrum of different living conditions for the flora and fauna.

Xeric Islands: Environmental Conditions on Inselbergs

Environmental conditions on inselbergs such as climate, moisture, or nutrient availability drive important ecosystem processes and modify the structure and composition of flora and fauna (Cantlon 1953; Phillips 1982). Especially microclimatic conditions often differ considerably from regional climate and steep gradients may occur within very short distances (Porembski et al. 1996). Beyond these fundamental observations, however, very little is known about diurnal and seasonal patterns of microclimate.As a contribution to the interdisciplinary framework of this book, the present chapter explores some of the important climatic interactions between inselbergs, the vegetation, and the atmosphere. Main aspects are the interception, retention, modification, and conductance of energy and water. Parallel to botanical and geological observations, the required micrometeorological data have been quantified by a long-term study in the Cote d’Ivoire, where special regard was paid to inselbergs in semideciduous rainforest. In order to emphasize the contrasting environmental conditions of these directly neighboring ecosystems, measurements were made on top of an inselberg, as well as within the surrounding rainforest during both wet and dry seasons under a variety of weather conditions. Following supplemental short-term investigations were accomplished in Cameroon, Nigeria, Benin, and South America (Venezuela).