Canopy structure, light microclimate and leaf gas exchange of Quercus coccifera L. in a Portuguese macchia: measurements in different canopy layers and simulations with a canopy model

Abstract

Summary.The structural characteristics of a diverse array of Quercus coccifera canopies were assessed and related to measured and computed light attenuation, proportion of sunlit foliage, foliage temperatures, and photosynthesis and diffusive conductance behavior in different canopy layers. A canopy model incorporating all components of shortwave and longwave radiation, and the energy balance, conductance, and CO 2 and H20 exchanges of all leaf layers was developed and compared with measurements of microclimate and gas exchange in canopies in four seasons of the year. In the denser canopies with a leaf area index (LAI) greater than 5, there is little sunlit foliage and the diffuse radiation (400-700 nm) is attenuated to 5% or less of the global radiation (400-700 nm) incident on the top of the canopy. Foliage of this species is nonrandomly distributed with respect to azimuth angle, and within each canopy layer, foliage azimuth and inclination angles are correlated. A detailed version of the model which computed radiation interception and photosynthetic light harvesting according to these nonrandom distributions indicated little difference in whole-canopy gas exchange from calculations of the normal model, which assumes random azimuth orientation. The contributions of different leaf layers to canopy gas exchange are not only a function of the can- opy microclimate, but also the degree to which leaves in the lower layers of the canopy exhibit more shade-leaf characteristics, such as low photosynthetic and respiratory capacity and maximal conductance. On cloudless days, the majority of the foliage in a canopy of 5.4 LAI is shaded -70%-90% depending on the time of year. Yet, the shaded foliage under these conditions is calculated to contribute only about one-third of the canopy carbon gain. This contribution is about the same as that of the upper 13% of the canopy foliage. Computed annual whole-canopy carbon gain and water use are, respectively, 60% and 100% greater for a canopy of 5 LAI than for one of 2 LAI. Canopy water-use efficiency is correspondingly less for the canopy of 5 LAI than for that of 2 LAI, but most of this difference is apparent during the cool months of the year, when moisture is more abundant.

The leaf-level emission factor of volatile isoprenoids: caveats, model algorithms, response shapes and scaling

“…Leaf-scale species-specific estimates of E S can be directly used to simulate canopy and landscape level BVOC emission fluxes using soil-vegetation-atmosphere transfer (SVAT) models (e.g., Baldocchi and Meyers, 1998;Baldocchi et al, 1999) similar to the schemes widely used for simulation of plant carbon gain (Caldwell et al, 1986;Falge et al, 1997;Ryel, 1993). SVAT models are typically 1-D layered models or 3-D models that describe the variation in light, temperature and humidity in dependence on the amount of leaf area and leaf area distribution of the vegetation (e.g., Baldocchi, 1991;Baldocchi et al, 1999;Cescatti and Niinemets, 2004).…”

Testing a Mechanistic Model of Forest-Canopy Mass and Energy Exchange Using Eddy Correlation: Carbon Dioxide and Ozone Uptake by a Mixed Oak-Maple Stand

“…As a result, with respect to C02 assimilation, the canopy can be treated as a single big leaf or superleaf; that is, the photosynthetic properties of chloroplasts are assumed to be scaled within leaves according to PPFD absorption profiles within leaves, and similarly, the photosynthetic properties of individual leaves are assumed to be scaled with depth in the canopy in relation to canopy PPFD profiles. Stomata1 conductance, PPFD-compensation point, and leaf respiration also are assumed to be scaled with canopy PPFD profiles as discussed by, e.g., Caldwell et al (1986). Because of this, a canopy can behave as a single big leaf that absorbs the same amount of light as the canopy and assimilates C02 in accordance with the total photosynthetic machinery present throughout the canopy (Amthor 1994~).…”

Carbon assimilation and water-use efficiency by neighboring Mediterranean-climate oaks that differ in water access

“…The experimental goals were to: (1) characterize the seasonal courses of leaf and canopy gas exchange by the two oak species; (2) measure their seasonally integrated carbon uptake and WUE; and (3) partition any observed differences in WUE into physiological, structural, and phenological components (Cowan and Farquhar 1977, Mooney and Gulman 1979, Williams 1983, Caldwell et al 1986, Rambal 1993)…”