Dennis D. Baldocchi’s research while affiliated with National Oceanic and Atmospheric Administration and other places

Carbon dioxide, water vapor and other passive scalars are physically transferred between a plant canopy and the atmosphere by turbulence. Intense and intermittent sweep and ejection events transfer most of the mass. Although the capacity for turbulence to transfer material is high, mass transfer is coupled to the diffusive source or sink strength of the foliage and soil and is ultimately limited to a minimum level set by the supply of material, or the demand for it. The diffusive source/sink strength of material leaving or entering leaves and the soil is a function of many physical, biological and chemical attributes and processes. These attributes and processes include the amount and distribution of foliage, the leaf boundary layer and surface resistances, the turbulence and radiative regimes in the canopy, biochemical and photochemical reactions and the scalar concentration field within and above the canopy and inside leaves and the soil. Here we discuss how these factors contribute to turbulent transfer in a deciduous forest.

The objective was twofold: (1) to characterize the processes governing the deposition of pollutants to individual leaves and plant canopies and (2) to relate deposition to the pollutant's effects on a plant's physiology and growth. The focus is principally dry deposited, regionally distributed gases of importance because of either their phytotoxicity (e.g., ozone, O/sub 3/; sulfur dioxide, SO/sub 2/; nitrogen dioxide, NO/sub 2/; peroxyacetyl nitrate, PAN; hydrogen sulfide, H/sub 2/S; hydrogen peroxide, H/sub 2/O/sub 2/) or influence on biogeochemical cycling in terrestrial ecosystems (e.g., carbonyl sulfide; COS; ammonia, NH/sub 3/; nitric acid vapor, HNO/sub 3/). Where appropriate, the sedimentation of particles and the interception of cloud water by plant canopies and individual leaves are discussed.

A gaseous deposition model, based on a realistic canopy stomatal resistance submodel, is described, analyzed and tested. This model is designed as one of a hierarchy of simulations, leading up to a “big-leaf” model of the processes contributing to the exchange of trace gases between the atmosphere and vegetated surfaces. Computations show that differences in plant species and environmental and physiological conditions can affect the canopy stomatal resistance by a factor of four. Canopy stomatal resistances to water vapor transfer computed with the present model are compared against values measured with a porometer and computed with the Penman-Monteith equation. Computed stomatal resistances from a soybean canopy in both well-watered and water-stressed conditions yield good agreement with test data. The stomatal resistance submodel responds well to changing environmental and physiological conditions. Model predictions of deposition velocities are evaluated for the case of ozone, transferred to maize. Calculated deposition velocities of O3 overestimate measured values on the average by about 30%, probably largely as a consequence of uncertainties in leaf area index, soil and cuticle resistances, and other modeling parameters, but also partially due to imperfect measurement of O3 deposition velocities.