In this paper some preliminary remarks have been presented on the characteristics of turbulence within plant communities most influencing the exchange of physical quantities (energy and substance) between plant leaves and a surrounding air layer.
Under the assumption that turbulent theories adopted for studies of an atmospheric surfac e layer are applicable as for the study of turbulent characteristics in plant layers (z≤H), we have
l(z)=κ·z·(1-d/H)
K(z)=κ·V_*(z)·z(1-d/H)
V_*(z)=V_*(H)K(z)/K(H)·H/z Z≤H
τ(z)=ρ{V_*(H)K(z)/K(H)·H/z}²
where l, K, V_*, τ denote the scale of turbulent eddy, the turbulent eddy, the turbulent diffusivity, the frictional velocity and the shearing stress within plant communities, respectively. κ=0.4, H, d, ρ are the Karman constant, the mean tip height of plants, the zeroplane displacement and to density of the air, respectively.
Fig. 1 shows schematically the difference in turbulent characteristics between plant layers and the atmospheric surface layer. These differences are due mainly to the difference of the turbulent scale-height function between two layers. The scale of turbulent eddy within plant layers is affected not only by the height above ground surface but also by the plant density.
In order to examine the applicability of the theoretical results to plant layers, these results have been applied to the micrometeorological data for the fir forest, paddy field and wheat field obtained by BAUMGARTNER (1956), FRANCESCHINI (1959) and PENMAN. LONG (1960), respectively. The obtained results are summarized as follows:
The proportinal constant β governing the dependence of eddy scale on the height above ground surface is expressed in terms of quantities such as Karman constant κ and plant layer's constant γ.
β=κ·γ, γ=(1-d/H)
And it is expected reasonably that the plant layer's constants may vary in proportion to the zeroplane displacement in a range between the following limits
γ=1.0 at d/H→0,
γ=0.0 at d/H→1.
The comparison of the plant layer's constant evaluated by Eqs. (5), (10) for three kinds of the plant community is shown in Fig. 2, indicating that the theoretical results mentioned above may be applicable to studying the turbulent features within plant layers.
When the dependence of the turbulent diffusivity on the height is assumed to be as follows:
K(z)/K(H)∝(z/H)^a
a value of power index (a) more than a unity indicates that the momentum flux diminishes with distance to downward from a reference height, namely is absorbed with plant leaves. Fig. 4 shows there is the same pattern in vertical profile among wind velocity, turbulent diffusivity, shearing stress within plant layers. These profiles are similar to those for a turbulent velocity (‹u'²›^1/2) reported by WATERHOUSE (1955), NAKAGAWA (1956), respectively. As shown in Fig. 5, a good agreement of momentum fluxes evaluated by the two methods makes it clear that Eq. (9) can be applied with a little error for evaluating the vertical profiles of the momentum flux and the frictional velocity most controlling the turbulent diffusivity within plant layers. Fig. 6 indicates that the decrease of the momentum flux within plant layers becomes steeper with an increase of the wind velocity in above the air layer, as expected by PENMAN. LONG (1960).
The experimental results have shown that the theoretical relations can be applied qualitatively to understand the turbulent characteristics in plant layers. However, it is highly desirable to carry out further investigations both theoretically and experimentally concerning the problem whether the theoretical