Modelled influences of non-exchanging trichomes on leaf boundary layers and gas exchange

The two main resistances in the exchange of gases between plants and the at mosphere are stomatal and boundary layer resistances. We modeled boundary l ayer dynamics over glabrous and pubescent leaves (assuming non-exchanging t richomes) with leaf lengths varying from 0.01 to 0.2m, and windspeeds of 0. 1-5.0ms(-1). Results from theoretical and semi-empirical formulae were comp ared. As expected, boundary layer thickness decreased with decreasing leaf length and increasing windspeed. The presence of trichomes increased leaf s urface roughness, resulting in lowered Reynolds numbers at which the bounda ry layer became turbulent. This effect is especially important at low winds peeds and over small leaves, where the Reynolds number over glabrous surfac es would be low. We derived a new simple dimensionless number, the trip fac tor, to distinguish field conditions that would lead to a turbulent boundar y layer based on the influence of trichomes. Because modeled rates Of CO2 a nd H2Onu exchange over turbulent boundary layers are one or more orders of magnitude faster than over laminar boundary layers, a turbulent boundary la yer may lead to increased carbon uptake by plants. The biological trade-off is potentially increased transpirational water loss. However, in understor y habitats characterized by low windspeeds, even a few trichomes may increa se turbulence in the boundary layer, thus facilitating photosynthetic gas e xchange. Preliminary field data show that critical trip factors are exceede d for several plant species, both in understory and open habitats. (C) 2001 Academic Press.