Two types of ventilated porometers compared on broadleaf and coniferous species.

Two ventilated porometers (diffusion and steady-state) were compared on four broadleaf and five coniferous species. The diffusion porometer gave consistently lower conductance values for both types of species, reflecting a direct stomatal response to low chamber humidity. At high conductance values, the porometers produced a linear and nearly equal response, but the diffusion porometer was less sensitive at low conductance values. This was due to lower air flow (20% of the velocity in the steady-state porometer) and water vapor sorption (by its acrylic plastic chamber). The broadleaf species had less variation (R(2) = 0.81) than did the coniferous species (R(2) = 0.61), but, with the latter, there was better correspondence between the two porometers, possibly due to sampling technique. Conductance values were clustered by species.