Abstract
A ventilated, transient-type porometer was designed to nondestructively measure the average stomatal conductance of conifer seedlings. The porometer consists of a low-power microprocessor unit and a stainless steel chamber which encloses the seedling. Conductance is calculated from the measured rate of increase in chamber relative humidity above the ambient level. Consequently, chamber air does not have to be dried prior to measurement. Direct readout of stomatal conductance is provided by means of a liquid crystal display and a serial (RS232) port. Porometer calibration over a range of humidities showed that the effective chamber volume was consistently 25% larger than the actual chamber volume. Stomatal conductance measurements made on Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings using the porometer agreed well with average values obtained with a ventilated diffusion porometer using four needle samples. The porometer proved to be very convenient to use in a forest clearcut environment, with the measurement procedure taking about 1 min. Measurements can be made on a seedling over the growing season because the use of the instrument causes no injury to stem, branches, and needles.
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