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Stomatal response to changes in leaf-to-air water vapor pressure difference (D) was studied in needles of the current year's shoot of three-year-old seedlings of Picea sitchensis (Bong.) Carr., Pinus contorta Dougl. ex Loud., Larix x eurolepis Henry, and Pinus sylvestris L. Both eight-week- and ten-month-old shoots of P. sylvestris were studied. Stomata of all the species responded by closing to some degree as D was increased over the range 0.4-2.0 kPa. Ten-month-old shoots of P. sylvestris showed the smallest reduction (7.5%), and shoots of P. sitchensis the largest reduction (64.6%) in stomatal conductance. However, in no species was stomatal closure sufficient to cause a reduction in transpiration (E) as D increased. Net photosynthesis (A) declined linearly as D was increased and as a result the ratio of E/A increased linearly in all species. Only the stomata of P. contorta and L. x eurolepis behaved in an 'optimal' way, i.e., estimated values of dEdA were approximately constant as D increased. For P. sylvestris, shoots of both ages, dE/dA increased markedly with D, whereas in P. sitchensis it declined. Explanation of these data does not require a mechanism of stomatal closure involving a site that senses the vapor pressure deficit outside the leaf. However, it is unlikely that a simple 'feedback' response involving bulk leaf water potential can explain the responses measured because changes in needle water potential were less than 0.1 MPa during an experiment.

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Present weather sensors are becoming increasingly important as a means to augment networks of automated weather stations and extend the capability of manned observations. The classification of hydrometeors is one of the principal tasks that is addressed by present weather sensors. In this paper we discuss new laser based technology for this purpose. The system improves upon current precipitation monitors by using a derivative of Phase Doppler Anemometry techniques to accurately determine particle speed and size. The instrument is also capable of distinguishing between liquid droplets and solid polycrystalline hydrometeors and can be used to estimate visibility. The incorporation of this technology into a meteorological station with other sensors, such as temperature and relative humidity probes leads to the accurate classification of particle type. The example data shown is taken from tests in Leicestershire, England and Utah, USA and shows the differences between solid and liquid precipitation events.

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