Photosynthesis in Sitka Spruce (Picea sitchensis (Bong,) Carr.): VII. Measurements of Stomatal Conductance and 14 CO 2 Uptake in a Forest Canopy

Abstract

Diurnal and seasonal variation in rates of photosynthesis (Fc) and stomatal conductance (Ks) of needles of different ages and position were measured in situ in a Sitka spruce forest canopy to define the location of the main sources of water vapor and sinks of CO2. Diurnal variation in Fc was overwhelmingly determined by the incident photon flux density at all levels in the canopy, so that there was no apparent correlation with vapor pressure deficit or shoot water potential. Fc was more dependent on temperature in spring, when the optimum temperature was 10.2.degree. C, than in the summer, when the optimum temperature was .apprx. 23.0.degree. C. Fc was largely independent of Ks at values of Ks above about 0.2 cm s-1 at all photo flux densities in both spring and summer. Diurnal variation in Ks was independent of photon flux density above about 300 .mu.E m-2 s-1 and of shoot water potential above about -16 bar. Ks decreased linearly with increasing vapor pressure deficit in the range 2-12 mb. Seasonal variation in both Fc and Ks during summer and autumn was largely determined by the stage of development of the current-year shoots, and by prevailing environmental variables, particularly photo flux density affecting Fc and vapor pressure deficit affecting Ks.