PHOTOSYNTHESIS OF BLACK SPRUCE, JACK PINE, AND TREMBLING ASPEN AFTER ARTIFICIALLY INDUCED FROST DURING THE GROWING-SEASON

Light-saturated photosynthesis following artificial frosts was monitor ed for black spruce (Picea mariana (Mill.) BSP), jack pine (Pinus bank siana Lamb.), and trembling aspen (Populus tremuloides Michx.). None o f the species exposed to -9 degrees C in May or to a and -9 degrees C in August recovered within the 23- and 14-day monitoring periods, resp ectively. Black spruce and jack pine treated at -6 degrees C in May re covered within 5 and 23 days, respectively. Black spruce treated at -3 degrees C in August recovered within 10 days. Frosts were applied to the upper and lower canopies of mature black spruce and jack pine in J une and to mature trembling aspen in July. For black spruce, the lower canopy did not recover whereas the upper canopy partially recovered o ver the 10-day monitoring period. For jack pine and trembling aspen, t here were no differences in recovery between canopy levels. Jack pine treated at -5.5 degrees C recovered within the 10-day monitoring perio d whereas at -8.5 degrees C, it only partially recovered. Although rec overy period varied with species, phenological state, and frost temper ature, gradual recovery of photosynthesis over 5-21 days seems a reaso nable modelling algorithm for boreal tree species when growing season frosts lower than -3 degrees C occur. However, cooling rates in our ex periments were greater than those that normally occur in nature.