EFFECTS OF SUMMER FROSTS AND SUBSEQUENT SHADE ON FOLIAGE GAS-EXCHANGE IN PEATLAND TAMARACK AND BLACK SPRUCE

In this study, we examined the effect of summer frost on light-saturated net photosynthesis (P(net)) and related parameters in 20-year-old black spruce (Picea mariana (Mill.) B.S.P.) and tamarack (Larix laricina (Du Roi) K. Koch) at a peatland site in central Alberta. Summer frosts were simulated in the field using a self-contained freezing chamber. Some trees were shaded from direct sunlight after freezing. Shaded trees received about 20% of full sunlight at midday. We found that the response of photosynthesis consisted of two phases: a depression phase, where photosynthesis declined progressively, and a recovery phase, where photosynthesis recovered gradually. The length of the depression phase varied with species but not with the degree of freezing. For both species, depression in P(net) was primarily related to decreased mesophyll conductance to CO2, and a full recovery of P(net) took more than 11 days. Post-frost shade enhanced recovery in P(net) after a -3.5-degrees-C frost in tamarack, but had no detectable effect after a -6-degrees-C frost in tamarack or a -3-degrees-C frost in black spruce.