Effects of summer frosts and subsequent shade on foliage gas exchange in peatland tamarack and black spruce

Abstract

In this study, we examined the effect of summer frost on light-saturated net photosynthesis (Pnet) and related parameters in 20-year-old black spruce (Piceamariana (Mill.) B.S.P.) and tamarack (Larixlaricina (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 Pnet was primarily related to decreased mesophyll conductance to CO2, and a full recovery of Pnet took more than 11 days. Post-frost shade enhanced recovery in Pnet after a −3.5 °C frost in tamarack, but had no detectable effect after a −6 °C frost in tamarack or a −3 °C frost in black spruce.