Deep undercooling and winter hardiness limitations in conifers and conifer mistletoe parasites

Abstract

This study examines the occurrence and ecological significance of deep undercooling in conifers and conifer mistletoe parasites. Differential thermal analysis was used to detect low temperature exotherms (LTE's) that result from the freezing of deep undercooled water. LTE's near -40°C were found in winter collected stem and leaf tissue of the dominant timberline species of the Colorado Rocky Mountains, Engelmann spruce (Picea engelmannii) and subalpine fir (Abies Zasiocarpa), and in numerous other woody species having an elevational limit at or below timberline. Based on a 5° C/1000 m temperature lapse rate, -40°C temperatures are predicted at timberline elevation in Colorado. Therefore, the winter hardiness limitation of deep undercooling may be a factor determining the abrupt and uniform cessation of upright tree growth in this region. The occurrence and variation of LTE's were studied in provenance collections of three boreal forest conifer species, white spruce (P. glauca), black spruce (P. mariana), and jack pine (Pinus banksiana). LTE's were found in the leaf tissue of all 71 acquisitions of white and black spruce between -43 and -49°C, and in 6 out of 20 acquisitions of jack pine between -45 and -47°C. LTE 's were not found in the stem tissue of any acquisition of the three species. The four most northern acquisitions of white spruce showed less variability in the LTE temperature (-47.8 ± 0.5°C) than the four most southern acquisitions (-45.8 ± 2.6°C). This suggests natural selection pressures in the north have favored trees with LTE's in leaf tissue at lower temperatures. Seeds of two conifer dwarf mistletoe parasites collected in Colorado; Arceuthobium americanum and A. cyanocarpum , avoid freezing in winter by deep undercooling all freezable tissue water to near -33°C. Seeds of A. americanum cooled to -40°C, below the LTE temperature, had 0% germination. In contrast, seeds cooled to -30°C, above the LTE temperature, had 52 % germination, similar to 54% for control seeds held at -2°C. Winter injury temperatures for the vegetative tissue of six conifer dwarf mistletoe parasites and Phoradendron juniperinum collected in Colorado and Arizona ranged from -25 to <-70°C. The vegetative tissue of the mistletoe species studied do not avoid freezing by deep undercooling and, therefore, may have maximum winter hardiness levels even lower than those determined.