Low Temperature Exotherms and Woody Plant Distribution1

Abstract

Stems of 49 woody species native to North America were collected from the field in Minnesota in January and subjected to controlled freezing tests. Characterization of the freezing process of xylem tissues by differential thermal analysis revealed that some species had no exotherms at low temperatures, some had small exotherms between –41° and –47°C, and others had large exotherms in the same temperature range. Generally very hardy species with ranges extending into northern Canada and Alaska had no exotherms. Species with small exotherms were native to the northern United States and southern Canada, and large exotherms were generally characteristic of the least hardy species studied. The low temperature exotherms occurred at the same temperature that results in xylem death of most of the plants studied. Plants with low temperature exotherms tend to have ring-porous xylem and the temperature of the low temperature exotherms was correlated with the minimum temperatures of the boundaries of the northern range of the species tested.