Effects of high temperatures on fine roots of mature red pine (Pinus resinosa) trees

The goals of this project were to measure and describe the fine-root dynamics of red pine (Pinus resinosa Ait.) following a low-intensity prescribed fire and to determine the thermal death point of fine roots of mature trees in the field by using a new technique. During the single low-intensity backing fire temperatures in the lethal range (>60 °C) were reached only at the surface of the mineral soil. Largely as a result the fire had no significant effect on the fine root system to a depth of 25 cm, as determined by a combination of video imaging in minirhizotrons and soil coring. To understand better the susceptibility of roots to injury by fire, individual fine roots of red pine trees were exposed to high-temperature (45–62.5 °C) water for 0.5, 1, 2 or 4 min. Root death was assessed 1, 2 and 3 weeks following exposure to high temperatures. Heat-induced mortality of fine roots was apparent within 1 week of exposure to temperatures of approximately 52.5 °C and higher, the amount of mortality directly relating to exposure time. The effect of exposure time was minimal below this temperature after 1 week. Two weeks after exposure there was a general decrease in root survival with increased time of exposure, even at the lowest treatment temperature. Because non-treated roots also began dying 3 weeks after the study began, effects of heat exposure must be evaluated before other causes of root mortality become significant. The technique helped define the time– temperature relationship of heat-induced mortality of red pine roots in an ecological context. Low intensity fires with a short residence time—e.g., our experimental fire—do not produce the necessary combination of high temperature and time of exposure to induce significant root injury in red pine.