Effect of bark properties on the cambium cell viability of Eucalyptus species under low radiative heat exposure

Abstract

Tree bark reduces energy transfer from the bark surface towards the cambium during fires. To date, the heat reflection, absorption, and transmission properties of bark has been interpreted mainly in terms of elapsed time to increase temperature in the region of the cambium in response to external heating. To better understand bark protective properties, this study combines a known energy input and a cambium cell viability measure of heat impact, to develop an understanding in terms of characteristics that alter the heat capacity of bark. In this way three Eucalyptus species with different bark types are compared in terms of their heat absorption and transmission responses to heating.Stem sections from freshly felled trees of the three Eucalyptus species with contrasting bark types were exposed to a heat source that produced a constant 10 kWm−2 at the bark surface, that was applied until the cambium reached 60 °C. After stems cooled the viability of cambial cells from both the heated and non-heated sides of the stem were tested to determine the extent of heat impairment of cell function. Radiant heat between 3.5 and 13.6 MJ m−2 was required for the cambium to reach 60 °C, with more heat required for thicker and wetter bark, among the stem sections tested.The capacity of bark to protect the viability of cambium cells was strongly associated with bark moisture content, as well as with bark thickness. Bark with higher moisture content required more energy to heat the cambium to 60 °C, but once heated the moist bark retained the heat for longer than drier bark, resulting in a higher degree of cambial cell impairment. The critical specific heat capacity of bark varied between 3444 and 4657 J kg−1 °C−1 depending on the bark type. When exposed to a constant 10 kWm−2 heat source the critical exposure time for loss of cambium cell viability was between 20 and 40 min depending on bark type. This study provides a strong scientific basis to predict the tree impacts of heat released from fuels during planned fire and during wildfire over a range of fireground conditions – with applications to limit tree damage and increase survival.