FireStem II: A 2-D heat transfer model for simulation of stem damage in prescribed fires

Background/Question/Methods

In this work, we present a software tool that allows fire managers to predict tree mortality and stem-damage in the occasion of a prescribed forest fire. When prescribing a fire, it is important for managers to predict tree mortality and the extent of stem damage, given the fuel conditions and burn design. Firestem II is a physically-based thermodynamic 2-D model of tree stem injury as a function of external heat forcing. It provides increased capability for predicting fire-induced mortality and damage before a fire occurs. It resolves the remaining moisture in the stem, maximal temperature and degree of charring, which are further integrated to an overall damage extent and viability index.

FireStem II allows the heat forcing from the fire to be prescribed as a dynamic time series and be heterogeneous around the stem. Other current 1-D models do not account for differential heating rates and damage responses around stems. A complete girdling of the tissue around the stem is needed to drive tree mortality. The realism and dynamics of this model allow it to be linked to a fire behavior model, that will provide it with detailed heating rates as produced in different heights above ground and at different locations throughout the burn area, and take into consideration the uneven heating in the circumferential direction. For example, as the fire-line passes a tree, it burns the front surface most intensely, and turbulence wake at the lee side of the stem captures heat that leads to increased damage at the back, while the sides of the stem may be less affected.

Results/Conclusions

A prescribed fire was conducted in the Pine Barrens State Forest in NJ. We used data from that prescribed fire to examine the extent of the damage to trees in the burned stand. We calculated the depth of necrosis of trees at different locations in the stand, and of different species and sizes. We looked at various heights of trees, in order to be able to simulate the damage of the whole trees. The results will allow us to improve current knowledge about the effects of the targeted fire conditions during a prescribed burn on damage and mortality of trees of different species and ages.