Forest fires often do not constitute a direct lethal threat to trees, but rather, leave behind trees with a variety of impairments, subsequently affecting their physiology. Important links between heat-induced structural tissue injuries and post-fire physiological tree responses have recently been established. However, it is assumed that the hot and dry air associated with fires can impair tree physiology also in a non-structural way. Abrupt shifts of atmospheric conditions during fires create an extraordinary high leaf-to-air vapor pressure deficit, which potentially favors high tensions (i.e. low water potentials) and, consequently, embolism formation in the xylem. We analyzed heat plume-driven losses in hydraulic conductivity for an angiosperm (Fagus sylvatica) and a coniferous species (Larix decidua) after exposing young trees to experimental fires. Embolism rates were measured in three different plant segments (branches, upper and lower stem) to assess spread patterns. Further, the influence of preexisting (drought-induced) tensions in the xylem on embolism formation during the fire was assessed by conducting experiments on trees of different pre-fire drought stress levels. Fire exposure caused substantial hydraulic conductivity losses of up to 80% in F. sylvatica. Embolism spread into all tree segments and formation was facilitated by pre-fire drought stress. In contrast, the fire treatment did not initiate additional conductivity losses in L. decidua as embolism rates in fire exposed trees ranged at the same levels as in solely drought-stressed control trees. These findings indicate that the xylem anatomy of conifers (tracheids with valve like bordered pits) offers high protection against embolism propagation, while vessel-bearing species can sustain high rates of embolism formation during fires, even in non-distal tree parts. Knowledge of species-specific fire effects will help to better under post-fire physiological functionality and mortality mechanisms.
