Abstract
Background
In the dry montane forests of western North America, quaking aspen (Populus tremuloides Michx.) is an iconic tree species, where it exists in vegetation communities of predominantly evergreen trees. While wildland fires can cause high mortality among conifer saplings and may kill the aboveground portions of aspen, this species is distinctive in its ability to resprout. Resprouting is a key physiological process that enables whole plant survival following fire through the production of new stems. However, there is limited pyro-ecophysiological knowledge of what levels of fire intensity induce top-kill, resprouting, and whole plant mortality in young trees. Quaking aspen is a prolific resprouting tree species that can resprout at all age classes, making it an ideal species to assess this information on saplings.
Results
Here we present the findings of a pyro-ecophysiology experimental approach designed to assess the degree of top kill, fire-induced whole plant mortality, and physiological traits following experimental fires. We applied discrete levels of fire radiative energy (FRE, MJ m−2) using custom fuel beds of pine needles and woodchips, followed by monitoring the post-fire responses over a 42-week period. None of the FRE treatment levels resulted in whole plant mortality and all saplings that experienced top kill resprouted. The moderately high fire intensity groups (FRE levels: 2.0 MJ m−2 and 3.0 MJ m−2) produced a greater number of resprouts (+ 126%) than the lower intensity groups (FRE < 1.0 MJ m−2), but the highest fire intensity group (FRE level: 4.0 MJ m−2) produced a lower number of resprouts compared to the lower intensity group (+ 59%). Post-fire diameter growth of the main stem exceeded that of the unburned controls in the low fire intensity groups (FRE levels: 0.3 and 0.6 MJ m−2). Post-fire height growth exhibited FRE thresholds for the resprouts, with FRE doses above 1.0 MJ m−2 exhibiting notably higher heights (e.g., + 200% at week 8) compared to the resprouts associated with FRE doses 0.3 MJ m−2 and 0.6 MJ m−2. In the main stem, the difference between chlorophyll fluorescence in top killed versus surviving crowns (at FRE = 1.0 MJ m−2) was associated with a notable decline (e.g., 75% at week 4).
Conclusions
Our results provide predictive information that managers could use to make informed decisions on the potential impacts of fires on aspen saplings. Given aspen’s ability to regenerate following fires, whereas many other tree species do not, it may have a competitive advantage under more severe fires. At the landscape scale this could lead to a shift in tree community composition, especially considering the observed survival in the aspen saplings even at high fire intensity levels. Land managers seeking to restore aspen in decadent aspen forests should use surface fires of moderately high fire intensity (i.e., 2.0 MJ m−2 < FRE < 3.0 MJ m−2) to maximize production of suckers. Future fire-induced tree mortality model research should include a focus on improving data associated with small diameter trees.