A primary objective of prescribed fire in southeastern pine forests is the elimination of resprouting hardwoods, such as sweetgum (Liquidambar styraciflua), from the understory. Previous studies suggested that growing season burns result in higher understory mortality than dormant season burns and although seasonal differences in starch reserves in roots were posited as the mechanism, this has not been explicitly tested. We used two-year-old, pot-grown sweetgums to evaluate mechanisms proposed to explain differences in mortality and physiological performance driven by season of burning. Using thermal imagery, we compared energy release in different seasons of burn. We expected similar fire intensity in different seasons of burn because differences attributed to higher ambient temperatures and lower fuel moistures in the growing season would be relatively small. We compared root starch concentrations of burned sweetgum throughout the year. We expected that trees burned during the growing season would exhibit the lowest starch concentrations after burning and that this would also result in higher mortality. We measured biomass and starch concentrations throughout the year to understand whether burned trees could recover starch reserves. We expected that regardless of season of burn, surviving trees would replenish starch reserves to pre-burn concentrations.
Results/Conclusions
In contrast to previous studies, sweetgum burned in the dormant season exhibited higher mortality than growing season (22 vs 4%, respectively). While our experimental approach does not reflect the dynamics of a prescribed fire conducted in the field, we did not find a seasonal effect on fire radiative energy release and there was no difference in the fire intensities between trees that died and those that survived. Root starch reserves decreased after growing season burns both after leaf flush and during resprouting; however, mortality did not differ from the control and starch depleted by leaf flush was replenished rapidly. This suggests that newly expanded leaves quickly begin recoup carbon capitol expended by their construction. By the end of the growing season, root starch concentrations were similar among treatments and root biomass was higher than pre-treatment, indicating that annual prescribed burns are unlikely to deplete sweetgum starch reserves. Based on the results of our study, we find that differences between top-kill in different season was minor compared to the effect of a failure to top-kill. The mechanism driving the higher dormant season mortality remains to be identified.