The bigger they are the harder they fall: Plant carbon balance explains size-dependent responses to fire

Background/Question/Methods

Fire is critical to the maintenance of many ecosystems because it is able to limit the success of certain plant species while promoting others. To effectively use fire as a management tool, it is important to understand the differences in plant responses to fire, but we lack a mechanistic framework from which to build this understanding. To this end, we studied the effects of fire on whole-plant carbon balance of nine species ranging from herbs to trees. Fire results in the loss of aboveground biomass and photosynthetic area, and therefore carbon uptake is severely compromised when it is most critically needed for recovering lost biomass. We tested the hypothesis that post-fire carbon balance is negatively correlated with plant stature across and within species, and that this pattern can largely explain inter-specific and intra-specific patterns of post-fire reproduction, biomass recovery, and replenishment of carbohydrate reserves. Destructive harvests of a range of stems sizes were used to determine allometric equations for estimating total biomass and leaf area, based on stem height. We measured height and maximum photosynthetic rate before burning, and for resprouts for four months after fire. With these data, we estimated potential whole-plant carbon assimilation over the course of resprouting.  

Results/Conclusions

In general, large growth forms were more negatively affected by fire. Across species, mean maximum photosynthetic rate was higher in resprouting plants than in unburned controls, however when tested within species, this effect was not significant for tree species. More importantly, larger individuals recovered a smaller proportion of their pre-burn leaf area, both within and across species. As a result, fire caused large plants to undergo a greater carbon deficit, relative to small plants. These results suggest that larger growth forms may be at a disadvantage under patterns of frequent fire, providing a mechanism by which frequent fire can promote species diversity of understory vegetation.