Acute physiological stress and mortality following fire in a long-unburned longleaf pine ecosystem
One important legacy of fire exclusion in ecosystems dependent upon frequent fire is the development of organic soil horizons (forest floor) that can be colonized by fine roots. When fire is re-introduced, the forest floor is often consumed by fire and heavy overstory mortality, often delayed by months, results. We hypothesized that the delayed post-fire tree mortality is a manifestation of a cascade of physiological stresses initiated by root damage that can also magnify the impact of other kinds of damage. We investigated the physiological impact of forest floor consumption on longleaf pines (Pinus palustris Mill.) subjected to a wildfire in 2005 in a long-unburned (>50 years) forest by measuring forest floor consumption, whole tree water use, and leaf chlorophyll content. Ten of the 23 study trees died within three years post fire. Post-fire sap flux was unrelated to crown scorch, but was negatively correlated with forest floor consumption. A segmented linear regression revealed declines in sap flux until a threshold of 31 % forest floor consumption, after which further consumption had no additional effect on tree water use. Trees with >30 % forest floor consumption beneath their crowns were more than 20 times as likely to die as those with less consumption. Chlorophyll content in needles that flushed post fire was negatively correlated with crown scorch (R² = 0.60, P = 0.009) though all trees with scorch also experienced varying degrees of forest floor consumption. Our results suggest that the consumption of the forest floor with the likely concomitant loss of roots initiated a decline spiral, driven by an inability to supply sufficient water to the crown. Though we did not measure loss of stored carbohydrates in consumed roots directly, we infer that this likely effect, coupled with decreased crown photosynthetic capacity, eventually resulted in substantial overstory tree mortality.