Abstract
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 (R 2 = 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.
Highlights
The natural history of forests is a history of fire (Spurr and Barnes 1980); fire management has often ignored this history with catastrophic consequences (Dombeck et al 2004)
We did not detect any relationship between forest floor consumption and chlorophyll content; all trees with significant crown scorch (>10 %) exceeded the threshold of forest floor consumption (31.3 %), which was associated with loss of transpiration and mortality described above
Combustion of the forest floor and the resulting root damage in longleaf pines is consistent with a cascade of chronic stressors resulting in tree mortality months to a few years after fire
Summary
The natural history of forests is a history of fire (Spurr and Barnes 1980); fire management has often ignored this history with catastrophic consequences (Dombeck et al 2004). Conifers adapted to frequent fire regimes possess many traits that confer resilience to low intensity surface fires, including self pruning limbs, thick bark, insulated buds, and reserves of stored carbohydrates that facilitate repair of damaged crowns When repeatedly burned, these ecosystems are characterized by low fuel loads, little to no forest floor accumulation, and short fire residence time (Varner et al 2005). While mature longleaf pine mortality rates are low in surface fires (Boyer 1979), the smoldering ground fires that can occur in stands with a well developed forest floor have resulted in up to 80 % overstory pine mortality (Varner et al 2005) Loss of these trees can be catastrophic for ecosystem structure, composition, and function in these biologically diverse ecosystems (Kirkman et al 2004, Mitchell et al 2006, O’Brien et al 2009). In order to test the hypothesis that forest floor consumption results in acute water stress, we measured whole tree water use and leaf chlorophyll content in trees with varying degrees of forest floor consumption and crown scorch, both prior to and after fire, and tracked mortality for three years post fire
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