Smoldering duff fire impacts on longleaf pine growth and defense

Background/Question/Methods

In many fire-prone forests, fire exclusion has resulted in substantial forest floor accumulations surrounding the tree base.  These accumulations pose a challenge to managers interested in reintroducing fire because of the prolonged heating associated with smoldering of the underlying duff.  In particular, managers are concerned that excessive heating may contribute to a reduction in tree growth and defense, which may lead to subsequent tree mortality.  Previous research has detected acute reductions in root non-structural carbohydrates associated with increased smoldering fire durations.  However, research examining the longer-term effects on growth and defense has not been conducted. Our study compared the effects of three smoldering fire treatments (root only, stem only, root and stem) and two control treatments (no fire and surface fire only) on longleaf pine (Pinus palustris) growth and defense. We cored 20 similar sized trees (30-40 cm diameter at breast height) per treatment.  We used ANOVA to test for significant treatment effects on basal area increment and measures of vertical resin duct structures (e.g. resin duct production, % resin duct area) ten years before and after treatment.

Results/Conclusions

We did not detect any significant treatment effects on either basal area increment (P > 0.79) or resin duct measures (P > 0.59) in any of the ten years following treatment.  Based on our results, we suggest that longer-term growth and defense response was not impacted by the smoldering duff treatments. However, these measures were highly variable among trees and variation in the preceding ten years was positively correlated with growth and defense measures of the subsequent ten years, R² = 0.71and R² =0.71, respectively. For instance, fast growing trees with more resin ducts in the first ten years had faster growth and more resin ducts in the subsequent ten years, suggesting that genetic differences may be more important than impacts associated with smoldering duff fires.  As a result, we suggest that duff reduction fire treatments around the bases of longleaf pines may serve as a way to mitigate potential negative impacts from subsequent prescribed fire treatments without impeding stem growth or defense.