Contingent resistance in longleaf pine (Pinus palustris) growth and defense 10 years following smoldering fires

Abstract

In many fire-prone woodlands and forests, fire exclusion has resulted in substantial litter and duff accumulations capable of long-duration smoldering once fire is reintroduced. While previous research has shown that the soil heating from smoldering fires resulted in short-term reductions in coarse root non-structural carbohydrates and latewood growth, information on the long-term effects of smoldering fire is lacking. 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 ten years after treatments. We cored 17–29 similar sized trees per treatment and measured growth as basal area increment and defense as resin duct properties (e.g., resin duct size, % resin duct area). We used generalized linear mixed models to determine the influence of smoldering treatments and climate on basal area increment and resin duct properties. Variation in basal area increment and resin duct size during the previous ten years were positively correlated with the subsequent ten years, r2=0.71 and r2=0.65, respectively. Additionally, temporal variation in growth and defense were related to climatic factors. Growth had the strongest correlation with current August temperatures (r=−0.50), and defense had the strongest correlation with current December temperatures (r=−0.64). Basal area increment was best predicted by summer and fall Palmer Drought Severity Index, while resin duct size was best predicted by the interaction of treatment and precipitation during the previous November. During the three driest years following treatment smoldering duration had a negative relationship with basal area increment (p<0.0085; r2=0.08) and resin duct size (p<0.0003; r2=0.16). While longleaf pine growth was generally resistant to long-duration soil heating following years of fire exclusion, the proceeding growth response was contingent on the post-fire climate. Longleaf pine resin duct size was sensitive to the effects of smoldering fire, and potential negative impacts may be exacerbated by lower precipitation. Restoring fire to long-unburned sites proceeded by warmer and drier climate conditions may result in reduced longleaf pine growth and defense that may contribute to increased probability of mortality during subsequent disturbances.