Longleaf pine (Pinus palustris Mill.) woodlands are dependent on frequent fire to maintain their structure, resilience, and ecosystem services. Fire is known to impact bark beetles and tree defenses in other pine species, but few studies have focused on longleaf pine. This study assesses the effects of frequent fire (2-year fire return interval), fire exclusion (≥14 years since fire), and fire reintroduction (1-3 fires after a 14-year absence) on bark beetles and tree defenses. The study was conducted in mature longleaf pine stands (>80 years old) at the Jones Center at Ichauway in southwestern Georgia, USA. Treatments were repeated at two sites with substantially different soil water holding capacities which affect forest growth and tree density (henceforth mesic and xeric). Our objectives were two-fold. First, we quantified effects of fire regime on bark beetle abundance for species (Ips spp.) known to colonize stressed trees and sometimes considered pests on conifers. Second, we assessed tree defenses, specifically physical quantity and chemical properties of resin, the trees’ primary defense against bark beetles and associated fungi.
Results/Conclusions
Preliminary results show that site characteristics may interact with fire regime in important ways affecting both bark beetle abundance and tree defenses. There was a significant interaction between fire treatment and site on Ips catches. In the mesic site, fire-exclusion traps yielded significantly fewer Ips beetles than the frequent-fire treatment (average counts 1,350 and 2,020). At the xeric site, fire-exclusion traps yielded more Ips than the frequent-fire treatment (3,205 vs. 1,139). These contrasting effects of fire treatment supports the hypothesis that the lack of fire leads to additional stress on trees and increases attractiveness to Ips; additional investigation is needed to understand results at the mesic site. Total resin flow quantity was significantly different between sites (5.73 g/day in mesic vs. 7.75 g/day in xeric), but there were no significant differences among fire treatments. Chemical analysis of resin for terpenoid compound identification, composition, and concentrations is ongoing. These results suggest that site soil properties may be more important than fire regime for tree physical defenses, at least in relatively early stages of fire exclusion.