Long-term sensitivity of ponderosa pine axial resin ducts to harvesting and prescribed burning

Forest restoration treatments primarily aimed at reducing fuel load and preventing high-severity wildfires can also influence resilience to other disturbances. Many pine forests in temperate regions are subject to tree-killing bark beetle outbreaks (e.g., Dendroctonus, Ips), whose frequency and intensity are expected to increase with future climatic changes. Restoration treatments have the potential to increase resistance to bark beetle attacks, yet the underlying mechanisms of this response are still unclear. While the effect of forest restoration treatments on tree growth has been studied, less is known about their impact on resin-based defenses. We measured axial resin ducts in the earlywood and latewood of ponderosa pines (Pinus ponderosa) in western Montana, USA, 20 years before and after the implementation of restoration treatments, with the aim to elucidate changes in the yearly and interannual investment in resin duct defenses following treatments and their sensitivity to climate. Two experiments were established in 1992: a moderate thinning and a retention shelterwood, with 35 % and 57 % basal area reduction, respectively. Each experiment comprised four treatments with three replicates per treatment: cutting only, cutting followed by prescribed burning in either spring or fall or under wet or dry duff moisture conditions, and an untreated control. Cutting treatments stimulated a long-term, sustained increase in resin duct production, more pronounced in the earlywood, which we attribute to a higher availability of resources due to reduced tree density. Prescribed burning following cutting induced a short-term increase in resin ducts, likely aiding in the compartmentalization of fire-killed cambium and enhancing the resistance of fire-injured trees to bark beetle attack. However, the fire-induced spike in duct production was not related to the degree of crown scorch. Treatments had little effect on climate-defense relationships, as ducts remained positively correlated to winter precipitation and, though less significantly, negatively correlated to spring maximum temperature. Our findings show that by reducing stand density, forest restoration treatments induce the synthesis of resin ducts, which are key in mitigating vulnerability of ponderosa pine to mountain pine beetle (D. ponderosae) attacks, thus promoting forest resilience to multiple disturbances.