Dwarf mistletoe infection interacts with tree growth rate to produce opposing direct and indirect effects on resin duct defenses in lodgepole pine

Research Highlights: I sought to disentangle the influences of tree age, growth rate, and dwarf mistletoe infection on resin duct defenses in lodgepole pine, Pinus contorta Douglas ex Loudon, revealing the presence of direct positive and indirect negative effects of mistletoe on defenses. Background and Objectives: For protection against natural enemies, pines produce and store oleoresin (resin) in 'resin ducts' that occur throughout the tree. Dwarf mistletoe, Arceuthobium americanum Nutt. ex Engelm. (hereafter "mistletoe"), is a widespread parasitic plant affecting the pines of western North America. Infection by mistletoe can suppress pine growth and increase the probability of insect attack-possibly due to a reduction in resin duct defenses or in the potency of chemical defenses at higher levels of mistletoe infection, as reported in Pinus banksiana Lamb. However, the influence of mistletoe infection on defenses in other pine species remains unclear. I hypothesized that mistletoe infection would induce greater resin duct defenses in P. contorta while simultaneously suppressing annual growth, which was expected to reduce defenses. Materials and Methods: Using increment cores from P. contorta trees occurring in a subalpine forest of Colorado, USA, I quantified tree age, annual growth, annual resin duct production (#/annual ring), and cross-sectional area (mm² of resin ducts/annual ring). Results: Mistletoe infection increased with tree age and had a direct positive relationship with resin duct defenses. However, mistletoe infection also had an indirect negative influence on defenses via the suppression of annual growth. Conclusions: Through the combined direct and indirect effects, mistletoe infection had a net positive impact on resin duct production but a net negative impact on the total resin duct area. This finding highlights the complexity of pine defense responses to natural enemies and that future work is needed to understand how these responses influence overall levels of resistance and the risk of mortality.