Phloem and defence traits in relation to tree size and age: Implications for host selection by bark beetles

Tree size often predicts the occurrence of bark beetles that breed in the inner bark (phloem) of trees. A notable example is mountain pine beetle, Dendroctonus ponderosae, which primarily kills large, old trees during outbreaks. The tree mortality pattern is often attributed to increasing phloem thickness with tree size, although tree size-dependent defences may also be important. Age effects may confound interpretations of tree size effects. We examined phloem and anatomical defence traits in single-cohort lodgepole pine, Pinus contorta, stands as a function of tree size and age within and across young mature stands (trees 25–45 years old) and old stands (115–140 years old). Phloem thickness increased linearly with tree size within and across stands, more strongly for trees in young stands that had thicker phloem than same-sized trees in old stands. Phloem nitrogen was greater in young stands and did not vary with tree size. Resin duct investment was generally greater in larger trees within stands, but young trees had more and larger ducts than old trees. These results are consistent with canopy dominance of larger trees within stands that can acquire more carbon for both growth and defence; relationships between tree size and both phloem and defence traits were weaker across stands of different ages. Greater defence in young trees may reflect greater risk and costs of herbivory that might be associated with higher phloem nitrogen in younger stands. Our results suggest that the best phloem resources (quantity and quality) are found in relatively large trees in young stands, which is a poor match for the pattern of attacks by mountain pine beetles that favour large trees in old stands. Lower defences in old stands may also explain the pattern of tree mortality by beetles. Other benefits of large trees merit more attention in understanding the importance of tree size to bark beetles.