Variation in Anti-Insect Defenses of Three Coniferous Tree Species across Temporal and Spatial Gradients of the Colorado Front Range

Recent epidemics of bark beetles (Curcurlionidae: Scolytinae) have not only revealed limitations in our understanding of bark beetle-pine host interactions, but have also revealed the presence of large variations in conifer defenses across seemingly small spatial scales. This variation has clear potential to affect the dynamics of bark beetle epidemics since coniferous tree defenses play an important role in bark beetle population dynamics. At the same time, evidence suggests that bark beetle epidemics are a strong form of selection on conifer defenses as beetles selectively attack less defended trees. This selection by bark beetles on tree defenses could create a mosaic of tree resistance across the landscape where forests impacted by more frequent or recent intense bark beetle infestations would have greater anti-bark beetle defenses. To elucidate the role of tree defenses in resistance to bark beetles, I investigated the effect of bark texture of limber pines (Pinus flexilis) on beetle attack densities and characterized the vertical resin ducts of limber pine and lodgepole pine (Pinus contorta) trees that were bark beetle susceptible (killed by bark beetles) to trees that were resistant (survived attack). Following these investigations, I characterized variation in resin defenses (characterizing both resin duct and resin chemical traits) of limber, lodgepole, and ponderosa (Pinus ponderosa) pines across spatial (elevational) and ontogenetic (tree age) gradients of the Colorado Front Range. I found increasing proportional coverage by smooth bark was negatively related to bark beetle attacks/m2 on limber pines. I also found resistant trees for both lodgepole and limber pine had significantly more resin ducts in recent xylem growth rings than susceptible trees. Resin duct production was also positively related to tree radial growth rates suggesting that both tree size/age (which are strongly associated with radial growth rates) and resource availability across spatial and temporal gradients is an important influence on pine tree anti-insect defenses. Similar to other forms of anti-insect defense (i.e. secondary chemistry, leaf toughness) both stem coverage by smooth bark and resin duct production are influenced by plant ontogeny whereby younger or smaller trees have greater defenses than older or larger trees. While historical selection pressure by bark beetles may have influenced current patterns of pine defenses, past and present human activities have strongly affected tree age structure in forests across elevation in the Colorado Front Range and elsewhere. At present, higher elevation forests tend to consist of older trees on average than lower elevation forests thereby creating an elevational gradient in pine anti-insect defenses that can be best explained by the underlying gradient in tree ontogeny and allometry