Bark beetle resistance in trees associated with increased resin duct allocation: cumulative support from four western U.S. pine species

Background/Question/Methods

Elevated global temperatures and associated droughts have been implicated in widespread bark beetle-caused tree mortality.  While tree resistance to bark beetle attack through some defense traits (resin flow, resin chemistry) has been established, data on these traits are often temporally-limited (1-2 y), time or cost intensive to measure, and variably informative. However, recent studies are beginning to highlight the importance of the relatively simple, retrospective defense trait measures of resin ducts to assess tree resistance to bark beetle-attack.  Here we synthesize existing studies that have demonstrated the importance of resin ducts traits (production, density, and size) in four pine species (Pinus contorta, P. edulis, P. flexilis, and P. ponderosa) to the susceptibility and mortality to bark beetles. Additionally, we begin to examine some potential contributing factors of resin duct variation within P. edulis, including tree size, irrigation treatments, and site productivity.      

Results/Conclusions

In all four pine species, allocation to xylem resin ducts was negatively associated with the probability of a successful bark beetle attack. In a P. ponderosa study from northern Arizona, trees that survived had 51% more resin ducts in the last 5 y of radial growth than trees that died.  Another resin duct study conducted on P. edulis in New Mexico found similar differences with live trees having 26% more resin ducts than dead trees.  However, both of these studies did not directly examine trees that demonstrated bark-beetle resistance.  A more recent study on P. contorta and P. flexilis in Colorado, which specifically included bark-beetle resistant and susceptible trees, corroborated the previous studies and found resistant trees to contain 23-37% more resin ducts than susceptible trees.  Preliminary results suggest that resin duct traits in P. edulis varied by tree size and site productivity, but small trees were not responsive to irrigation treatments after six years.