PS 28-134 - Pine Resin Duct Defenses Against Bark Beetles Respond to Experimental Fire in Saplings and Mature Trees

Abstract: The two main disturbances that affect pine trees in the Pacific Northwest are fires and bark beetles. Few studies have linked fire behavior and intensity with post-fire responses in conifer defense. Resin acts as both a mechanical and chemical barrier in preventing bark beetles from entering trees, and resin ducts serve as the sites of resin production and the network of resin delivery. We assessed the response of vertical resin ducts to known fire intensities in saplings and mature trees of Pinus ponderosa to investigate the interactions between wildfire and defense against bark beetle attack.

We assessed sapling cross-sections from Pinus ponderosa collected 21 months following a combustion laboratory experiment with plants exposed to a fire radiative energy (FRE) of 0.7 MJ m^-2 and unburned controls. We measured the number of vertical resin ducts per xylem area in the year following fire. We found that fire nearly doubled the number of resin ducts in the year post-burn, from a mean resin duct density of 53.1 ducts cm^-2 in unburned controls, to 101.2 ducts cm^-2 in burned saplings. One cause for this increase we observed was an increase in traumatic resin duct production.

We also measured resin ducts of mature Pinus ponderosa trees in the field after a prescribed fire at the University of Idaho Experimental Forest in plots where FRE varied from 1.3 to 13.3 MJ m^-2. We measured resin ducts in increment cores from 31 trees that survived fire and from unburned controls. We found a significant increase in duct production, duct size, and duct area in burned trees relative to controls in the year after fire. Increased resin duct metrics were mostly driven by trees which received relatively lower FRE. Notably, in the second-year post-burn we observed no effect of fire on resin ducts.

Understanding the responses of both saplings to laboratory fire experiments and mature trees to fire in the field will improve our ability to predict how trees will respond mechanistically under future fire scenarios. Our results show that burned trees increase resin duct metrics in response to fire, for both sapling and mature tree life stages, at least in the year immediately following fire. Future research plans include resampling trees burned in the field 10 years after that fire to investigate long-term resin duct responses, and to explore interactive effects of drought stress with experimental fire on sapling resin ducts.