THE IMPACTS OF REMOVING WILDFIRE ON PINUS PONDEROSA RESISTANCE AGAINST DENDROCTONUS PONDEROSAE

Disturbance and disturbance interactions drive multiple ecosystem processes over multiple spatial and temporal scales. I studied the role of frequent, low-severity fire in Pinus ponderosa (ponderosa pine) forests in altering the success of the irruptive insect Dendroctonus ponderosae (mountain pine beetle (MPB)). I had three overarching research questions: (1) What are the best predictors of ponderosa pine resin flow, a trait related to MPB attack success, and how do these predictors relate to tree growth?; (2) Are predictors of resin flow also predictors of MPB attack success and are they affected by changes to wildfire disturbance regimes?; (3) How do forest management treatments affect individual tree defense and forest resistance to a MPB outbreak? I first show that the best predictors of resin flow are resin duct size and total resin duct area, both of which increase with tree growth rate. Next, I show that resin ducts are good predictors of MPB attack success. Based on field data in natural stands subjected to wildfires, I then show low-severity fire induces resin duct production, and that resin ducts decline when fire is excluded. Therefore, low-severity fire can trigger a long-term induced resistance that may increase tree survival from subsequent herbivory. Lastly, I investigated the consequences of removing fire from fire-dependent ponderosa pine forests on resistance to a MPB outbreak 10 years after experimental density and prescribed fire treatments. Annual growth and resin ducts increased after thinning, but contrary to results from natural fires, prescribed burning did not affect resin duct production, although slight changes in resin quality that may reduce attack success were observed in the burn-only treatment. MPB caused ~50% tree mortality in the control compared to ~20% in the burn-only, and almost no morality in the thin-only and thin-burn. These results suggest that thinning in fire-excluded stands is the most effective management to reduce MPB. However, the positive effect of natural fire on ponderosa pine defense combined with anticipated changes in regeneration and stand dynamics after fire suggest that fire treatments should not be ruled out as effective means to increase long-term forest resilience to bark beetles