Changing growth response to wildfire linked to climate variables in old-growth ponderosa pine trees

Background/Question/Methods

Old-growth ponderosa pine forests are experiencing changes in fire-frequency, fire intensity, and climate. These novel conditions are likely to alter tree responses to wildfire. However, growth responses to natural wildfires in unmanaged, old-growth forests (as opposed to prescribed fires or thinning treatments in managed, 2^nd growth forests) have rarely been studied.  There is also surprisingly little known about the long-term effects of multiple wildfires versus fire-exclusion, or whether responses to wildfire are changing over time, and how tree age or size-class may affect these responses.  The common view is that short-term growth responses to individual fires may vary with fire intensity, degree of tree tissue damage, and soil nutrient release, but that long-term responses tend to be positive for surviving trees because of density reduction and lower competition.  However, these views have rarely been tested in multi-aged, unmanaged stands over long periods.
Results/Conclusions We used basal area increments computed from tree-rings to compare diameter growth responses between trees in paired burned (3-4 fires) and unburned stands (0-2 fires) at four remote sites in the Selway-Bitterroot region of Idaho.  Long-term (44-85 year) responses to multiple wildfires, and short-term (5 and 10 year) responses to individual wildfires were analyzed separately. The relationship of short-term fire responses to tree age, tree size-class, and climate conditions in the months prior to fire were also studied. Contrary to expectations, three out of four sites showed no significant differences in long-term growth responses between burned and unburned stands, while a fourth site showed higher growth in the burned stand.  Short-term responses to individual fires showed an interesting trend over time. Wildfires were more likely to produce negative responses (reduced growth in the burned stand compared to the unburned stand) in recent years that had drier winters and warmer summers. These responses were not systematically related to tree ages or sizes at the time of fire. Climate-driven negative growth responses to wildfire in old-growth forests could have important ramifications for forest productivity and carbon-balance under future climate scenarios.