Interacting disturbances: Unexpected increases in coast redwood mortality from synergistic effects of fire and a non-native pathogen

Background/Question/Methods

Understanding how natural and anthropogenic disturbances interact is of growing importance because of the increasingly important role disturbances such as habitat fragmentation, exotic species, or global change play in structuring ecological communities. Phytophthora ramorum, which causes the emergent disease sudden oak death (SOD), is a non-native pathogen causing widespread tree mortality in coastal forests of California and Oregon.  In the absence of the pathogen, species composition in these forests is shaped by a number of biotic and abiotic factors, including the endemic disturbance of wildfire. Large wildfires in California in 2008 provided an opportunity to test the interactions between P. ramorum and wildfire and their separate or synergistic impacts on forest composition.  Here we ask whether the interacting effects of P. ramorum and wildfire increase mortality for three dominant species in redwood forests that differ greatly in their vulnerability to the pathogen and to the effects of fire. The 2008 Basin Complex and Chalk fires in Big Sur, CA, burned more than 40% of the 280 forest monitoring plots established throughout the region in 2006-2007.  In 2009 we surveyed every stem alive at plot establishment in 2006-07 to determine mortality one year following the fire.

Results/Conclusions

In the absence of fire, all three species suffered little mortality between 2006-07 and 2009, except for tanoaks in pathogen-infested plots.  Fire increased mortality, but mortality in tanoak and bay laurel was much higher (48% and 65% basal area lost, respectively) than in redwood (only 8%) in burned plots.  Redwood mortality varied substantially by tree stature, with mortality risk more than doubling for intermediate-sized stems in infested forests. Widely used predictive models of tree mortality following fire linking tree size and bark thickness to survival under-predicted redwood mortality in infested areas, even though the pathogen does not impact redwood health and redwood is typically resilient to wildfire.  Only by including disease impacts to the forest was it possible to successfully predict post-fire redwood mortality in infested stands. Elevated fire-related redwood mortality in the presence of P. ramorum is not due to direct effects of the pathogen on redwood, but rather the ways that SOD influences fire severity through impacts on forest structure and fuel availability. This is a startling example of how the synergistic effects of an invasive pathogen and wildfire spill over to negatively impact a dominant forest species that is usually resistant to either disturbance alone.