Forest wind disturbances are an under-appreciated influence on regional carbon cycles

Background/Question/Methods

One of the major shifts in thinking about forest dynamics since the time of Lucy Braun has been the recognition that most forests are in a non-equilibrium state, generally as a result of having been disturbed.  This has a broad range of implications for structure, habitat, species composition, and other population and community level characteristics.  At the ecosystem level, we have only recently recognized how important disturbances are, collectively, as drivers of regional carbon dynamics.  Indeed, uncertainty about how disturbances influence North American forest carbon dynamics remains one of the biggest lacunae in our knowledge of regional carbon cycles.  Moreover, climate change may bring altered frequencies and severities of wind events.  An important paper by Jeffrey Chambers and colleagues documenting the ‘carbon footprint’ of Hurricane Katrina suggested that roughly 100 Tg of carbon may have been transferred from living to dead pools by the hurricane, an amount similar to the total carbon fixation by vegetation in the conterminous 48 states.  Zeng et al (2009) later retrospectively estimated that for the 20th century, hurricanes probably accounted for a mean of 18-20 Tg of carbon transferred from living to dead.  Clearly wind disturbances may be important, but these studies have explored only one type of wind disturbance.  Consequently, my lab has recently been investigating how tornado disturbances to forest might compare to and augment the carbon impact of hurricanes.  Tornadoes are very much smaller but much more numerous than hurricanes.  

Results/Conclusions

We have recently estimated that a Southern Appalachian tornado transferred ~ 0.212 Tg of carbon from living to dead pools.  We have quantified the number, path length, path width, and severity across a representative sample of other tornadoes to scale up.  At the national level of impact (e.g. ~ 1200 tornadoes annually), we estimate that the ‘carbon footprint’ of all U.S. tornadoes is perhaps 7-10 Tg of carbon, or roughly 40-50% of that of hurricanes.  This leaves the carbon impact of large derecho events (typically 1-3 annually in the U.S.) completely unexplored.  If tornadoes and hurricanes together may shift ~ 30 Tg C annually from living to dead, and if the unexplored impact of derecho events is intermediate between these other types of storms, wind events in total may be shifting 45 Tg C annually from living to dead.  Clearly, wind disturbance indeed may be a major influence on regional carbon dynamics for eastern North America