Wind disturbance & carbon cycling
Our recent focus on carbon cycling in wind disturbed areas is not only a new topic, but a big shift in focus from community-level to ecosystem-level consequences of wind disturbance. The global carbon cycle is known fairly well, but regional-scale carbon cycling is more poorly understood. In North America, forests are major influences on the carbon cycle, and forest disturbances are one of the biggest unknowns, in terms of how carbon cycling is affected. Much attention has been concentrated on the impacts of fire, but rather little on the effects of wind. We have yet to obtain major funding to pursue this topic, but we have been able to conduct several pilot studies.
To demonstrate the feasibility of documenting soil respiration in wind damaged forests, Uma Nagendra has led a pilot study in our Boggs Creek study site during Summer and Fall 2013. The basic arrangement of the study was to sample soil respiration along a gradient of disturbance severity, which we had quantified soon after the 2011 tornado. Going into the study, we expected that the more severely-disturbed areas would have less cover in 2013, and therefore higher soil temperatures, which in turn should drive higher rates of soil respiration. In contrast to our expectations, however, the first two rounds of measurement showed that soil respiration was not higher in more severely disturbed plots.
This surprising result has prompted us to reconsider other expectations, up to the broad question of when disturbed areas switch from being carbon sources to the atmosphere to being carbon sinks; we now think that such a switch may occur within the first few years rather than the 5-10 year time frame we had previously envisioned.
A second way in which we are approaching the topic of wind disturbance and carbon cycling is at a much larger scale. We have been attempting to quantify the amount of forest area annually disturbed by tornadoes within the U.S. Regardless of severity, the simple area affected by wind disturbances will be one major determinant of the carbon effect of such disturbances. Leaving hurricanes to other researchers, we are estimating tornado track length through several different land use types, including forest.
Once the total tornado track length through forest is documented, we will then use several approaches to convert the length to area, based on estimates of tornado track widths, although this parameter is particularly poorly documented. We also need to estimate how much of the time tornadoes are actually on the ground; i.e. how much of the track is actually disturbed vs areas where the vortex has temporarily lifted off the ground. To accomplish these objectives we are combining GIS analysis of tornado tracks, with national databases on tornado track lengths. This work is ongoing, but we expect that we will find that annually, perhaps 200,000 – 300,000 ha of forest area are affected by tornadoes in the U.S.
The above estimates of area impacted by tornadoes can be taken a step further. From our GIS analyses of tornado track air photos (see Research page on landscape-scale analyses), we estimated that the north GA April 2011 tornado shifted 0.212 Tg of carbon from living to decomposing, in a track of 73.9 km (with on-the-ground damage across 58.3 km). Scaling up from these numbers to the total of >300 tornadoes in the late April 2011 outbreak, we estimate the 3.33 Tg of carbon was shifted from living to decomposing by that tornado outbreak. Further analyses are underway to further scale up to the national total for a given year.