Tree & neighborhood scale patterns of wind damage

Work by Peterson or students in our lab now covers at least 15 different wind-disturbed forest sites in eastern North America, scattered among Pennsylvania, Connecticut, Missouri, Minnesota, Tennessee, and Georgia. 

study sites

Not all of this work has yet been published, and in fact a “synthesis” paper presenting findings from all of these sites together, is underway.  The most robust patterns are probably to be expected:  larger trees are usually (not always!) more vulnerable to wind damage than smaller trees; there are interspecific differences in levels of wind damage, although size is probably a more important factor, and the numerous other influences (soil type, soil saturation, stand age, exposure) makes it difficult to get an accurate picture of how species differ.  This image shows moderate-severity damage at a site in northern Georgia; note that most of the largest trees are down, while most of the saplings and understory trees are still standing.

 blowdown

There are interesting and perhaps counter-intuitive trends spatially across stands, in which trees growing at long-established edges, or elsewhere that have had a lengthy history of wind exposure, tend to be more resistant to wind damage than those trees that lives a “sheltered” life in a closed stand.  There are hints from other researcher’s studies that some species may preferentially lose limbs in high winds, and thereby avoid uprooting or major trunk breakage.  Resprouting after wind damage is very common among most of the hardwood species, and resprouts may in some cases make major contributions to the regenerating stand.  The long-term viability and stability of sprouts on damaged parent trees, however, is unknown. 

One recent finding from work in a series of different-sized gaps in Allegheny National Forest (northwestern Pennsylvania) showed that gap size and severity covary; i.e. larger gaps also tend to be more severely disturbed.

 size severity

This finding has not been previously described, and it will be interesting to see if other studies find similar patterns.

Another trend that we’ve documented is that in a given wind-damage site, treefall directions seem to be more strongly determined by the wind than by the topography; trees do not necessarily fall downhill. 

tree fall direction

Perhaps more surprising, though, is a currently-unexplained pattern:  larger trees tend to fall slightly more in a clockwise direction than smaller trees.

 small vs large

Most work (including our own) has thus far simply presented damage patterns “as is”, using the data from plots that experienced varying levels of wind damage.  However, following groundbreaking work by Charlie Canham, Michael Papaik, Roy Rich, and Lee Frelich, we are shifting the approach to studying damage patterns by partitioning our data into differing levels of severity to ask questions about how damage patterns vary from lower to higher severity.  Severity is the outcome or amount of damage from a disturbance, and is usually correlated with intensity, or the “power” of the disturbing agent; in the case of wind disturbance, intensity would be the speed and duration of the wind.  Severity and intensity are not perfectly correlated, however, because multiple factors determine whether trees stand or fall in a wind event.  Thus the newer approach seeks to estimate intensity (in the absence of anemometers!) using some kind of standardization of the severity (e.g. use one species and one size class to compare damage among many sample plots).