The greatest erosion occurs when all large trees are removed without providing a new parapet cover. Fort Conahey and Fort Urmston are unfortunate examples of earthworks that have suffered erosion as a result of tree removal. With the canopy removed, the leaf litter cover is washed away with the next storm. Erosion sets in, immediately softening the profile, blurring and eventually destroying features, such as gun platforms and embrasures. The erosion ends only when a new cover is established.
Large trees growing on or adjacent to the parapet, however, can uproot and damage the earthwork.
Tree throw on the Fish Hook line was not found to be a problem generally, but storm damage could be catastrophic. This suggests that earthworks in mature forest cover cannot simply be left alone as suggested by the manual, but must be actively maintained by selectively removing trees. High-risk trees would be thinned and removed, while low risk trees would be encouraged to maintain enough canopy to shed rainwater and replenish the leaf litter cover. From our observations in the field, however, the location of thrown trees appears entirely random. Certain tree species, such as white pine, appear to cause more damage than others when uprooting.