Friday, July 21, 2006
Toppled trees: every fallen tree has a history
There seems to be a spate of toppled trees around town. The recent heavy rains and flooding must be the cause. That appears to be what people are thinking and also reading (Boston Globe July 13). It does make sense — saturated soils are significantly weaker than normal dry soils as anyone who has planted a tree or fence post can attest. We've had drenching rains and many areas are still very soupy, even now in mid-summer.
The technical term for trees failing by pulling up their surrounding soil "plates" is "windthrow," which happens frequently when saturated soils combine with high winds. This is a major problem in many parts of the country such as Florida. I suppose there have been a few cases of genuine windthrow around town this year but most of the tree failures that I've seen have had a different cause. Let me explain my thinking.
Trees not pre-programmed
Trees grow every year that they're alive. They are not pre-programmed to die as we in the animal kingdom are. They simply keep getting bigger and bigger until their luck runs out. At some point the energy costs required to protect their ever-expanding structure exceeds their ability to capture energy from the sun and manufacture defensive chemicals. At that point, the ever-present fungi and insects start winning the battle, and decline sets in.
Declining trees will ultimately fail in some combination of three mechanisms. First, the limbs might break, further limiting the trees' ability to photosynthesize. Alternatively, the trunk might break very near the ground, and, if the base is obscured as you drive by, this might appear to be windthrow. Third, the roots will decay, allowing the tree to topple. This last will appear to be windthrow, because some roots and soil will still be attached visibly.
Double whammy for trees
I believe that exceedingly dry soils over this last winter and the saturated soils this spring and summer were a double whammy relative to decay in both the trunks and root systems of many local trees. Trees that were already drought- stressed probably lost root hairs and failed to grow replacements. Remaining fine roots were then drowned. Deprived of oxygen in prolonged flooded conditions, the roots of poorly adapted trees simply die. Losing those smaller roots reduces the tree's anchorage and ability to fight pathogens. I also suspect that many existing decay areas were able to expand rapidly under these conditions. Unable to outgrow the advancing decay, trees simply broke this spring.
Healthy trees in their prime sitting on a good site can still have bad luck. Lightning strikes routinely wipe out large white pines. I would be curious to know how often a strike lands without killing one around Carlisle. Microbursts, or sudden concentrated downdrafts during thunderstorms, are equally random and devastating. You might recall this happening on Punkatasset Hill (off Monument Street) a couple of years ago.
Hurricane might be welcome
On the flip side there are many trees and tree seedlings that would welcome the arrival of a good, old-fashioned New England hurricane. A hurricane would give them a chance to realize their dreams by providing bare soil and plenty of sunshine.
Exotic fungi, insects, climbing vines (notably bittersweet), acid rain and summer droughts are on a growing list of bad luck for our native trees. Of course, growing on a few inches of soil over ledge is exceedingly bad luck, but trees can't exactly pick up and move.
One cause for the apparent wave of tree failures during our recent modest summer storms might be the relatively mild recent winters. They have been very short on the icing and wet snow combined with high winds that typically pull down the weaker trees. These marginal trees have accumulated in the landscape. The weight gain from high wood moisture levels and the lush growth encouraged by the wet conditions keep increasing until a surprised robin lands on the branch and it then "snaps."
Growth vs. anchorage
There is a significant difference between the minimum roots and conducting tissues required for growth versus the roots and wood fiber required for adequate anchorage and support. One day the structure looks fine, the next day it's on the ground. Norway spruce in particular seems to have this problem.
Damage such as compacted soils, grade changes, severed roots and adjacent tree clearing all contribute greatly to tree loss in our constructed landscape. When these trees fail, it's often blamed on windthrow, but the trees know better. Extensive research is being conducted in Florida using massive wind machines. They're finding a strong correlation between genuine windthrow and poor planting methods that cause problems such as girdling roots. In contrast, both naturally seeded and properly planted trees hold up pretty well.
Every fallen tree has a history. It often looks like windthrow but the real cause is very often a quite different story.
© 2006 The Carlisle Mosquito