Strong wind gusts are a year-round occurrence up on our Penn State New Kensington ridge top. Summer thunderstorms, spring and fall weather fronts, and winter blizzards can each generate high velocity winds that can do serious damage to our trees. This past week, a racing cold front colliding with our first warm, humid spring air mass (wasn’t it nice?) triggered wind gusts that locally topped 90 miles per hour! It was a hurricane of a storm.Wind is increasingly being recognized as a vital, sculpting force in a forest. Even primal forests of Pennsylvania had a broad array of trees of different sizes and ages and sunlight requirements. It is thought that one reason for this age and species diversity was the occasional, but inevitable, impact of wind storms. Older trees were broken or wind-thrown, and younger trees in the then sunlit understory would race up into the canopy and replace them.
Many important tree species, like the white pine for example, need this regular release from the deep shade so that their seedlings can have a chance to grow into the giants of the forest. Disturbance and change was important for the forests’ ecological health and vigor.
Needless to say, though, the forest is not a good place to be during a wind storm!
On campus a large spruce tree was shattered in the recent storm. A month or so earlier, a two hundred year old white oak out on our nature trail was similarly damaged. This oak was the largest and, likely, the oldest tree along our trail. It had a four foot trunk diameter and a broad, spreading crown of thick branches. It had probably germinated after the local land clearing events of the early 1800’s and then somehow survived the extensive timber cuttings of early 1900’s. It stood over a cluster of much younger yellow poplar, white ash, and red and sugar maples.
Deborah and I had photographed the tree in the style of Georgia O’Keefe’s painting “The Lawrence Tree:” the view straight up the trunk ending in the chaotic geometry of the branches. We used this picture in several of the editions of the nature trail brochure and have it out on the Virtual Trail web site on the white oak species page.
This isn’t the first large white oak to fall out on the trail. One of almost equal size and age fell in a July 1999 microburst after having been killed by the repeated gypsy moth infestations of the late 1980’s and early 1990’s. The death and fall of this tree, though, opened up the canopy for a generation of fast growing poplars that grew in great leaps (6 or 7 feet a year!) once the limiting shading was removed.
It’s odd that these strong white oaks should be broken in storms while the softer wooded poplars around them survive. But, then, we have to remember that we have cleared out many, smaller fallen poplars over the years, too. It’s just that none of them had the presence or the history of either of these oaks.
The forest is constantly changing. There is the slow, incremental change caused by growth and competition for the limited sunlight reaching the understory trees. This incremental interaction of the trees has led to a gradual transition of the forest into increasingly mature stages of succession. The black pines and white ash are slowly dying out, and the white oaks and sugar maples are slowly rising to dominate the canopy. But there is also the sudden, violent catastrophe of a storm that re-sets the ecosystem and wipes out decades of gradual change in the space of a few seconds. Sometime in April we’ll go out and trim and clean up the fallen and broken trees on the nature trail. A little spring cleaning is good for the “house” and gets everyone ready for all that is yet to come.