The Venerable Trees of the Kentucky Bluegrass are extraordinarily long-lived. This past weekend, we lost one of the largest and oldest bur oaks in the region. The tree, at the corner of Versailles Road and Man O'War Boulevard on a farm, fell to the south. The death of this tree was most unusual. It was a huge and vigorously growing tree, and it simply fell over. Here is a preliminary analysis of what might have caused this tree to fall over.
Although this tree was ancient, it is rare for Bluegrass trees to simply fall over. More commonly, trees decline slowly after lightning strikes, or following soil compaction and mower damage. Of course, the most common cause of death is the bulldozer - development has decreased the population of bur oaks in Fayette County by over 90% in only 60 years.
I examined this tree as did Master Arborist Dave Leonard, and there are some clues to why this tree fell over. The conclusions are mine, but Dave pointed out several features of significance.
The most important clue is the exceptional weather just before the tree fell (weather data from a station <1 mile from the tree):
- From April 2 to 3, we had 6.40 inches of rain, including 5.17 inches in 24 hours, a record;
- Sustained winds from April 2 to 5 were 16 to 36 mph, with gusts of 47 mph on April 2 and 36 mph on April 3 from N and WNW;
- There were no tornadoes in the region, and no known microbursts. However, there is no nearby official Doppler radar and we cannot rule out a microburst.
The condition of the tree:
- This is an exceptionally large tree, 15.96 ft circumference 10.5 feet above ground (it wasn't possible to get a tape under the tree at 4.5 ft.) and tall, >92 ft.
- The tree was growing vigorously and rapidly, which is common in our ancient trees. Lateral branches averaged 9 inches new growth in 2014.
- The tree had a pronounced lean to the south. Such leans are common in ancient bur oaks in the Bluegrass.
- The tree is remarkably sound. Although there does appear to be some heart rot, there are few signs of extensive decay. We will know more when we dissect the tree.
- Structural roots (the major woody roots) on the north side of the tree are large and sound, with no decay. However, they broke off at 3-15 ft from the stem.
- There was evidence of surface structural root damage on the south side of the tree from long-term mower damage. We can't yet see deeper structural roots on the north side because the stem is in the way. This is important because in heavy wind, these roots would experience strong compression.
My tentative conclusion is that the tree fell because 1) surface structural roots on the south side were heavily decayed due to long-term mower injuries; 2) the soil was saturated from prolonged heavy rainfall; 3) wind gusts from the north compressed roots on the south side, and the remaining south side roots were not sufficient to absorb all the compressive force; 4) as the compressed roots on the south side failed, the weight of the tree exerted enough tension on roots on the north side that these roots failed, even though they were not decayed. We cannot, however, rule out a microburst that produced an intense force on the north side of the tree, compounded by the weakening of soil structure by heavy rain.
This is a preliminary analysis, and we will learn more as we dissect the tree.