How Old Is That Tree?
Published on March 08, 2016
We often hear claims about 200, 300-year old trees being removed or preserved in Arlington County, and, while we do have a handful of those, most trees are not that old. When aging or dating a tree, you have to take to take three major things into account: History, Site characteristics, and Species.
History
Urban foresters do informal tree ring counts on removed trees, and have employed outside support on comparing these ring counts to climate and other events. Rings will tell you the most information on the age of the tree, and you can often get within a decade of the tree’s birth.
That being said, doing some digging in the library can often get you a better idea of when a tree started growing. Here are some major aspects of Arlington County and events that strongly influenced trees in the area:
Civil War
While disturbance before the Civil war definitely happened, finding information on this is difficult. The war itself has sparse information, but one thing is common knowledge in aging trees. Sight lines for artillery and resource needs cause the removal of almost all trees in Arlington County. This means most of our trees are not likely to be older than 150 years. Where trees were preserved for shade for soldier, there is a chance for older-aged trees, and one of the two remaining old growth forests, on Arlington Cemetery, behind the Lee House, was preserved. This forest appears to have been unlogged since before the late 1700s, but is a striking exception to the rule.
Farming
Much of Arlington county was farmed, in the 1800s, and we have records of dairy farms in the county until the 1990s. These areas typically had very little trees, except as hedgerows or trees near the homes. Checking old maps, like the one on the right, from 1934, can tell you whether the land was used for forest or not. If the land use was farmland, you can exclude most ages beyond that date. We are blessed with an abundance of historical data, being in the DC region, and this can aide in this research.
Many digital scans of these maps are available on our website, as well, in our GIS Gallery.
Post-World War 2 development
The rush to build housing for new federal employees, and the new Pentagon brought with it large subdivisions in Arlington County. The Fairlington community was built during the war, to house thousands of new employees. If you drive through the community now, there are sizeable willow oaks and Darlington oaks gracing the streets. These trees were mere sprigs when this development was completed, but can only be 70-80 years old by now, despite their current size (see images on the left).
Fairlington then (1940s)
Recent disturbance
Home redevelopment, street projects, and other disturbance happens all the time in this busy county. Finding out from locals about projects can tell you whether a tree was removed and replaced. Neighborhood conservation projects and county tree planting programs have not always been around, and this information can often be gathered from neighbors around the tree.
Fairlington Now (2010s)
Site characteristics
The site in which a tree grows up greatly influences the speed of its growth. Poor sun exposure, scarce access to water and oxygen, nutrients, and other limiting factors can slow down the growth of a tree drastically. While a tree of the same species in the middle of a forest may only grow a conservative 0.2 inches a year in diameter, in an open field it may shoot up at 1 inch annually. This means street trees, which often do have restrictions, but more access to light, can often be much younger at the same size as forest-grown trees.
Species
One of the oldest broadleaf trees in the Eastern United States is a Black Tupelo (Nyssa sylvatica) in New England (News link) It’s almost 600 years old, but it’s only 30 inches in diameter. This species grows so slowly that one could not guess its age by looking at it. On the other side of the spectrum, the elm tree on the right, which was removed last year, at 40 inches in diameter, only has rings for 40 years of age.
Some species will not get very large. Finding a dogwood with a diameter larger than 15 inches is a rarity, and can indicate good growing conditions and old age. Knowing your species, and their biological characteristics can help greatly in providing estimates.
Young American Elm
Tools for dating trees
All of this information will give you an idea of the age of the tree. If you are doing in-depth research, you will need more information, so ring counts (the field of dendrochronology, for those Scrabble fans out there) are often the best approach. If removing the tree is not an option (which it often isn’t), coring a tree, or drilling a small hole to extract a rod, can be a great way to get an idea of its age. This technique extracts a sample of the tree rings, usually not harming the tree, which can then be analyzed. Many trees are hollow, however, and estimates are typically the only approach, along with the other tools above.
Tree core extraction. Source: International Environmental Data Rescue Organization
More information:
About Tree Rings, University of Arizona
What tree rings tell us about our environment
How old is my tree?