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Big City, Big Trees
Parrotia persica
What Is a Tree?
Imagine a tulip, a tomato plant, and an oak tree. One of these stands out from the others—in fact, it stands above the others. Yet even though that oak seems radically different, it too is a plant—a really big plant. Trees are among the longest-lived, most massive, and tallest organisms on the planet.
So how are trees distinguished from other plants, particularly big shrubs? Arborists generally define a tree as a plant with these characteristics:
- it can grow at least 10 feet tall
- it has a main woody stem that at 4 feet above ground is at least 3 inches in diameter
- it has a crown of branches at its top
- it does not die at the end of the growing season (that is, it is a perennial).
In order to grow tall, trees need to be sturdy enough to bear the weight of their branches and withstand wind and storms. Tree cells contain a substance called lignin, which makes the plant rigid and leads to the development of wood. With their woody stems, trees can grow much taller than tulips and tomato plants.
What Make a Tree a "Big" Tree?
The classification of trees as small, medium, and large is usually determined by how big a tree can grow by its peak of maturity. Trees are commonly considered big trees if their mature height is at least 40 feet, their canopy spreads to at least 40 feet wide, and their trunk has a diameter of more than 30 inches.
Some trees may take four decades to reach their full size; others grow very quickly, adding girth and/or height rapidly. Therefore, not all big trees are old trees. The invasive tree of heaven (Ailanthus altissima), for example, grows at a much faster rate than many native species—reaching 70 feet by 20 years of age.
Caucasian wingnut
The Benefits of Big Urban Trees
Trees do a tremendous amount for the urban environment. Their leaves absorb large amounts of carbon dioxide, a culprit in global climate change, and use it in the process of photosynthesis. Trees clean pollutants from the air, all the while releasing oxygen back into it. The bigger the tree, the greater the benefits: A large mature tree can absorb nearly 70 times more pollution and give off much more oxygen a year than a newly planted sapling.
Big trees help to cool cities in the summer, providing shade and reducing the air temperature around them. They soak up a lot of rainwater, which reduces storm runoff and soil erosion. Big trees break the force of strong winds, decrease glare from the sun, and reduce air conditioning and heating bills in buildings to which they are adjacent. Moreover, big trees serve as noise buffers, muffling the sounds of car horns and buses, and they provide habitats for wildlife, even in the midst of the busiest city.
What You Can Do
Since trees give so much to the community, why not do a little something for them in return?
Be a custodian to street trees: Trees in beds along sidewalks are known as street trees. Although most municipalities maintain these plants, they need your help. In hot weather, newly planted street trees require 10 to 15 gallons of water a week; older trees need even more. Give street trees a drink! Keep tree beds clear of litter, which blocks the roots' access to oxygen and water. Canine waste kills trees, so please curb your dog. Don't use rock salt, bleach, or cleaning agents near tree pits.
Advocate on behalf of trees: There are a number of organizations you can join to show your support for trees. Look online for tree organizations in your area (for example, in New York City, look for New York ReLeaf or Trees New York and check out the PlaNYC 2030 street tree initiative), and look into national groups such as the Alliance for Community Trees, National Arbor Day Foundation and American Forests.