Source: 10 most magnificent trees in the world (Worth a look!)
Although several cities have started re-planting programs (Chicago & Los Angeles, for example), the trend of city tree cover has been steadily down. Tree cover in cities in Michigan and North Carolina are only 27% of what they once were, while Philadelphia and Chicago are only 16%. There are several reasons for the decline:
- Sprawl. Cities and suburbs have expanded to accomodate growing population and appetite for larger houses with yards. Developers tend to mow down large, mature trees and later replant baby trees as accents for the houses.
- Maturity. A large proportion of the trees were planted immediately after WWII, in a swell of optimism and civic pride. That was 60 years ago, and many trees are reaching their natural lifespan.
- Tree injuries. Trees in urban areas are subjected to living with pavement over most of their root systems, mower injuries, and excavation and compactment of their roots. They also have to deal with greater pollution than their country counterparts. Urban trees are usually heavily stressed.
- Attrition. Trees die from all sorts of reasons - weather, increased pest population, etc, and cities have tended to replant smaller, easier to maintain trees.
- McMansions. Many consumers now prefer "no-lot line" and generally bigger houses, which don't leave room for the larger trees.
Urban trees are important for many reasons. They exhale oxygen, help prevent flooding and soil erosion, and provide shade and windbreaks. The shade keeps cities cooler. For example, in Atlanta, where 380,000 acres of trees were bulldozed over 25 years, the temperature in the city has risen 5 - 8 degrees Fahrenheit, compared to the countryside. This heat island effect can have drastic impact on human health during heat waves, which may increase in severity with global warming.
Trees also absorb carbon dioxide and particulate pollution. Street trees reduce the level of particulates by up to 60%. Large trees can take several decades to mature, but when they do, they absorb 60-70 times as much pollution as small trees. If we cut down trees to provide wood for fuel, we are putting ourselves in double trouble: creating particulate pollution while also destroying the way to absorb that pollution.
So we are entering a period of energy decline, where there will be increasing incentives to cut down trees for cooking and heating fuel, and our urban tree cover is already at historic lows in many cities.
But don't give up hope. After all, if they can replant Kenya.....