Results

In 2012 and 2013 Earthwatch engaged over 300 volunteers to measure over 5,000 trees in Cambridge, San Francisco, Chicago, and Atlanta. Join us in 2014 to help us learn even more.

  • Cambridge: 2,544 trees measured by 128 volunteers
  • San Francisco: 1,923 trees measured by 108 volunteers
  • Chicago: approximately 300 trees measured by 32 volunteers
  • Atlanta: approximately 300 trees measured by 35 volunteers


Cambridge, Massachusetts

Earthwatch volunteers have measured 13% of the 20,000+ trees in Cambridge. This includes over 60 different species, which is 46% of all the species in the city.


How much of Cambridge’s urban forest have we tracked?


Cambridge has a diverse urban forest with 131 different species of trees. Tree diversity is important in order to provide a greater range of benefits, and to prevent large-scale devastation by pests or diseases. By visiting the same street tree sites in 2008 and 2012, we found a slight increase in the diversity of tree species in Cambridge. The most common trees in Cambridge are Norway Maple, Honeylocust, Red Maple, Pin Oak, and Littleleaf Linden.

We are investigating the following research questions to improve knowledge about the dynamics and health of the urban forest.


How fast do the trees in Cambridge grow? What factors affect tree growth?


We have found that the average street tree in Cambridge grows 1/3 inches per year. We learned this by measuring a random and representative set of 684 trees in 2008 and again in 2012. 1/3 inches per year growth is within range of normal street tree growth rates throughout the United States, which varies between 1/10 and 4/5 inches per year.

We have also found that young trees grow more slowly than the average across all tree sizes. In 2013 Earthwatch volunteers measured data for young trees planted between 2008 and 2012. Average annual growth rates for the first two years after planting was about 1/10 inches in diameter, whereas average annual growth rates for trees 2−6 years after planting were close to 1/3 inches in diameter per year, similar to mature tree growth rates. The trunks of younger trees likely grow more slowly than larger trees because about 80% of their roots are removed in the process of transplanting them from the nursery to their new home. They need to put more resources into re-growing the roots they lost, so put less energy into growing trunk girth.

Tree growth also varies by species, suggesting that some may be better suited than others to grow in the harsh urban environment. Among the young trees, elms — including the native American elm — showed the fastest growth rates at 2/5 inches diameter per year. On the other hand, Serviceberry and Ginko showed the slowest growth rates, less than 1/10 inches per year.

The urban environment is not ideal for optimal tree growth, and we are identifying urban characteristics and how they affect growth of these trees. Our cities contain a lot of impervious surfaces (ex. paved roads, sidewalks, concrete), which reduces the amount of water that can reach tree roots, while also compressing and heating up the soil. As predicted, we have found that growth of many species decreases as impervious surface increases around the tree. Surprisingly, however, one species — Honeylocust — actually grows better with more impervious surfaces around it.

When comparing the growth rates of the young trees planted in different seasons, we found that trees planted in the spring and summer grew more than those planted in the fall. New England arborists already know the growing season for trees is limited to these three seasons, but these results may suggest an even more limited planting season.


What is the survival rate of urban trees?


The City of Cambridge plants 250-500 trees per year to both offset the annual loss of trees and increase the total population of street trees. Overall we found a high annual survival rate for street trees in Cambridge: 94.2% for young trees, and 96.4% for older trees.


  • Average street trees (across all ages) showed an annual mortality rate of 3.4% per year. This includes trees that were damaged by storms, pests, disease, or were removal for any other reason. The city does a good job replacing these trees, with over 40% replanting within 4 years.

  • Average young trees had a higher rate of mortality at 5.8%. However, some species fared better than others: less than 1% of Pin Oak trees died annually, whereas over 13% of London Planetree and Serviceberry died annually.

  • The environmental conditions a tree experiences within the first years after planting can have a large effect on its ability to survive. Among the young trees, annual growth rates were lowest for the trees planted in 2012, which are likely still recovering from transplant shock. Individuals planted in 2009 had slightly higher growth rates than trees planted in other years. We are currently looking at weather patterns to see if 2009 was a particularly mild year.

How much are our street trees worth?


Most people are familiar with the saying, "You can't put a value on nature." Well, actually we can. City trees provide ecosystem services, benefits to people such as air pollution reduction, storm water filtration, and climate change mitigation. The monetary benefits of these services can be quantified based on how much it would cost to provide the same services through engineered means. Over the next 100 years Cambridge's urban forest could provide as much as 280 million dollars in ecosystem services.


The potential for individual trees to provide ecosystems services depends on the size of the tree, and which species it is. As a tree matures and gets larger its value in terms of its ecosystem services grows. Maximizing an urban forest's ecosystem services means carefully selecting trees and maintaining those trees to a healthy mature size. This chart shows the variability in the annual ecosystem service benefits provided by the 5 most common tree species, which comes from differences in the number of trees, the sizes of each tree, and the characteristics of each species.