by John Pitkin, Chair, Greater Boston Group Executive Committee
Late last year the City of Cambridge reported that the city’s tree canopy shrank by 6.7 percent between 2009 and 2014. This finding was based on an analysis of an aerial LiDAR survey by researchers at the University of Vermont and documented in a report. This loss of tree canopy is alarming and should be of concern to everyone who lives in Greater Boston or hopes to in the future.
Cambridge’s green canopy is created by an urban forest of tens of thousands of trees growing along streets, in public parks and reservations, in private yards and on university and school campuses; and it spreads over roughly 30 percent of the total land area.
Cambridge’s has earned Tree City USA awards for 23 years running. Yet in spite of its efforts to maintain and care for trees, there was a net loss of about a hundred acres of tree cover over the five years of the study.
Trees die for reasons as varied as people do. Some die of the infirmities of advanced age, diseases, drought and other more or less natural causes. Others die from the effects of toxins such as road salt and methane leaks, pavement and dehydration, accidental and intentional injuries, poor care and simply being cut down for one reason or another, the result of human actions. There seems is no single cause for the decline, but in short it is a challenge for trees to survive in urban conditions.
And the loss of trees almost certainly did not end in 2014, the date of the City’s most recent aerial survey. It is obvious that the dry summers since 2014 have been hard on trees at all stages of growth, both here in Cambridge and across the region. There are strong indications that the impacts of climate change will continue to be hard on trees throughout the metropolitan region.
According to Cambridge’s landmark 2015 Climate Change Vulnerability Assessment, the annual number of 90 degree-plus days in the region is very likely to triple by the end of the next decade. The assessment identifies increased heat and flooding as the most significant near-term impacts of climate change. Heat stress on human health will become “much more severe”.
Increasing heat also imperils trees, with the greatest impacts on certain species including types of maples, lindens and pines. If these trees can survive and continue to grow, they can help us cope and live with a warming climate in the decades ahead.
It is no exaggeration that trees are fundamental to our wellbeing. They improve water and air quality, reduce flooding, cool air and pavement, and save energy needed to cool buildings. They contribute to better health, higher property values, habitat for birds, and natural beauty.
We are just one of many species that depend on trees. Trees themselves depend on other trees. They rely on each other to create the micro-climate, and support system needed to thrive. The loss of canopy itself worsens conditions for the surviving trees and raises the specter of a vicious cycle of worsening conditions and smaller, more spindly trees leading to a tree desert.
While this grim scenario may seem like a remote possibility, it bears remembering that vast areas of the earth were once covered by large forests, including much of the Mediterranean basin, most of the Middle East, Cyprus, Madeira and Iceland.
The simple truth is that trees are our silent allies in confronting a changing climate and that we need them as much as they need us.
Preserving and maintaining healthy trees in our metropolitan area is worth every effort. Trees can fend for themselves very well in natural forests. But in the urban landscapes we have created they need help from us. The sustained loss of tree cover in Cambridge is a signal that they urgently need more.