and the incidence of cardiovascular disease in humans.
Donovan was already aware of some seminal European studies
looking at human stress, illnesses and loosely defined “green space”
in cities. And there were other studies, including Richard Mitchell’s
work in Scotland, showing lower mortality rates near urban parks.
While Mitchell’s research revealed a big health boost to poor people,
Donovan’s work showed the sudden tree blight had a bigger impact on
wealthier neighborhoods, probably because those had the most trees
to lose. Overall, the counties that were hit by the borer suffered
15,000 additional deaths from cardiovascular disease and 6,000 more
from lower respiratory disease. Those figures represent a sizable 10
percent increase in expected mortality. It’s hard to say whether the
deaths were caused by worsened air quality or changes in stress
brought on by not having the tall, green, comforting trees to look at,
or both. If trees can move us so powerfully in their metaphoric reach,
as the veterans on the Salmon felt, then perhaps looking at sick or
dead trees is in itself stressful.
Toronto takes its 10 million trees very seriously, valuing its urban
forest at $7 billion. A recent study there showed the higher a
neighborhood’s tree density, the lower the incidence of heart and
metabolic disease. Putting it into raw economic perspective, the
health boost in those living on blocks with about 11 more trees than
average was equivalent to a $20,000 gain in median income. Lucky
residents were rich in trees.
Every tree helps. As the founding nature/brain researcher Rachel
Kaplan told me, “nature doesn’t have to be pervasive. One tree is an
awful lot better than no tree.” But more trees are best. The city of
Washington, D.C., and partner nonprofits have been trying to plant at
least 8,600 trees a year in an effort to increase the street canopy to 40
percent in the next two decades. New York City recently completed a
wildly ambitious campaign to plant a million trees, and Los Angeles,
Shanghai, Denver and Dubai are in the middle of similar ones.