Scientists
find connection between deforestation and drought in the Rockies and globally
BY
LANCE OLSEN
Alliance
for Wild Rockies
We
often think of forests as merely the passive beneficiaries of rain or snow- it
falls on them, and they grow because of it. This is one of the most basic
truths of the relationships linking forest and precipitation, and it applies
to the forests of the Northern Rockies as much as it applies to any part of
the planet.
But
that's not the whole story of forest, water, and climate. In the Northern
Rockies as much as in any other region, forests return a lot of the water to
the atmosphere from which it fell. At that point, the moving air masses we
know as prevailing winds will carry this recycled water away, perhaps only to
the next mountain, but perhaps to distant points downwind.
This
part of the story begins where the roots of trees beneath the forest floor
catch water fallen from the skies above. In his 1970 Scientific American article
on the water cycle, the British physicist H. L. Penman wrote, "Here, at
the point of water uptake by the roots of plants, begins the problem with
respect to water in the biosphere that makes all other water problems seem
trifling."
Why
is this such a big issue? Because a forest's capture of water by the roots of
its trees - and other plant life - pours a huge river of water back to the
atmosphere.
In
his book Fresh Water, E. C. Pielou says, "Vegetation pumps an
enormous amount of water from the soil into the air; few people realize how
much, because the whole process is invisible. For example, a single hectare of
Douglas-fir forest spews out about 59 tons of water vapor in the course of a
sunny summer day. "This process is called transpiration, and has been
well known ever since that word entered the vocabulary of botany.
Now,
everyone will recognize at once that cutting one tree will not have much
affect on the volume of water returned to the atmosphere. But large-scale
cutting, which we've seen for a quarter century across western North America,
has far-reaching implications in the realm of climate and climate change. Why?
Simply because once forests pump rain (or melted snow) back to the atmosphere
prevailing winds then carry it downwind where it can fall again as rain at
nearby points or thousands of miles away.
The
clear implication is that logging snips off valuable pumps that help deliver
water to places located far downwind. Scientists estimate that up to
two-thirds of the precipitation falling on the continental land masses is
delivered to inland destinations by vegetation. And the result of large
scale pump-snipping has a name: drought. Since the 1980s, scientists
have been finding this deforestation-drought scenario confirmed again and
again.
For
example, after reviewing evidence already available from research in the Sahel
and Amazon regions by 1985, WorldWatch founder Lester Brown could say,
"Knowing what we do about the extent of deforestation over the past
generation and about the way the hydrologic cycle works, it would be
surprising if climate were not changing." Where forest cover has been
removed, drought had shown up at sites downwind of it. And, by 1985, the
Forest Service said, "this is an issue our agency should consider."
Brown's
plain spoken summary would be echoed across the world. Just last year, in
describing the impact of upwind deforestation on a downwind nature preserve
in Costa Rica, University of Alabama-Huntsville researcher Robert Lawton would
say, "This isn't a dodgeable effect. It's a straight hand off." But
the Forest Service has still not considered it.
The
forests of the Northern Rockies play two roles in this same unsurprising and
undodgeable process. They are watered by upwind coastal forests stretching
from California north to Alaska, and, in turn, they pump out water that
prevailing air currents carry to the downwind plains - and points beyond.
Although drought is certainly normal to our region, the past four decades of
scientific inquiry have turned up evidence aplenty that tampering with forests
can worsen normal drought and may even create it where normal fluctuations of
wet and dry do not.
Most
contemporary reports of links from forest to climate emphasize the role of
carbon dioxide as a greenhouse gas, but skip mention of forests' powerful
contribution within the hydrologic cycle. Like other greenhouse gasses, carbon
dioxide is and will be an important influence on forests, and will be
influenced by them. As greenhouse-forcing of climate change proceeds, it will
force many
forest species to flee to higher altitudes and higher latitudes; at least some
tree species will suffer a die-off at the southern and low elevation margins
of their current range. These forest deaths will in turn free more carbon
dioxide into the atmosphere, where it will then exert more pressure on the
surviving forests.
These
and other consequences of greenhouse-forcing make it impossible for the
conservation community to ignore the impact of climate change brought by
carbon dioxide and other greenhouse gasses. But we also can't ignore the
climate change brought by what scientists call land-use forcing of climate
change, and land use that includes the cutting of forest takes us back to the
very basics of how this planet's climate works.
For example, in a 1992 article for the Journal of Geophysical Research, Eric F. Wood and co-authors reported that, "The redistribution of solar energy over the globe is central to studies of climate and climate change. Water plays a fundamental role in this redistribution through the energy associated with evapotranspiration - the transport of atmospheric water vapor, and precipitation." In forcing changes on forests, and thereby on the fundamental climatic role of the hydrologic cycle, we have created a situation where we now have two kinds of climate change to reckon with, and not just one.