Download where will the tropical rainforests be in 100 years?

The environment where tropical
rainforests occur is changing fast.
Their future, and possibly ours,
depends on how they will respond
where will the
tropical rainforests
be in 100 years?
W
Text and photos by Kenneth J. Feeley, Ph.D.
here tropical rainforests occur now is determined largely by
climate. Indeed, as the name itself makes clear, tropical
rainforests are found in the tropics (where it is hot) and
where it rains nearly all the time. The mean temperature of
Amazonian rainforests, for example, is more than 80° F (26.7°C) with yearly
rainfall averaging more than 80 inches (two meters). As a comparison, the
mean temperature in Miami is 75°F and annual rainfall is only 60 inches—
which is why Fairchild’s rainforest exhibit requires supplemental irrigation.
A s we look to the future, the question we need to ask is: If the thousands of
plant species that make up tropical rainforests can only grow where the
climate is “right,” what happens when the climate changes? One disturbing
possibility is that many species may simply go extinct as rainforests “die
back” and are replaced by savanna-like habitats supporting many fewer
species. In fact, some models, conducted by myself and others, have
predicted that the changes in climate predicted to occur during this century
alone may lead to the extinction of as many as 50 percent of all tropical
plant and animal species. But there may be a way out. Namely, rainforest
species may tolerate the coming changes to climate or, alternatively, they
may shift their distributions to follow the movement of favorable conditions.
Is it really possible that rainforest plant species may simply tolerate climate
change? Temperatures in tropical forests are generally predicted to increase
by an average of 7 to 8°F (4°C) over the next 100 years with many scientists
now forecasting even greater warming. Rainfall has a much stronger regional
signal, depending on many factors such as wind, topography and land
cover. As a consequence, changes in rainfall are much harder to predict than
changes in temperature, but the majority of the global climate models that
scientists use to predict climate change suggest that much of the tropics is
going to become dryer. Perhaps even more important, these models also
predict that many parts of the tropics are going to become much more
seasonal—with rainfall concentrated in distinct rainy seasons separated by
longer and stronger dry seasons during which there is little or no precipitation.
Higher temperatures coupled with less rainfall and drier dry seasons means
that many plant species will become increasingly stressed for water.
30 THE TROPICAL GARDEN
Research studies that my associates and I have conducted, as well those of
fellow FIU professor and Fairchild associate Steven Oberbauer and his team,
have shown that tropical rainforest species can be extremely sensitive to just
these sorts of climatic changes and that many trees grow markedly slower
when the temperature is warmer and especially when there is less rainfall
(Perhaps a bit surprisingly, the negative effects of less rainfall hold true even
in very wet forests where you might expect that plants would be at little risk
of water stress.)
The long-term implications of slower tree growth are still a topic of active
investigation but these results strongly imply that tropical plant species are
currently growing under their optimum climatic conditions and that any
change to the thermostat can have potentially negative consequences (see
the Fall 2009 issue of The Tropical Garden for more information on this
topic). This will be especially true given the extremely rapid pace at which
future climate change is predicted to occur, leaving little room for adaptation
or evolution in long-lived species such as trees.
As the old expression goes, “if you can’t handle the heat, get out of the
kitchen.” If tropical rainforest plant species are incapable of tolerating future
changes in climate, maybe they can change where they live on the planet—
for example, moving their distributions to follow the shifting location of their
optimal temperatures.
As temperatures get warmer, plants could feasibly remain within their
optimal temperatures by shifting their ranges to higher elevations or toward
the poles (north or south depending on which side of the equator the species
currently resides on) in order to occupy areas that are currently too cold for
them (Just to be clear, even though the word “migration” is sometimes used
in reference to this idea, it is distinct from what we classically think of as a
migration in that here I am not referring to individuals moving, but rather to
entire species gradually changing their distributions by differential
recruitment and mortality in the colder and hotter portions of their ranges.)
Supporting the idea that some plant species may change their distributions to
track favorable climate, scientists examining fossil pollen records and other
paleological evidence of where topical rainforests occurred in the past have
found indications that many species did in fact shift their distributions in
response to past climate change, for example moving down the Andes when
temperatures got colder and moving upslope when temperatures got warmer.
So plants have done it in the past, but can they do it again? Unfortunately,
the answer is most likely “No.” There are two things that will make it
difficult for plant species to shift their distributions and follow the climate as
they did in the past. The first is the nature of climate change itself. The
anthropogentically driven climate change that we are experiencing now is
occurring much faster than anything in the past. It may simply be impossible
for species to shift their distributions fast enough to keep up.
At the end of the last ice age temperatures in the tropics rose by
approximately 0.02°F per decade. Given the change in temperature with
elevation (approximately 3°F colder per 1,000 feet of elevation gain), this
would have required Amazonian plant species to shift their distribution up the
Andes at the rate of just six feet per decade. Since the 1970s, temperatures in
the tropics have increased by almost 2°F, or approximately 0.5 degrees per
decade—more than 30 times as fast as in the past. And changes in
temperature over the next 100 years are predicted to be even faster,
exceeding 0.7°F per decade or more than 40 times faster than in the past.
Inside the Peruvian cloudforest.
www.fairchildgarden.org 31
Can tropical plant species shift their distributions upslope the more than 20
feet per year that will be required to just keep pace with warming? Working
with an international team of biologists calling themselves the Andes
Biodiversity and Ecosystem Research Group or ABERG, I conducted a series
of studies tracking changes in the distributions of tree species growing in
Manu National Park along the eastern slopes of the Peruvian Andes down
into the Amazon. What I found is that the species there are in fact shifting
their distributions upslope as we predicted, but only at the rate of about 6 to
10 feet per year. This is simply not fast enough to keep pace with current or
future climate change. If this pace continues in the future we estimate that
the amount of area over which many of these species occur will be more
than cut in half as they lose habitat area along the hotter, lower portions of
their ranges and fail to make up for it along the cooler upper portions.
Of course the problem is even worse for the vast majority of tropical plant
species that are unfortunate enough not to grow close to the Andes or other
tropical mountains. In the tropics, temperature changes vary little with
latitude, so to keep pace with a 7 degree F temperature increase, lowland
species would need to move almost 1,000 miles to the north or south—the
approximate distance from Miami to Washington, D.C. And this is saying
nothing about the thousands of species confined to tropical islands such as
in the Caribbean or the South Pacific where moving is simply not an option.
The second factor that may keep species from shifting their distributions
and tracking favorable climates is that we humans have done a lot more
to the environment than just change the climate. We have already cut
down more than half of all the tropical rainforests and replaced them with
farms, soybean and oil palm plantations, and cattle pastures. And we
continue to cut down over 2.5 million acres of rainforest each and every
year. As a result, any areas that will be climatically suitable for a species
in the future may be largely or completely “unavailable” since we are
already using them. Or if there are any areas that will be climatically
suitable and available, we may have put up so many barriers in the form of roads, clearings or other
inhospitable habitats, that there will be no way for species to get there by natural gradual range shifts.
Fast climate change and loss of habitat due to human land use will make things difficult, but if species are
able to shift their distributions as required then everything will be fine and species won’t go extinct, right?
Unfortunately the answer is again “No.” Studies that I have conducted looking at the possible implications
of the range shifts predicted for thousands of Amazonian and Andean plant species have shown that even if
species are able to perfectly track their preferred climatic conditions, they will still almost all experience a large
net decrease in the amount of habitat available to them simply because there is less land at the colder high
elevations or high latitudes than in the expansive equatorial Amazon basin. (To visualize this, it may help to
think of mountains as pyramids with species moving up from the wide base to the peak or to think about
species from the large Amazon basin moving north into the Central American isthmus or down into the
cone of South America.) As they shift their distributions toward the cold and habitat area decreases, many
species will likely go extinct. That said, shifting distribution and facing a risk of extinction is clearly still a better
option than staying put and facing near-certain extinction.
So what can be done? Is there any way to save our tropical rainforests in the face of climate change? The
first step is clearly to slow the pace of climate change. I say “slow” and not “stop” because we have
unfortunately already reached a point where halting climate change is no longer a realistic option. Our
climate is already changing and we are already committed to more change. In other words, even if
everyone everywhere cut their carbon emissions to zero tomorrow, temperatures would continue to rise
just due to the momentum of the system and built-in time lags. If you paid attention to the Copenhagen
climate summit last month you may have noticed that nobody talked about stopping climate change but
rather about ways to limit future warming to “just” 3.6°F (2°C). This warming would still necessitate plant
species moving upslope more than 1000 feet in elevation or almost 500 miles to the north or south.
Even achieving the modest goal of just 3.6°F warming will be a daunting challenge, necessitating that we
all do our part. We all need to reduce our personal contribution to global warming, be it through changing
32 THE TROPICAL GARDEN
ABOVE: Looking
down the Alto
Madre de Dios
River in Peru.
our driving patterns, reducing our energy consumption or eating lower on the food chain (meat and cattle
production is a leading cause of both greenhouse gas emissions and deforestation).
Many people have grown discouraged looking for the silver bullet or the one big fix that will get us out of
this climate change mess. Assuming that a viable geo-engineering strategy won’t be implemented any time
soon, the fix is more realistically going to come in the form of lots of small, and at times seemingly
insignificant, changes. Of course, small changes only work if lots of people are participating, so beyond just
changing our own behaviors we must also all become advocates and recruit others (including corporations
and policy makers) to do their part.
But given that the goal of these lifestyle changes is to slow and not halt climate change, is there anything
else that can be done to prevent the loss of species due to the warming we are already committed to? The
answer is that we must work to enable species to respond to future climate change. In other words, we
should strive to not make things harder for the plants than they already are.
BELOW: The
author standing
above treeline
looking down
into a valley in
the high Peruvian
Andes that has
been largely
converted to
agriculture.
Most important, steps must be taken to limit habitat loss and preserve as much intact tropical forest as
possible, for example by slowing deforestation and increasing the amount of protected area and/or
increasing the strength of enforcement for areas already protected by law. Every time an acre of tropical
rainforest is cut down we push species several steps closer to extinction; we reduce the amount of habitat
available to them now; we reduce how much habitat will be available in the future; we add extra barriers
to the movement of species; and we lose genetic diversity that may hold the key to species adapting to
climate change. What’s more, every time we cut down rainforest we actually contribute to the problem and
cause even more carbon dioxide to be emitted into our atmosphere, causing even more warming. For each
acre of forest cut, more than 50 tons of carbon is released. In fact, if you add up the millions of acres of
tropical forest cut down each year, it is responsible for approximately 20 percent of all anthropogenic
carbon dioxide emissions—more than all our trains, planes and automobiles combined.
A relatively new conservation mechanism that appears to hold great promise to help reduce deforestation
and simultaneously fight climate change is REDD, or Reduced Emissions from Deforestation and forest
Degradation. If put into practice, REDD would allow for greenhouse gas emitters in developed countries
(such as the United States), to offset some of their emissions by paying
developing countries (including nearly all tropical countries) to not cut
down their forests. There are still many kinks to be worked out (for
example, how would funds be distributed and how can it be guaranteed
that deforestation doesn’t just move, or “leak,” to other areas resulting in
no net benefit), but if done right REDD could be a major step forward and
help to reduce climate change and save rainforest diversity.
Another tool in the fight to save rainforest diversity is “assisted migration,”
or “active translocation,” of species. In assisted migration, scientists would
combine knowledge of species’ growing requirements with global climate
models to determine where species’ habitats will likely be in the future.
The species would then be actively transplanted into the new habitats,
thereby facilitating the range shifts that will be required of them in the
future and helping them to bypass the many barriers that could hinder
natural range shifts. Assisted migration remains extremely controversial,
since it theoretically has the potential to intentionally introduce invasive
exotic species and push other species closer to extinction.
As this debate progresses, botanic gardens such as Fairchild are playing
an important leadership role, helping to guide the choice of candidate
species to migrate, developing the safest possible protocols for
transplanting species, and potentially even serving as a sort of “halfway
house” to help plant species get established in their new ranges while at
the same time monitoring against invasions.
The sad truth is that, no matter what we do in the future, our past actions
have already condemned some species to extinction. But through the
collective actions of scientists, citizens, institutions and policy makers, we
hope we can keep extinctions to a minimum and preserve some of the
richness of tropical rainforests for future generations to enjoy.
www.fairchildgarden.org 33