What initially prompted your research on forests
and nutrient cycling?
I started with this innate fascination for natural systems. I've always
been attracted to the question of how the natural world works. I then
moved toward the kind of questions that are more are important for
understanding how humans are affecting natural systems.
That's a path toward studying complex ecological systems, and then you
almost have to choose whether to have your feet wet or dry. By that I
mean, I started out studying water systems and then slowly moved up
onto dry land.
Then if you're interested in climate change, as I was, you very quickly
figure out that tropical forests, like in the Amazon or Central
America, are especially important in the earth's climate system. Plus,
they're just terrifically fascinating, complex, diverse, and they're
highly sensitive to human activities.
Since the 1990s, you've specialized in
unpolluted tropical and temperate forests. What's the importance
of these unpolluted forests?
We started off studying temperate forests, trying to understand the
frame of reference for a natural temperate forest. Most studies before
us had looked at polluted temperate forests, and they'd describe
essential processes—cycling of nitrogen or other
nutrients—and the observation was always that these ecosystems
were highly perturbed by human disturbance. European forests, North
American forests, the answer was always human disturbance.
But nobody had a frame of reference for a temperate forest that hadn't
been disturbed by humans. So we had the idea to travel back in time,
before the industrial revolution. We did this by going to the most
unpolluted temperate forest we could find in the world, in Southern
We tried to characterize how such truly natural forests work, to derive
a kind of baseline for natural forests. And our results changed how
people think of the nutrient cycles that sustain these forests, and how
pre-industrial conditions are understood and characterized in climate
"We want to reduce our understanding to
mathematical expressions that can then go into these
models and allow us to better forecast how nutrients
control the carbon cycle."
Once we did this with temperate forests, we decided to compare them to
tropical forests, which appear to work completely differently. And so
we began studying tropical forests around the world.
When did you publish the work on temperate
forests in southern Chile, and how was it received?
That was in 1995 (Hedin LO, Armesto JJ, Johnson AH, "Patterns of
nutrient loss from unpolluted, old-growth temperate
forests—evaluation of biogeochemical theory," Ecology
76: 493-509, March 1995), and received widespread attention
(including an award from the Ecological Society of America). The work
showed that we had not previously understood how the unpolluted
nitrogen cycle works in these natural forests.
We spent about five years studying these pristine, undisturbed forests,
and we ended up learning entirely unexpected things about the nitrogen
cycle. For example, our findings helped resolve some long-standing
paradoxes and problems for which we hadn't had solutions.
One paradox was the question of why so many forests in the world are
limited by nitrogen. We found that nitrogen limitation can be generated
in a quite unexpected way—by the slow leak of organic forms of
nitrogen from the soil.
Could you fill us in on the importance of the
nitrogen cycle in forests, since it's so critical to your work and
to this concept of nutrient cycle?
Nitrogen is the key nutrient that controls the machinery of
photosynthesis. That's why you use nitrogen to fertilize the tomatoes
you grow in your garden. Nitrogen allows the photosynthetic machinery
to be built and to function. Nitrogen cycles in natural ecosystems; it
goes from the plant to the soil and back up to the plant. It can be
plentiful or it can be extremely scarce, depending on the local soil
conditions. So nitrogen is perhaps the most important nutrient
controlling plant growth in the world.
If you want to understand how plants work, how forests work in the
climate system, you need to understand the nitrogen cycle—how an
abundant or scarce nitrogen defines exactly how the forests respond to
the greenhouse gas carbon dioxide in the atmosphere, whether it can be
taken up and stored, or not. Such knowledge defines how we think of
these systems as productive or unproductive, and how to manage them in
the face of human perturbations such as climate change.
Do human pollutants affect the availability of
That's the point. In disturbed parts of the world, in the parts with a
lot of humans and a lot of pollutants, there are large amounts of
nitrogen pollutants. That's why we wanted to get away from them in our
work on unpolluted Chilean forests.
In our 2002 Nature paper, "Nitrogen loss from unpolluted South
American forests mainly via dissolved organic compounds (Perakis SS,
Hedin LO, 415: 416-9, 24 January 2002), we pointed out that
almost all the studies that had been done previously to determine how
human pollutants worked in these systems didn't show how the systems
worked naturally. And because of that, they didn't have a baseline
against which they could compare and judge the extent of human
Did you expect that paper to be so highly
I suspected it might. The original paper in 1995 on temperate forests
was highly cited and this one put the earlier findings in a broader
How has your research evolved in the years
since that 2002 Nature paper?
Since then we've realized that the nitrogen and phosphorous cycles in
tropical forests behave very differently from what the traditional
models suggest. The traditional models come from temperate systems, and
we have to develop a whole new set of rules and theories for tropical
environments. And that's been very exciting. We have some major
findings on that.
Can you tell us about those?
I may be complicating this too much, but let me try to explain. It's
well known that tropical forests are major sinks for carbon dioxide,
which is, of course, a greenhouse gas. There's no question that the
nitrogen cycle will limit how much carbon dioxide can be taken up in
tropical forests, and in particular a part of the nitrogen cycle called
If plants run out of nitrogen, they can fix it from the atmosphere.
It's a fantastic process, a symbiosis between plants and microbes that
are housed within nodules that are attached to the plant root that can
bring in lots of fertilizer nitrogen naturally. We are beginning to
think that this process is essential for allowing tropical forests
store carbon over time.
What we found in tropical forests that we study in Panama was a big
surprise: the factor that controls this process of nitrogen fixation is
not phosphorous, a major nutrient, as everyone had thought, but instead
a trace metal, molybdenum. Molybdenum is a part of the enzyme
nitrogenase, which is responsible for nitrogen fixation, and it depends
on molybdenum. It has molybdenum atoms in its active site. And what we
found is that molybdenum is the factor that controls whether fixation
can happen or not in these soils.
"...nitrogen is perhaps the most important nutrient
controlling plant growth in the world."
It was kind of breathtaking. That's the first time that was ever shown
in highly productive tropical forests. We published that last year in
Nature Geoscience. The first author, Alexander Barron, was a
student of mine (Barron AR, et al., "Molybdenum limitation of
asymbiotic nitrogen fixation in tropical forest soils," 2: 42-5,
Were there any unexpected or serendipitous
events in your research—moments when your findings were
determined as much by luck as anything else?
Many times. I'll give you two examples. The first was the work in
Chile. As a graduate student I realized that the work on nutrient
cycles in temperate forests was almost entirely done in polluted areas
of the northern hemisphere, but if we wanted to actually learn about
natural systems and how those are organized, we would have to do it in
the far southern hemisphere. As we said, those were the most unpolluted
in the world.
Within a week of thinking about that, I had a South American faculty
member walk into my office, and he said, "Hi, I'm Juan Armesto and I
study forests in Southern Chile." We collaborated for 10 years after
Another example is on the molybdenum finding on nitrogen fixation. As
it turns out somebody in Panama was running an eight-year experiment
testing different kinds of fertilizers to see how the forest responded
to them. We were fortunate to be able to take advantage of that
experiment. But we were also fortunate because one of the experimental
treatments was a mixture of trace metals, which is not usually done.
People tend to add nitrogen and phosphorous and that's it, but they'd
also added trace metals.
And the curious thing—this is really beautiful—was that
when phosphorous fertilizer was added to this forest, we still found an
increase in nitrogen fixation in those plots. That went with the
traditional model that phosphorous is the primary governing factor on
fixation in tropical soils. But then, when we looked at the trace metal
treatment, we found the same response. That was very curious. You
wouldn't expect the same response from both phosphorous and trace
Here's the serendipity. We realized that molybdenum was the most likely
trace metal that influenced fixation, and in phosphorous fertilizer
there's often contamination by trace metals. Guess what that was?
Molybdenum, at about one ten-thousandth the concentration of the
phosphorous itself. We went back and did the experiment again but this
time using super-clean, pure phosphorous, and we added that to the
forest and we got no response. That was the key experiment that pointed
to the sole influence by molybdenum.
That means that in this system, and we're still working to generalize
this, but in this system, it was molybdenum that was controlling the
fixation. That was serendipity, but we had to have an open mind to
really appreciate it.
Did you find that some researchers were
reluctant to accept such a radical finding?
First of all there's certainly been a lot of interest from all over the
world. We have gotten emails from China, Africa, Brazil, and a lot of
other tropical countries. We have gotten supportive comments about how
beautiful it was to be able to separate out the effects of molybdenum
verses phosphorous. Others maybe misunderstood our findings by thinking
that we were arguing that phosphorous is never important, which we did
Our current view is that in some environments maybe it's molybdenum and
in some maybe it's phosphorous. It just points to how poorly we know
tropical forests even though they're really central to our climate
Does this uncertainty about tropical forests
affect the modeling of climate systems worldwide?
This becomes a very important question. There are things we know about
tropical forests and things we don't. We know they're an important sink
for carbon dioxide, but we don't know how nutrients will affect the
ability of forests in the future to continue taking up carbon dioxide.
Will this carbon sink persist over time or will it become saturated?
That's where these nutrients come in and they're absolutely critical.
It's a fundamental challenge.
There's a very complex chain of reasoning for why we care about these
nutrients, and when we put them into state-of-the-art models, the
nutrients really make a difference. Yet there are so many
unknowns—how the nutrients affect the forests, how the forests
store the carbon, and how the storage of carbon affects the climate
If you increase molybdenum concentration
further will it continue to increase nitrogen fixation?
That's one question that came up a lot after we published these
findings. And the answer is, we don't know that and I hesitate to go
there. I'd just be speculating. Right now we're doing some experiments
that will get at that, or partly get at that.
How would you describe the ultimate goal of
Ultimately what want to do is build a set of rules for how nutrients
govern these highly complex ecosystems, and build them into what's
called earth system models. We want to reduce our understanding to
mathematical expressions that can then go into these models and allow
us to better forecast how nutrients control the carbon cycle.
The carbon cycle on land, which is what we're studying, is the most
poorly known part of the global carbon cycle. We're behind the people
who study it in oceans in many ways.
What would you like to convey to the general
public about your work?
That with global climate change and the human influence on the
environment, we now live in a time when we have to learn to manage our
environment. In doing so, we have to understand how the environment
works as a complex system. We can't take shortcuts. Instead we really
need to understand how it works at a deep, fundamental level. That's
what we're trying to do.
Lars. O. Hedin, Ph.D.
Department of Ecology and Evolutionary Biology
Princeton Environmental Institute
Princeton, NJ, USA
KEYWORDS: TROPICAL FORESTS, TEMPERATE FORESTS, NUTRIENT CYCLING, NITROGEN
CYCLE, NITROGEN, CHILE, POLLUTED FORESTS, PRISTINE FORESTS, FERTILIZER,
PHOTOSYNTHESIS, SOIL, CARBON DIOXIDE, PHOSPHOROUS, NITROGEN FIXATION,
MOLYBDENUM, CARBON SINKS, EARTH SYSTEM MODELS, GLOBAL CARBON CYCLE.