Princeton University - Council of Science & Technology

POPE PRIZE

Essay 1
The Unknowns of the Tropical Forest
Jon W. Benner

Essay 2
A Few Words about Spin one Half
Thomas S. Jackson

Essay 3
How Do You Grow Organic?
Ben Jones

Essays 4, 5 & 6
Competing against the Body
Irrationally Rational
Beyond the Genome
Laura A. Shackelton

The Unknowns of the Tropical Forest

I'd never thought of temperate deciduous forests as sterile before spending a semester in Panama. On my return from the Panamanian tropics last May, I went for a walk through the Institute Woods in Princeton, New Jersey. My walk felt like a stroll through the recovery ward of a hospital. The bright fluorescent lights were replaced by the white sunlight washing the forest floor. The evenly spaced patient rooms took the form of orderly, regularly arrayed yellow poplars, and the hushed voices of the patients' visitors were the polite chirpings of birds, anxious not to interrupt fellow chirpers. Above all there was a sense of space, the feeling that you could close your eyes, spread out your arms, and sprint through the forest without so much as being scraped by an offending branch.

Walking through a lowland forest in Panama, on the other hand, feels more like pushing through Grand Central Station with four suitcases and three small children in tow on the day before Thanksgiving. It's not so much that the understory is dense - there are thorny bushes, and sometimes thick undergrowth - but more that everything in the forest is conspiring to occupy your space and ensure you don't appropriate their resources. Everything is fighting, tooth and claw, to eke out an existence in one of the most species-rich ecosystems on the planet. You can feel the activity and the vibrance as you push through the forest. You see a marauding band of Eciton burchelli, the army ant scourge of Neotropical forests, swarming through the leaf litter and coating the ground with a carpet of black bodies, preceded by frantically fleeing, leaping, flying, running spiders and grasshoppers and any other creature unfortunate enough to be in the path of the raiding column. Eight species of insectivorous bird flit around the front of the swarm, picking off unlucky invertebrates who in their rush to escape the ants are heedless of attack from the air. Clouds of parasitic flies swirl in the air above the swarm front, laying eggs on any spider that pauses for long enough to serve as a landing pad. The eggs will hatch into larvae that will slowly consume their host alive until they hatch into adult flies. Life is hard in the forest, and any weakness is exploited. The number of stings received on a short walk through the forest offers an unnecessary reminder that you are not a welcome guest. The forest in Panama is a tangled skein of tree branches, lianas, sunflecks and hot, heavy air, inhabited by millions of species all jostling for space and a life long enough to pass on their genes. Yet, for all of the awe we humans feel walking through these forests, our knowledge of how these forests are assembled, grow, and change is surprisingly superficial. To almost any question you can ask about the Panamanian tropical forest, the answer would have to be "No one really knows."

I'm riding in the back seat of a Toyota Hilux pickup truck on a pot-holed section of the Pan-American highway with Monica, a fellow undergraduate at Princeton. We're halfway through a ten-hour drive from Panama City to the province of Chiriqui, where we'll stay in the cloud forests of the continental divide for two weeks. In the driver's seat of the truck is Henry, a retired military doctor from North Carolina and one of the world's foremost experts on weevils, the largest group of organisms in the order Coleoptera, the beetles. Marianne, a 70-year-old American expatriate and expert in Panamanian forest ecology, is in the passenger seat. It's about 90 degrees Fahrenheit outside and the landscape is desolate and shimmering with heat waves - baked brown hills with dry grass, a few scrubby shrubs, and a brown pall of smoke from the ubiquitous grass fires. Panamanians set fire to their property in the dry season to clear scrubby growth, but the fires pay little heed to property boundaries. The desert-like landscape we're jolting through is not "natural", although it's been like this for more than 8,000 years, according to pollen records from nearby lake sediments. Just like today's residents, when the first humans migrated across the Bering Strait and eventually reached Panama a few thousand years later, they set fire to the landscape - perhaps because open land provided better game hunting, or perhaps for chiefs to survey their domain and spot approaching enemies from afar. Whatever the reason, the seasonal dry forest could not survive the high fire frequency, and grasses and shrubs permanently replaced the trees.

Tires squeal and the truck lurches off of the road, coming to an abrupt stop in the gravel under a tree. The passenger door flies open and Marianne springs onto the hood and reaches up to grab a low-hanging branch. The leaves are compound - many small leaflets arrayed along a central midrib compose one leaf. After a quick inspection of the pattern of veins in the leaflets, Marianne proclaims in a triumphant voice: "A primitive mimosoid legume!" Henry is silent, perhaps awestruck. Monica and I must look in need of explanation.

"It's in the legume family," Marianne says. "A nitrogen fixer." She clarifies, "No one really knows if legumes are actually fixing nitrogen."

No one really knows? This statement surprises me. Why hadn't people figured this out already?

"Researchers haven't paid much attention to tropical forests," Marianne says. "Most ecologists live in northern industrialized countries, and it's not that easy or convenient to study tropical forests. We really know very little about the tropics."

The case of the legumes is a prime example of the gaps in our understanding of tropical forests. The legumes comprise a family of plants, Fabaceae (also called Leguminosae), which evolved in the tropics approximately 200 million years ago. In the tropics, legumes are a highly successful family of trees. Legumes outnumber trees from any other family in many lowland Neotropical forests, and in the Brazilian Amazon Fabaceae is the most diverse of any plant family. However, leguminous trees have been minimally successful at spreading north and south into the temperate zone The particular tree that Marianne found on the side of the Pan-American highway is in the sub-family Mimosoidae, one of three sub-families in Fabaceae - the other two are Caesalpinioideae and Papilioniodeae. Like all plants, legumes need nitrogen to grow. Nitrogen is a critical component of amino acids, which are the building blocks for proteins. The enzymes that regulate all plant physiological activities, including the enzymes necessary for photosynthesis, are made of proteins. Usually, plants obtain all of their nitrogen from ammonium (NH₄⁺) and nitrate (NO₃^-) in the soil; these forms of nitrogen are called "fixed" nitrogen. In many ecosystems worldwide, plant growth is limited by low levels of fixed nitrogen in the soil - if these "nitrogen-limited" plants could obtain more fixed nitrogen, they would be able to construct more of the enzymatic machinery necessary to photosynthesize and construct sugars to meet their energy requirements.

Leguminous trees have a unique characteristic: they can "fix" nitrogen gas, N2, from the atmosphere into ammonium, which they can use for growth. Nitrogen fixation is a highly unorthodox strategy across all life forms because it presents unique challenges: only one type of enzyme - nitrogenase - has been found that can break the triple covalent bond between the two nitrogen atoms in N₂ gas. Nitrogenase is denatured in the presence of oxygen, so nitrogen fixation must take place in oxygen-free pockets - in other words, the atmosphere must be completely excluded from the site of nitrogen fixation, otherwise nitrogenase no longer functions properly. Because of the substantial energy required to break the triple bond, as well as the requirement that no oxygen be present, it is extremely difficult to break apart the nitrogen atoms in N₂ gas. However, the atmosphere is 80% N₂ gas. As a result, there should be a huge incentive in nitrogen-limited ecosystems for plants to be able to access the atmospheric reservoir of N₂ gas - if they could, they would overcome the limitation of low soil nitrogen levels and would be able to outcompete other plants. Many leguminous trees are able to access this atmospheric reservoir by forming a mutualistic pact with bacteria living in the soil. Through a unique set of chemical signals, the tree and bacteria "agree" to form a symbiosis, and the bacteria set up shop in specialized housing structures called nodules in the tree's roots which exclude oxygen. Once settled, the bacteria use nitrogenase to split N₂ , and the resulting fixed nitrogen is transferred to the tree. In return, the tree feeds the bacteria sugars it needs for growth. Therefore, housing and feeding the N-fixing bacteria cost the tree some of its valuable carbon from photosynthesis, but in return the tree gets nitrogen, which is crucial for growth.

It's a tidy story, and it makes perfect sense that such a pact would be a successful strategy for trees that are nitrogen-starved and are willing to trade carbon for nitrogen. However, here enters what ecologists call the "paradox of nitrogen fixation": in the tropics, where nitrogen fixing legumes are widely successful, soils are chock full of nitrogen - nitrogen has been accumulating since the soils were last scoured to the bedrock by glaciers millions of years ago. In the temperate zone, where nitrogen fixers are practically absent, the soils have very little nitrogen - there has been very little time for nitrogen to accumulate since the end of the Pleistocene glaciation a mere 11,000 years ago. Why, then, are leguminous trees so successful in the tropics, where their nitrogen fixing ability is rendered superfluous by the vast soil N reserves? According to current understanding, legumes should thrive in the temperate zone, where a nitrogen fixing strategy would give a tree a substantial competitive advantage over other trees that must rely on the meager soil reserves. Some ecologists have used a "chicken or the egg" argument to explain this paradox, claiming that tropical soils are rich in nitrogen because there are so many nitrogen fixing legumes there. If this were the case, why would the legumes continue to be so successful? If feeding and housing the bacteria really diverts the tree's sugars supply from other activities, such as growth, they should be at a competitive disadvantage to the trees which just rely on soil nitrogen, and they shouldn't dominate tropical forests.

Of course, the paradox of nitrogen fixation is only a paradox because ecologists don't understand it. This lack of understanding is deeper than just a little confusion over the physiology of nitrogen nutrition in tropical trees. Instead, it suggests that we have very little understanding of how trees the world over obtain a nutrient critical to controlling forest growth. We can split the atom, but we can't answer the most fundamental questions about the controls on forest growth and tree species distribution across the planet.

The question of nitrogen fixing trees was not the last time I heard the answer "no one really knows" during our three-month stint in the Panamanian tropics. It seemed that wherever we turned, there were more questions than answers. In the high elevation cloud forests on the Continental Divide in Chiriqui, "no one really knows" was everywhere. The forest was unlike anything I'd ever experienced: the trees were stunted, barely distinguishable forms under the thick mats of mosses and lichens. The mossy ground held the impression of our footsteps briefly before slowly rising and obscuring the marks of our passing. It was perfectly quiet and still; the only movement was the wraiths of fog blowing through the dark green canopy. Occasionally the pat of a water drop landing on a leaf in the understory - condensed fog from a canopy leaf - or the trill of an unseen bird in the branches, obscured by fog, would ripple the quiet. Else, just muffled silence, and the feeling that the tooth and claw struggle to stay alive which was so apparent in the lowland forests was somehow subdued in these dripping forests. Why were these forests so stunted? Was it too cloudy for the trees to get adequate light to grow? Had most of the critical nutrients washed out of the soil from the incessant rain and fog drip? How will these forests respond to a warming climate, or to high levels of nitrogen pollution washed out of the atmosphere in rain? What role does the thick canopy moss play in the nutrient cycles of these forests? These are all questions without solid answers, but their interest is not merely academic: the effects of human-caused environmental change are already being noticed in cloud forests. Lowland tree and bird species are expanding their ranges to higher and higher elevations as the climate warms. Nitrogen pollution is projected to increase drastically over the coming century in the tropics as developing countries industrialize and agricultural nitrogen fertilizer use continues to rise. Because high elevation tropical forests are highly efficient at collecting and storing water needed to support human societies in the lowlands, they likely will be preserved even as the surrounding lowland forests are cleared for agriculture, grazing, development, and lumber. In Chiriqui, this trend is painfully apparent. On a clear day, you can stand in the road cut on the Continental Divide and look south over broad, grassy slopes dotted with fires descending to the Pacific Ocean. These slopes were once thickly forested. Now, only the cloud forest remains, perched on the high ridges, luxuriant growth filtering water out of the blowing fog for humans.

Ecology, and in particular tropical ecology, is struggling to catch up with other branches of science. In the rush to discovery since the Scientific Revolution, the rate of accumulation of knowledge of our larger natural environment has not kept pace with other disciplines. With good reason, research has focused on improving the human condition. Physics breakthroughs have resulted in technological advances, and biomedical research has alleviated human suffering. Because research to understand nature has less tangible and immediate benefits to humanity, progress has languished over the past centuries. Notable exceptions include the work of great 19th century naturalists like Charles Darwin and Alfred Russell Wallace, whose groundbreaking ideas on evolution have set the intellectual tone for the last two centuries. However, for the most part tropical ecology has focused on the description and collection of exotic new plants and animals, and even these efforts have resulted in a woefully incomplete assessment of the sheer diversity of species in the world's tropical regions. Only with the recent realization that we can't take our larger planetary environment for granted has research in the natural world spurted forward to make up for lost years. Ecologists are now simultaneously trying to document what they see in nature (species lists and discoveries of novel organisms), describe how nature works (how do trees get enough nitrogen to grow?), and predict how nature will respond to human-caused changes in the global environment - a tall task, and an urgent one.

In a time when we can fly faster than the speed of sound, describe the structure of the electron clouds around nitrogen atoms in our DNA, and talk to other humans thousands of miles away on cellular telephones, we do not understand the most basic questions about forests. We cannot answer seemingly simple questions like "Why are there more than 400 species of trees in a Peruvian lowland forest, and only 48 in a forest in central New Jersey?" The vast gaps in our knowledge of tropical forests will need to be filled in if we are to do anything but passively respond to anthropogenic disturbance of these ecosystems. By the time our children travel to Panama and ask, "Why are there so many nitrogen fixing trees?" and "Why do cloud forests have stunted growth?" there will need to be answers. The answer "no one really knows" will soon be unacceptable.

References

Crews, T.E. 1999. The presence of nitrogen fixing legumes in terrestrial
communities: evolutionary vs. ecological considerations.
Biogeochemistry 46: 233-246.

Foster, R.B., and Hubbell, S.P. 1990. The floristic composition of the Barro
   Colorado Island forest. In: Gentry, A.H. (Ed.). Four
   Neotropical Rainforests (pp. 85-98). Yale
   University Press, New Haven, Connecticut, U.S.A.

Postgate, J. 1998. Nitrogen fixation. University of Cambridge Press,
Cambridge, U.K.