THE SPECIATION OF TREES

FROM:  THE NATIONAL SCIENCE FOUNDATION 
Tracing the evolution of forest trees

Evergreen tree in Hawaii offers clues into survival of tropical ecosystems
There are at least 60,000 identified tree species in the world, "but we know next to nothing about how they got here," Elizabeth Stacy says. "Trees form the backbone of our forests, and are ecologically and economically important, yet we don't know much about how speciation happens in trees."

Speciation, the evolutionary process by which new biological species arise, fascinates Stacy, an associate professor of biology at the University of Hawaii Hilo, and forms the core of her research. The National Science Foundation (NSF)-funded scientist is focusing on the origins of the many forms of Metrosideros, a diverse genus of forest trees, and on one of its species in particular--Hawaii's M. polymorpha--as a model for studying diversification.

The Hawaiian Islands were formed and continue to be formed from volcanic activity, which makes them an ideal place to study speciation. Because the islands are so isolated, their plant and animal species almost certainly colonized for the first time millions of years ago when wind, ocean currents, birds and insects carried early specimens there.

"Hawaii is a fantastic place to study evolution and the origins of species," Stacy says. "It's like its own planet, its own evolutionary experiment."

Metrosideros comprises trees and shrubs found predominantly in the Pacific Rim region. The name means "iron heartwood," and derives from the ancient Greek metra, or "heartwood," and sideron, or "iron." Stacy is trying to discern the relationships among the many forms of this genus in Hawaii and learn how reproductive barriers arise between diverging populations.

"Over time, Metrosideros has diversified into five species," she says. "M. polymorpha is by far the most abundant. It's unusual for its huge ecological breadth. You can find it in almost every habitat in Hawaii. It's everywhere."

Insights into the evolution of such long-lived trees as these could have important implications for future conservation practices in Hawaii, and possibly elsewhere.

"Because it is so abundant and dominant, Metrosideros is a keystone species for many of Hawaii's terrestrial environments," Stacy says. "It is an important resource for native birds and insects. Insights into how the many forms of Metrosideros originated and how different they are from each other today can reveal insights into the same for the many animals that use Metrosideros. Understanding the ecological needs of species is an essential first step in their conservation.

"Conservation biology has gained an appreciation for evolution," she adds. "Over the last decade, people have grown to appreciate that we need to pay attention to the processes that give rise to species. Speciation is literally the origin of the biodiversity that we are concerned about saving. To really think about long-term conservation, we need to be aware of these evolutionary processes."

Stacy is conducting her research under an NSF Faculty Early Career Development (CAREER) award, which she received in 2010. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization. NSF is funding her work with about $750,000 over five years.

Her project uses molecular genetic methods to "try to unravel the very shrouded evolutionary history of Metrosideros in Hawaii," she says. "We're experimenting with novel molecular markers--previously inaccessible genes and gene regions--to get a clearer picture of how the forms of Metrosideros are related, both within and across islands."

Uncovering the evolutionary relationships among closely related trees is especially difficult because of their tendency to hybridize, and thus share the same genetic material, she adds.

Also, she and her team are conducting experiments in the field and in the greenhouse with seedlings of various forms, exposing them to different stresses to compare their differences.

"These experiments are revealing insights into how long-term exposure of tree populations to Hawaii's famous environmental gradients can lead to diversification, and they reveal which specific environmental factors, for example, water, light and wind are most important for causing the differences among the forms of the tree," she says.

"Lastly, we are looking at their reproductive barriers: can you two make 'babies' with each other, and how fit are your 'babies?"' she adds. "How well do your offspring survive, and reproduce compared to everyone else in your population? We do a lot of hand-crossing, or hand pollination, where you take pollen from one tree and pollinate another."

These crosses allow examination of the both prezygotic (before fertilization) and postzygotic (after fertilization) barriers that accumulate between diverging populations on the way to speciation.

"I posit that adaptation of this widespread tree to Hawaii's highly varied environments has led to the evolution of partial reproductive isolating barriers between forms that are adapted to different habitats," she says.

As part of the grant's educational component, she is encouraging her students to participate in research through field and lab projects. The team also has established Ho'oulu Lehua, a community-based organization that provides hands-on environmental education for youth with projects that address real conservation issues in the native forests of East Hawaii Island.

The goal of Ho'oulu Lehua, under the leadership of CAREER technician Jennifer Johansen, is to strengthen connections between Hawaii's young people and native forests through restoration activities based on scientific understanding and cultural traditions.

"This island has 11 of 13 climate zones," she says. "We have desert, and wet forests and bogs. Because we are in this amazing evolutionary laboratory, I think we excel in engaging our students with authentic research experiences outside. You can't do this stuff in a lab."

-- Marlene Cimons, National Science Foundation
Investigators
Elizabeth Stacy
Related Institutions/Organizations
University of Hawaii at Hilo

AMOUNT OF WATER TREES NEED AND THE CHANGING ATMOSPHERE

On the ground: looking into Harvard Forest's trees from a less lofty perch.  Credit: NSF Harvard Forest LTER Site

FROM:  NATIONAL SCIENCE FOUNDATION
Changing Atmosphere Affects How Much Water Trees Need

Spurred by increasing levels of atmospheric carbon dioxide, forests over the last two decades have become dramatically more efficient in how they use water.

Scientists affiliated with the National Science Foundation's (NSF) Harvard Forest Long-Term Ecological Research (LTER) site report the results in this week's issue of the journal Nature.

Harvard Forest is one of 26 such NSF LTER sites in ecosystems from deserts to grasslands, coral reefs to coastal waters, around the world.

Studies have long predicted that plants would begin to use water more efficiently, that is, lose less water during photosynthesis, as atmospheric carbon dioxide levels rose.

A research team led by Trevor Keenan and Andrew Richardson of Harvard University, however, has found that forests across the globe are losing less water than expected and becoming even more efficient at using it for growth.

Using data collected in forests in the northeastern United States and elsewhere around the world, Keenan and Richardson found increases in efficiency larger than those predicted by state-of-the-art computer models.

The research was done in collaboration with scientists from the USDA Forest Service, Ohio State University, Indiana University and the Karlsruhe Institute of Technology in Germany.

"This could be considered a beneficial effect of increased atmospheric carbon dioxide," said Keenan, the first author of the Nature paper.

"What's surprising is we didn't expect the effect to be this big. A large proportion of the ecosystems in the world are limited by water--they don't have enough water during the year to reach their maximum potential growth.

"If they become more efficient at using water, they should be able to take more carbon out of the atmosphere due to higher growth rates."

While increased atmospheric carbon dioxide may benefit forests in the short-term, Richardson emphasized that the overall climate picture would remain grim if levels continue to rise.

"We're still very concerned about what rising levels of atmospheric carbon dioxide mean for the planet," Richardson said.

"There is little doubt that as carbon dioxide continues to rise--and last month we just passed a critical milestone, 400 parts per million for the first time in human history--rising global temperatures and changes in rainfall patterns will, in coming decades, have very negative consequences for plant growth in many ecosystems around the world."

How do increasing carbon dioxide levels lead to more efficient water use?

The answer, Keenan said, is in the way photosynthesis works.

To take in the carbon dioxide they need, plants open tiny pores, called stomata, on their leaves. As carbon dioxide enters, however, water vapor is able to escape.

Higher levels of carbon dioxide mean the stomata don't need to open as wide, or for as long, so the plants lose less water and grow faster.

To take advantage of that fact, commercial growers have for years pumped carbon dioxide into greenhouses to promote plant growth.

To test whether such a "carbon dioxide fertilization effect" was taking place in forests, Keenan, Richardson and others turned to long-term data measured using a technique called eddy covariance.

This method, which relies on sophisticated instruments mounted on tall towers extending above the forest canopy, allows researchers to determine how much carbon dioxide and water are going into and out of the ecosystem.

With more than 20 years of data, the towers at the NSF Harvard Forest LTER site--which have the longest continuous record in the world--are an important resource for studying how forests have responded to changes in atmospheric carbon dioxide levels, scientists say.

"A goal of the NSF LTER program is understanding forest ecosystems and the basis for predicting fluxes of energy and materials in these ecosystems," said Matt Kane, program director in NSF's Division of Environmental Biology, "as well as distributions of forest biota as a result of global climate change."

"Findings from this study are important to our understanding of forest ecosystems--and how they can be managed more effectively now and in the future."

Though more than 300 towers like Harvard Forest's have sprung up around the globe, many of the earliest--and hence with the longest data records--are in the northeastern United States.

When the researchers began to look at those records, they found that forests were storing more carbon and becoming more efficient in how they used water.

The phenomenon, however, wasn't limited to a single region. When the scientists examined long-term data sets from all over the world, the same trend was evident.

"We went through every possible hypothesis of what could be going on, and ultimately what we were left with is that the only phenomenon that could cause this type of shift in water-use efficiency is rising atmospheric carbon dioxide," Keenan said.

Going forward, Keenan, who is now at Macquarie University in Sydney, Australia, is working to get access to data collected from yet more sites, including several that monitor tropical and arctic systems.

"This larger dataset will help us better understand the extent of the response we observed," he said.

"That in turn will help us build better models, and improve predictions of the future of the Earth's climate.

"Right now, all the models we have underrepresent this effect by as much as an order of magnitude, so the question is: What are the models not getting? What do they need to incorporate to capture this effect, and how will that affect their projections for climate change?"

The research was also supported by NOAA. Field measurements at the sites, which are part of the AmeriFlux network, have also been funded by the U.S. Department of Energy and the USDA Forest Service.

-NSF-

SEED DISPERSAL AND ENVIRONMENTAL CONDITIONS IN AFRICAN FORESTS

Nouabale-Ndoki National Park in Central Africa, site of the scientists' research. Credit: Wikimedia Commons

FROM: NATIONAL SCIENCE FOUNDATION
Seeing the Forest for the Trees: Seed Dispersal, Environmental Conditions Matter in African Forests
Nouabale-Ndoki National Park is a tree-dotted enclave in Central Africa's Republic of Congo. Heavy logging surrounds the park, but it still has one of the largest intact forests in Africa. In recognition, it recently became a UNESCO World Heritage Site.

Trees--thousands of them--make up a forest. How did Nouabale-Ndoki's trees become so numerous, and how do they stay that way?

The answer, say biologists, lies far below the tree canopy, in the soil where seedlings sprout.

Today in the journal PLOS ONE, scientists report results of an extensive seedling experiment in Nouabale-Ndoki National Park.

The research, which involved sowing 40,000 seeds of five tree species, is a new look at "seeing the forest for the trees."

The findings, which show what limits seedling growth, are important to reforestation efforts in areas that have been logged.

Every tree can produce hundreds of thousands of seeds in its lifetime, but on average, only one seed survives to adulthood, says John Poulsen of Duke University, a co-author of the journal paper.

Other paper co-authors are Connie Clark, also of Duke, and Doug Levey, formerly of the University of Florida and now a program director in the National Science Foundation's (NSF) Division of Environmental Biology.

Which seeds have the best chance of making it to old age?

"There are basically two ways to look at successful seedling recruitment [survival]," says Levey. "Species may be seed-limited or establishment-limited."

A tree species is seed-limited if its ability to grow is determined by whether its seeds reach a particular location on the ground. The seeds may arrive on the wind or simply by falling from trees.

Establishment-limited trees are those that depend on the environment around them, rather than on seeds landing in just the right spot. If the soil is too wet or there is too much shade, a species is establishment-limited.

To test the importance of these two limitations on seedling recruitment, the scientists sowed tens of thousands of seeds.

They chose the species randomly, which allowed the results to be generalized to all tree species, not just the most common ones, says Poulsen.

The seeds were planted in different amounts in plots that stretched across an area the size of the state of Rhode Island.

Latter-day Johnny Appleseeds, the researchers couldn't do it alone, however.

"We hired a small army of indigenous, Mbendzélé hunter-gatherers," says Clark. "These families could easily locate seeds, and we were the beneficiaries of their intimate knowledge of the forest's natural history."

After the seeds were planted, the ecologists watched them grow into seedlings over two years.

They found that only a small fraction of seeds, some 16 percent, became seedlings. An even smaller amount, about six percent, survived to reach their second birthdays.

When numbers of seeds were at one end of a spectrum--rare or abundant--the trees' recruitment was seed-limited.

"When seeds were at intermediate densities," says Levey, "the chance of recruitment was influenced by environmental factors such as soil type and sunlight."

The importance of seed- and establishment-limitation changes over time, Levey says. "As individual trees get older, they need the correct soil and light exposure [become more establishment-limited]."

Not that different from our changing needs for the right nutrients and enough light as we reach our sunset years.