Roughly 10 percent of all plant species are orchids, making them the largest plant family on Earth. But habitat loss has rendered many threatened or endangered. This is partly due to their intimate relationship with the soil. Orchids depend entirely on microscopic fungi in the early stages of their lives. Without the nutrients orchids obtain by digesting these host fungi, their seeds often will not germinate and baby orchids will not grow. While researchers have known about the orchid-fungus relationship for years, very little is known about what the fungi need to survive.

Image right and below: Flowers (right) and leaves (below) of the orchid Goodyera pubescens, commonly known as the downy rattlesnake orchid, endangered in Florida. (Photos by Melissa McCormick/SERC)

Biologists based at the Smithsonian Environmental Research Center in Edgewater, Md., launched the first study to find out what helps the fungi flourish and what that means for orchids. Led by Melissa McCormick, the researchers looked at three orchid species, all endangered in one or more U.S. states. After planting orchid seeds in dozens of experimental plots, they also added particular host fungi needed by each orchid to half of the plots. Then they followed the fate of the orchids and fungi in six study sites: three in younger forests (50 to 70 years old) and three in older forests (120 to 150 years old).

Image right and below: Leaf (right)  and flowers (below) of Tipularia discolor, the cranefly orchid, endangered in New York and Massachusetts, and threatened in Michigan and Florida.

After four years they discovered orchid seeds germinated only where the fungi they needed were abundant—not merely present. In the case of one species, Liparis liliifolia (lily-leaved twayblade), seeds germinated only in plots where the team had added fungi. This suggests that this particular orchid could survive in many places, but the fungi they need do not exist in most areas of the forest.

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Image right: These ancient corn cobs date roughly from 6,500-4,000 years ago. A is Proto-Confite Morocho race; B, Confite Chavinense maize race; and C is Proto-Alazan maize race.. (Photo by Tom Dillehay)

Some of the oldest known corncobs, husks, stalks and tassels, dating from 6,700 to 3,000 years ago were found at Paredones and Huaca Prieta, two mound sites on Peru’s arid northern coast. The research group, led by Tom Dillehay from Vanderbilt University and Duccio Bonavia from Peru’s Academia Nacional de la Historia, also found corn microfossils: starch grains and phytoliths. Characteristics of the cobs—the earliest ever discovered in South America—indicate that the sites’ ancient inhabitants ate corn several ways, including popcorn and flour corn. However, corn was still not an important part of their diet.

Image left: Wild forms of Zea mays are called ‘teosinte’. Over time, selective breeding modifies teosinte’s few fruitcases (left) into modern corn’s rows of exposed kernels (right). (Photo courtesy John Doebley.).

“Corn was first domesticated in Mexico nearly 9,000 years ago from a wild grass called teosinte,” Piperno says. “Our results show that only a few thousand years later corn arrived in South America where its evolution into different varieties that are now common in the Andean region began. This evidence further indicates that in many areas corn arrived before pots did and that early experimentation with corn as a food was not dependent on the presence of pottery.”

Understanding the subtle transformations in the characteristics of cobs and kernels that led to the hundreds of maize races known today, as well as where and when each of them developed, is a challenge. Corncobs and kernels were not well preserved in the humid tropical forests between Central and South America, including Panama—the primary dispersal routes for the crop after it first left Mexico about 8,000 years ago.

“These new and unique races of corn may have developed quickly in South America, where there was no chance that they would continue to be pollinated by wild teosinte,” Piperno says. “Because there is so little data available from other places for this time period, the wealth of morphological information about the cobs and other corn remains at this early date is very important for understanding how corn became the crop we know today.”

“Preceramic corn from Pardones and Huaca Prieta, Peru,” Grobman, A., Bonavia, D., Dillehay, T.D., Piperno, D.R., Iriarte, J., Holst, I. 2012. . PNAS early online edition, week of Jan. 16, 2012.

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Image right:  American Mistletoe (Phoradendron flavescens) by Mary Vaux Walcott, 1923.  (Image courtesy Smithsonian American Art Museum)

* Mistletoe is found mainly in tropical or temperate areas. Its name refers to species of parasitic plants from families in the order of flowering plants known as Santalales. There are some 1,300 species of mistletoe worldwide.

Image left: The mistletoe Phoradendron serotinum. (Photo by Jorg and Mimi Fleige)

* The word “mistletoe” is thought to derive from the Anglo-Saxon “mist” or “mistel”, meaning dung, and “tan”, meaning twig, or “dung twig”. This derivation stems from the fact that mistletoe is mostly spread by birds, through their droppings.

* Birds also squeeze mistletoe seeds from the fruits before eating them and wipe the seeds on a branch. Mistletoe seeds are covered in a sticky substance so they stay put on a limb until they sprout.

* Mistletoe is semi-parasitic, meaning it invades a living branch of a host tree or bush with a shallow root (called  a “hastorium”) and absorbs food, minerals and water, and also produces food through photosynthesis in its evergreen leaves.

* Viscum album, the European mistletoe, and Phoradendron serotinum, from North America, are the two mistletoe species most commonly harvested and sold during the Christmas holidays.

Image right: The mistletoe Vicsum album (Photo by Malcom Storey)

* Mistletoe is considered a pest in many areas of the world. A host tree or bush heavily infested with mistletoe can be stunted or even die. Still, mistletoe provides an important food source and a nesting place for a variety of bird species.

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Image right: The Smithsonian’s Barro Colorado Island was the site of the first large-scale, long-term forest dyanmics plots. Now there are 42 forest dynamics plots worldwide that use the same methodology, the Smithsonian Institution Global Earth Observatory system managed by the Center for Tropical Forest Science. (Photo by Marcos Guerra)

“Air pollution is fertilizing tropical forests with one of the most important nutrients for growth,” said S. Joseph Wright, staff scientist at the Smithsonian Tropical Research Institute in Panama. “We compared nitrogen in leaves from dried specimens collected in 1968 with nitrogen in samples of new leaves collected in 2007. Leaf nitrogen concentration and the proportion of heavy to light nitrogen isotopes increased in the last 40 years, just as they did in another experiment when we applied fertilizer to the forest floor.”

Nitrogen is an element created in stars under high temperatures and pressures. Under normal conditions, it is a colorless, odorless gas that does not readily react with other substances. Air consists of more than 75% nitrogen. But nitrogen also plays a big role in life as an essential component of proteins. When nitrogen gas is zapped by lightning, or absorbed by soil bacteria called “nitrogen fixers,” it is converted into other “active” forms that can be used by animals and plants. Humans fix nitrogen by the Haber process, which converts nitrogen gas into ammonia—now a principal ingredient in fertilizers. Today, nitrogen fixation by humans has approximately doubled the amount of reactive nitrogen emitted.

Image left: The Smithsonian’s Barro Colorado Island was the site of the first large-scale, long-term forest dyanmics plots. Now there are 42 forest dynamics plots worldwide that use the same methodology, the Smithsonian Institution Global Earth Observatory system managed by the Center for Tropical Forest Science. (Photo by Marcos Guerra)

Nitrogen comes in two forms or isotopes: atoms that have the same number of protons but different numbers of neutrons. In the case of nitrogen, the isotopes are ¹⁴N and ¹⁵N, although only about one in 300 nitrogen atoms is the heavier form. Imagine nitrogen in the ecosystem like a bowl of popcorn. Normally the ratio of popped (light) to unpopped (heavy) kernels stays the same, but when someone starts to eat the popcorn, the lighter, popped kernels get used up first, increasing the ratio of heavy to light kernels (or ¹⁵N/¹⁴N in the case of the ecosystem). Light nitrogen is lost through nitrate leaching and as gases such as N2, and various forms of nitrous oxides or “noxides,” some of which can be important greenhouse gases. In the fertilization study in Panama, mentioned earlier, N₂O emissions were tripled.

“Tree rings provide a handy timeline for measuring changes in wood nitrogen content,” said Peter Hietz from the Institute of Botany at the University of Natural Resources and Life Sciences in Vienna, who faced down a tiger when sampling trees in a monsoon forest on the Thailand-Myanmar border. “We find that over the last century, there’s an increase in the heavier form of nitrogen over the lighter form, which tells us that there is more nitrogen going into this system and higher losses. We also got the same result in an earlier study of tree rings in Brazilian rainforests, so it looks like nitrogen fixed by humans now affects some of the most remote areas in the world.”

“The results have a number of important implications,” said Ben Turner, staff scientist at STRI. “The most obvious is for trees in the bean family (Fabaceae), a major group in tropical forests that fix their own nitrogen in association with soil bacteria. Increased nitrogen from outside could take away their competitive advantage and make them less common, changing the composition of tree communities.”

“There are also implications for global change models, which are beginning to include nitrogen availability as a factor affecting the response of plants to increasing atmospheric carbon dioxide concentrations,” said Turner. “Most models assume that higher nitrogen equals more plant growth, which would remove carbon from the atmosphere and offset future warming. However a challenge for the models is that there is no evidence that trees are growing faster in Panama, despite the long-term increases in nitrogen deposition and atmospheric carbon dioxide.”

Decades of atmospheric nitrogen deposition have caused major changes in the plants and soils of temperate forests in the U.S. and Europe. Whether tropical forests will face similar consequences is an important question for future research.

The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The Institute furthers the understanding of tropical nature and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems. Website: www.stri.org.

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Images: Photos of Michaux’s sumac by Lytton Musselman

Because Michaux’s sumac grows only in areas with few trees where the vegetation has been disturbed, it has long been assumed that its seeds germinate naturally following exposure to the high-temperatures of a brush or forest fire. Decline of this plant has been attributed to the prevention and suppression of brush and forest fires by humans. In Virginia it grows in only two places: on the grounds of the Virginia Army National Guard Maneuver Training Center in Fort Picket and a mowed railway right-of-way in an undisclosed location.

In a recent series of germination experiments, the scientists exposed different sets of Michaux’s sumac seeds to dry heat temperatures of 140, 176, 212, 248 and 284 degrees Fahrenheit, some sets for 5 minutes and other sets for 10 minutes. (The temperatures were determined based on maximum wildfire surface temperatures and burn times recorded in southeastern U.S. forests.) The researchers found that temperatures above 212 degrees F. killed the seeds. Lower temperatures had virtually no impact on breaking the seed’s dormancy.

The highest germination rates—30 percent—occurred after sulfuric acid was poured on Michaux’s sumac seeds and allowed to scarify (dissolve and weaken) the seed coats. This finding, from an experiment done in 1996, has led the researchers to their next experiment using birds. “We are going to feed the seeds to quail and wild turkey to determine if that breaks the seed dormancy,” says Bolin, a research associate with the Department of Botany at the Smithsonian’s National Museum of Natural History and an assistant professor at Catawba College in Salisbury, N.C. Seed passage through the digestive tracts of frugivorous (fruit eating) birds (and exposure to the acid in the bird’s stomachs) may break the physical dormancy of these seeds and help disperse them as well, the scientists write.

The paper “Germination of the federally endangered Michaux’s sumac (Rhus michauxii), authored by Jay F Bolin, Marcus E Jones (Norfolk Botanical Garden, Norfolk Va.,) and Lytton J Musselman (Old Dominion University, Norfolk, Va.) appeared in the Summer 2011 issue of “Native Plants Journal”

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The research was led by scientists from the Centre for Ecology and Hydrology and the University of Cambridge, UK. The results were published online today in the scientific journal “Nature Climate Change.”

Image right: Leaf litter around the buttress roots of a tropical tree at the study site in Panama.

The researchers used results from a six-year experiment in a rainforest at the Smithsonian Tropical Research Institute in Panama, to study how increases in litterfall–dead plant material such as leaves, bark and twigs which fall to the ground–might affect carbon storage in the soil. Their results show that extra litterfall triggers an effect called ‘priming’ where fresh carbon from plant litter provides much-needed energy to micro-organisms, which then stimulates the decomposition of carbon stored in the soil

Lead author Emma Sayer from the Smithsonian Tropical Research Institute and the Centre for Ecology and Hydrology said, “Most estimates of the carbon sequestration capacity of tropical forests are based on measurements of tree growth. Our study demonstrates that interactions between plants and soil can have a massive impact on carbon cycling. Models of climate change must take these feedbacks into account to predict future atmospheric carbon dioxide levels.”

Image left: Undergrowth showing leaf litter at the Smithsonian Tropical Research Institute study site. (Photos by Emma Sayer)

The study concludes that a large proportion of the carbon sequestered by greater tree growth in tropical forests could be lost from the soil. The researchers estimate that a 30% increase in litterfall could release about 0.6 tonnes of carbon per hectare from lowland tropical forest soils each year. This amount of carbon is greater than estimates of the climate-induced increase in forest biomass carbon in Amazonia over recent decades. Given the vast land surface area covered by tropical forests and the large amount of carbon stored in the soil, this could affect the global carbon balance

Tropical forests play an essential role in regulating the global carbon balance. Human activities have caused carbon dioxide levels to rise but it was thought that trees would respond to this by increasing their growth and taking up larger amounts of carbon. However, enhanced tree growth leads to more dead plant matter, especially leaf litter, returning to the forest floor and it is unclear what effect this has on the carbon cycle.

“Soils are thought to be a long-term store for carbon but we have shown that these stores could be diminished if elevated carbon dioxide levels and nitrogen deposition boost plant growth,” Sayer adds.

Co-author Edmund Tanner, from the University of Cambridge, said, “This priming effect essentially means that older, relatively stable soil carbon is being replaced by fresh carbon from dead plant matter, which is easily decomposed. We still don’t know what consequences this will have for carbon cycling in the long term.” –Source: Center for Ecology and Hydrology

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Image left: A single gene controls the repeated evolution of red color patten mimicry in passion-vine butterflies.

Now several research teams that include Smithsonian scientists in Panama, have discovered that Heliconius butterflies mimic each other’s red wing patterns through changes in the same gene.

Not only does this gene lead to the same red wing patterns in neighboring species, it also leads to a large variety of red wing patterns in Heliconius species across the Americas that result when it is turned on in other areas of the wings.

Because different butterfly species evolved red wing patterns independently, resulting in a huge variety of patterns we see today, researchers thought that different genes were responsible in each case.

Image right: Heliconius butterflies were reared in butterfly houses at the Smithsonian Tropical Research Institute’s facilities in Gamboa, Panama.

“The variety of wing patterns in Heliconius butterflies has always fascinated collectors,” said Owen McMillan, geneticist at the Smithsonian Tropical Research Institute, “People have been trying to sort out the genetics of mimicry rings since the 1970’s. Now we put together some old genetics techniques and some newer genomics techniques and came up with the very surprising result that only one gene codes for all of the red wing patterns. The differences that we see in the patterns seems to be due to the way the gene is regulated.”

First the team used genetic screens to look for genes that are turned on differently in butterflies with red wing patterns and lacking in other butterflies without this pattern. When they discovered a promising gene, they used stains to show where this gene was expressed on butterfly wings showing different patterns. They found the gene to be expressed exactly where red pigment occurs in the wings in every case. The match was so perfect that they could identify subtle differences in red patterns between species using these stains.

They combed genetic libraries—gene banks— to see if the gene they found matched genes characterized in other studies. “We found that the same gene that codes for the red in Heliconius wings was already identified as a gene called optix that is involved in eye development in other organisms,” said co-author Heather Hines, “It is intriguing that the ommochrome pigments that color these wings red are also expressed in the eye. How the optix gene codes for wing color raises a host of new questions.”

“Tropical biologists have been striving for centuries to explain what it is that makes life in the tropics so biologically diverse,” said STRI Director, Eldredge Bermingham, “Now this group has discovered that a single gene underlies one of the most spectacular evolutionary radiations in nature! Perhaps the genetic basis for diversity will turn out to be far more simple than we expected.”

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Image right: Seed and pod of the common nutmeg. (Photo by Steven Paton)

The reproductive success of any fruiting plant depends upon how effectively its seeds are dispersed yet tracking and mapping individual seeds carried off by the wind or ingested by animals is nearly impossible. Today, ecologists studying forest dynamics rely mostly on theoretical models to calculate the area of seed distribution for specific plants. New tracking technology however is changing that.

In the first stage of their experiment, the scientists collected fresh seeds from a common Panamanian nutmeg tree (Virola nobilis) and fed them to captive toucans (Ramphastos sulfuratus) at the Rotterdam Zoo. Toucans gulp nutmeg seeds whole, the outer pulp is processed in the bird’s crop, and the hard inner seed is then regurgitated. Five zoo toucans fed 100 nutmeg seeds took an average of 25.5 minutes to process and regurgitate the seeds.

Image left: A wild toucan in the rainforest at Gamboa, wearing a backpack containing a GPS transmitter and accelerometer. (Photo courtesy Roland Kays)

Next, in Panama, the scientists netted six wild toucans (four R. sulfuratus and two R. swainsonii) that were feeding from a large nutmeg tree in the rainforest at Gamboa. They fitted the birds with lightweight backpacks containing GPS tracking devices (these devices recorded the bird’s exact location every 15 minutes) and accelerometers which can measure a bird’s daily activity level.

When matched with the seed-regurgitation time of the zoo toucans, the GPS data indicated the wild toucans were probably dropping nutmeg seeds a distance of 472 feet, on average, from the mother tree. Each seed had a 56 percent probability of being dropped at least 328 feet from its mother tree and an 18 percent chance of being dropped some 656 feet from the tree.

Image right: Researchers Reinhard Vohwinkel, left, and Martin Wikelski attach a high-tech backpack to a wild toucan. The backpacks are designed to fall off the birds after ten days. (Photo courtesy Roland Kays)

In addition, the accelerometer revealed that the toucans’ peak activity and movement was in the morning followed by a lull at midday, a secondary activity peak in the afternoon, and complete inactivity at night. This is a normal pattern of for tropical birds.

“Time of feeding had a strong influence on seed dispersal,” the scientists write. “Seeds ingested in morning (breakfast) and afternoon (dinner) were more likely to achieve significant dispersal than seeds ingested mid day (lunch).” This observation explains why tropical nutmegs are “early morning specialists” with fruits that typically ripen at early and mid-morning so they are quickly removed by birds.

Ideally, the scientists observed, nutmeg trees could increase their seed dispersal distances by producing fruit with gut-processing times of around 60 minutes.

This spatially explicit map of the probability of see dispersal away from feeding trees by toucans shows seeds have a 56-percent chance of being dispersed more than 328 feet (100 meters) and an 18-percent chance of being moved more than 656 feet (200 meters).

The article, “The effect of feeding time on dispersal of Virola seeds by toucans determined from GPS tracking and accelerometers,” was published in the journal Acta Oecologica. It was authored by Roland Kays of the New York State Museum and the Smithsonian Tropical Research Institute; Patrick Jansen of the Smithsonian Tropical Research Institute and the Center fro Ecosystem Studies in Wageningen, The Netherlands; Elise Knecht of the Center for Ecosystem Studies, Wageningen; Reinhard Vohwinkel of Avifaunistische Untersuchungen, Germany; and Martin Wikelski of the Smithsonian Tropical Research Institute and the Max Planck Institute for Ornithology, Germany. –John Barrat

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