What can bat poop tell us about past tropical landscapes?

Rachel Reid is a postdoctoral researcher at Washington University in St. Louis. She uses isotope chemistry to answer questions about ecology, geology, and conservation – including questions that can help build reference models for ecological restoration. Note: This blog is republished with permission from Amigos (No. 91 May 2019), the newsletter of Las Cruces Biological Station.

 Many people head to Costa Rica for spring break to see monkeys and sloths at Manuel Antonio National Park or to try their hand at surfing in the Pacific. While we did stop to gawk at the crocodiles that hang out under the bridge over the Tárcoles River with a busload of tourists, the goal of our trip diverged significantly from the spring break crowd – we were heading off the beaten path to southern Costa Rica to collect samples of modern and ancient bat guano (aka poop).

Bats sometimes visit the same caves over thousands of years, and the accumulated piles of guano offer a unique opportunity to study past environments. Just like a core of sediment from the bottom of a lake or the ocean, a core of bat guano collected from a cave contains useful information about the past, both recent and distant. The material at the bottom of the core is the oldest and that at the top is the youngest, so by sampling the length of a core, we can essentially take a short, stinky walk back in time.

We are interested in detecting changes in bat guano chemistry (particularly the carbon isotope values) through time as a way of evaluating what type of vegetation would have been on the landscape in the past. This works because information about the plants at the base of the food chain gets propagated up to the plant-eating insects and then to the insect-eating bats whose guano we’re sampling.

Bat Food Chain

Like other animals, bats and insects both gain carbon and nitrogen through the food they eat. Bats eat insects, which are in turn eating the local vegetation. Different types of plants have different carbon isotope values, such that most trees and shrubs (C3 plants) have much lower carbon isotope values than most grasses (C4 plants). Shifts in tropical bat guano carbon isotope values, therefore, are indicative of landscape-level changes in vegetation between more open, grassland plants and tropical forest.

How does bat poop inform conservation?

In the late 1940s, southern Costa Rica was nearly 100% forested. We know this from aerial photos – the earliest ones are from 1948. In later years, aerial photos show that most of that forest was cleared for coffee plantations; two thirds of it was cleared by 1980, for example.

This recent deforestation has motivated forest restoration efforts such as the creation of biological corridors and international scientific studies. Nonetheless, several studies (such as this and this) suggest that extinction rates in this region may be lower than would be predicted from recent habitat loss. One explanation for this could be that the regional flora and fauna evolved for several thousand years in a mixed forest and non-forest landscape managed by humans. By piecing together records of past vegetation from bat guano cores, we’ll be able to gain a better picture of what the landscape would have looked like in the past and potentially refine landscape-scale conservation and restoration targets.

For this first trip, our goals were to visit several caves to collect samples and to scout out future sampling opportunities. Southwestern Costa Rica has the highest concentration of karst caves in the country, so we were in the right place. In four days of fieldwork we visited three different caves (two of them twice!), collected 77 cm of core material, and took dozens of samples of modern bat poop.

At Bajo los Indios Cave, also known as Corredores, along the Rio Corredor, we ventured into a restricted, elevated chamber in hopes of finding deeper, more protected accumulations of guano. We were disappointed to find that even in this higher chamber, the cave was very wet and muddy and any significant guano accumulations appeared to have washed away. We collected a guano/mud core anyway and we’ll see what we can learn from it.

Bat Guano Team by JF

The bat guano team. From left to right: Leighton Reid & Christy Edwards (Missouri Botanical Garden), Rachel Reid & Alice Xu (Washington University in St. Louis), and Jeisson Figueroa (Organization for Tropical Studies). Photo by Jeisson Figueroa.

Taking a guano core by JF

Leighton Reid uses a peat corer to extract a sample of bat guano from a karst cave. Photo by Jeisson Figueroa.

One additional important piece to our project is to try to get a better idea of what modern insectivorous bats, such as the mesoamerican mustached bat (Pteronotus parnellii mesoamericanas), are eating. We’ll then use that information to better interpret our results back in time. We’re excited to start analyzing samples!

This pilot study was generously funded by grants from the Living Earth Collaborative and from the International Center for Energy, Environment and Sustainability.

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Virtual field trip to the Guajira desert and the Serranía de Macuira in northern Colombia

James and Thibaud Aronson describe the natural and cultural context of a little-known area of northern Colombia, home to the Wayuu people and a microcosm of arid lands worldwide.

Colombia is one of the world’s seventeen megadiverse countries.  In a few hours of travel, one can go from the sweltering Amazonian lowlands to the snow-capped peaks of the Andes. It even has a true desert, a small peninsula called la Guajira, shared with Venezuela, which constitutes the northernmost point of South America.

For most of the last 50 years, the Guajira was notoriously dangerous, principally because of drug trafficking, but things have improved in recent years. We traveled there last month, shortly after the first big rains the region had received in several years. ​ And we found that it’s a poignant example of the plight of drylands globally and their peoples.

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The Guajira peninsula, in northern Colombia, including the authors’ itinerary.

Our trip actually began in Panama, which was part of Colombia until 1903. While much smaller, Panama is also a country of contrasts. Much of the Pacific coast used to be covered in seasonally dry tropical forest, and some fragments persist today in and around Panama City itself, while the forests of the Caribbean slope, a mere 50 km away, are much wetter. A curious switch occurs near the Colombian border, where the wet forests then extend down the Pacific coast of Colombia and Ecuador – the famous Chocó-Darien rainforest, one of the wettest and most diverse tropical forests on Earth.

Meanwhile, the seasonally dry forests continue along the 1,000 km long Caribbean coast of Colombia and give way to semi-desert and then true desert (annual rainfall < 250 mm), lined by a coast with mangrove forests, and a series of lagoons and bays where flamingos and ibises add a shock of color.

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Mangroves in Bahia Hundita, Alta Guajira, showing desert woodland with tree cacti (Stenocereus griseus) and various legume trees growing on the sandstone bluffs in the background.

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Roseate spoonbills, great egrets, and a white ibis sharing a coastal wetland near Uribia.

As if this wasn’t enough contrast, halfway along the Caribbean coast rises the Sierra Nevada de Santa Marta, Colombia’s tallest mountain range, reaching 5,700 meters (18,700 feet) above sea level at the highest peak. It takes only about two hours to drive from its foothills, where toucans and monkeys chatter in the majestic trees, to Riohacha, the gateway to the desert.

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A brown-throated three-toed sloth (Bradypus variegatus) hanging by one arm in a Cecropia tree in Tayrona National Park, at the base of the Santa Marta mountains.

Alta Guajira’s desert trees and woodlands

The Alta Guajira is arid indeed, but it hosts trees, remarkable both in their exuberant diversity and their abundance, considering the high temperatures and meager rainfall. We saw what we consider true desert canopies, such as we have described in other posts. However, no desert flora exists in isolation, and indeed the kinship to the ecosystem type known as Seasonal Dry Tropical Forest (SDTF; see map above) seems to be strong.

The dominant trees of the Guajira are species of Prosopis, Caesalpinia, Vachellia (formerly part of Acacia s.l.), Parkinsonia and other legume genera, accompanied by Bursera, Capparis relatives, Bignoniaceae, and other species common in the dry forests of Central and South America, and 3 kinds of tree cacti (Stenocereus, Pilocereus, and Pereskia), growing close together, often covered in climbing vines. In particular, it was interesting to see bona fide desert woodlands dominated by two well-known legume trees, Prosopis juliflora and Vachellia farnesiana, which are widespread and often strongly invasive in other parts of the world, but not here! Fascinating biogeographical and ecological questions abound in this poorly explored region, many of which are relevant to conservation and restoration.

Regarding  landscape ecology in the region, the vegetation is curiously like a patchwork, alternating between dense desert woodlands, nearly pure tree cacti stands, sometimes with a dense grass cover, and sometimes not, and frequent saline flats where nothing grows. In our opinion, the human element, namely land and resource use history, is paramount to understanding what one sees when travelling here and trying to ‘read’ the landscapes.

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Mixed patch of tree cacti and spiny legume trees with a surprising amount of grass understory. Elsewhere under similar stands, for no clear reason, there is no grass cover at all.

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A track of the Alta Guajira, near Nazareth, at the base of the Macuira hills where the notorious Prosopis juliflora, known in Colombia as Trupillo, is so exuberant and long-lived it forms a natural tunnel above this track.

 

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Prosopis juliflora colonizes newly exposed beach dunes, in areas where the shoreline is receding. Here, at Camarones, it occurs alongside Calotropis procera, a woody weed of the Apocynaceae known in English as giant milkweed, and familiar throughout the Caribbean islands, the Middle East and drylands of Africa. It survives because of its toxic milky latex where most other plants get eaten out by livestock.

Other standouts are the beautiful Palo de Brasil, Haematoxylum brasiletto, with its unusual fluted trunks and Pereskia guamacho, an enigmatic ‘primitive’ tree cactus with true leaves and one of the most exquisite tasting fruits we know. This is one of the least well-known but most intriguing of all desert trees to our minds.

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Typical landscape of the northern Guajira desert woodlands, with an even-aged stand of one of the several neotropical legume trees known as Brazilwood: Haematoxylum brasiletto, or Palo de Brasil in Spanish.

Despite those common names, this species is in fact only found wild along Caribbean coastlines from Colombia and Venezeula, all the way north to both coasts of Mexico. The scientific name is thus a misnomer. The most famous Brazilwood tree is another legume, Paubrasilia echinata (= Caesalpinia echinata) that once grew abundantly along the Atlantic coast of Brazil, as a large tree with a massive trunk, reaching up to 15 meters tall. Today, it’s almost entirely gone in the wild, and mostly planted in gardens and along roadsides. It was prized for the bright red dye obtained from the resin that oozes from cut branches or trunks. The dye was widely used by textile weavers in the Americas and Europe in the 17th-19th centuries. The tree also provided the wood of choice for high quality bows for stringed instruments and was widely used for furniture making as well. So important was its economic value that the country was named after it, originally Terra do Brasil (Land of the Brazilwood), later shortened to Brazil. Recently it was designated as sole member of a new genus, as part of a comprehensive revision of the entire genus Caesalpinia, carried out by an international team of experts.

It’s curious that H. brasiletto bears the same common name as P. echinata, since the two trees are nothing alike, apart from their red sap and heartwood. Little literature exists for H. brasiletto, and we are embarking on some detective work to shed some light on this puzzle. We go into detail as these are both relatively fast-growing trees with great economic as well as ecological value. They would both be excellent candidates for inclusion in ecological restoration work and are both in dire need of conservation efforts.

Wayuu: Alta Guajira’s Indigenous People

This desert also hosts a fairly large human population. The Guajira is the home of the Wayuu, Colombia’s largest surviving indigenous group and, along with the Navajo, one of the last desert-dwelling peoples in the New World. These fiercely independent people, organized in 17 matrilineal clans, were never subjugated by the Spanish, and even today the Guajira region functions mostly in isolation from the rest of the country. As we were heading well off the beaten track, we needed a guide, a 4 x 4 jeep in good condition, and a skilled driver to navigate the meandering and unmarked desert paths.

Despite an ancient history of human presence, and some periods of intensive exploitation and intervention (such as a pearl harvesting boom that took place soon after European explorers arrived), the ecological condition of the region at the landscape scale is remarkably good. Indeed, apart from the salt works in the small town of Manaure, which produce two thirds of Colombia’s salt, and El Cerrejón, South America’s largest open-pit coal mine, in the south of the Guajira, there is no major industry.

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Typical traditional salt works at Manaure worked by hand by local men and women just as they have for generations.

And the isolated people who dwell here – fishermen, shepherds, and weavers – are right out of a Gabriel García Márquez story. Indeed the author, most famous for One Hundred Years of Solitude, grew up on Colombia’s northern coast, speaking both Spanish and the Wayuu language, Wayuunaiki. As we traveled deeper into the desert, we traversed small settlements with simple houses made of wood and yards surrounded by tree cacti hedges.

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The Wayuu village of Boca de Camarones, in the south of the Guajira peninsula, showing the living hedgerows of columnar cacti produced from tall stanchions. In the background, surrounding the homes, are Trupillos, and good specimens of Dividivi Libidibia coriaria (formerly called Caesalpinia coriaria).

This third caesalpinoid legume tree, closely related to the two Brazilwoods mentioned above, is the source of another lovely red dye, derived in this case from its pods. Until recently, there was an annual festival in Camarones, in honor of this formerly major economic plant product. The tree was also used as an important source of tannins. Like Paubrasilia echinata, it deserves more ethnobotanical and biogeographical studies.

Here, as in many other arid lands, goats and sheep are important for the Wayuu people, as a source of food and social currency. For example bride price during arranged weddings, and gifts for guests attending vigils of important elders and healers, are paid to this day in heads of live goats or sheep. Historically, mules and donkeys were very common as well, but now they are increasingly replaced by motorcycles.

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Small children following a flock of desert-hardy sheep in Boca de Camarones. The peaks of the Sierra Nevada de Santa Marta are visible in the background.

Crown jewel of the Alta Guajira

The crown jewel of this desert, its best kept secret, is the Serranía de la Macuira, a small mountain range (serranía meaning “small sierra” in Spanish) in the northeast of the peninsula. This miniature sky island is almost impossibly lush, thanks to moisture-bearing clouds that shroud its upper reaches. They feed streams that flow year-round, and sustain many kinds of trees that grow to well over 10 meters tall.

As one climbs the slopes of the Macuira, the humidity dramatically increases and the parched lowlands, with their desert woodlands, blend perceptibly into a seasonally dry tropical forest reminiscent of those we had seen in Panama. A little-known fact: seasonally dry tropical forests are the most endangered of all tropical forest types, and those in La Guajira are worthy of much greater research, conservation, and restoration.

Climbing higher still, the mid- and upper ranges of the Macuira seem like another world. Most astonishing of all, there is apparently an abrupt transition above 550 meters, and the higher reaches are covered in true cloud forest, with mosses, epiphytic orchids, tree ferns, and dozens of tree species that otherwise occur hundreds of kilometers away! This is probably the only place in the world where cloud forest is found less than 5 km from true desert. Fortunately – from a conservation point of view, but unfortunately for us – the upper peaks of all three peaks of the Macuira are sacred to the Wayuu, and completely off-limits, to native people and visitors alike. Try as we might, we were unable to get permission to hike up there.

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Seasonally dry tropical forest on the northeastern facing slope of the Macuira, where precipitation is much higher than in the surrounding lowlands occupied by desert woodlands.

Even though the whole Macuira is officially protected as a national park, the reality is more complicated. While walking inside the park, we encountered recently cut trees, the ubiquitous goats, and even a Wayuu man hunting birds with a slingshot in broad daylight. The beautiful continuous tree canopy covering most of the slopes stands in stark contrast to the severely eroded, nearly bare hilltops, on which stand small Wayuu homesteads. Still, the presence of clear ecotones speaks to mostly healthy landscapes.

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The severe erosion around a small Wayuu farm inside the Macuira National Park.

Alta Guajira’s ecological future

The pressures on the Guajira’s ecosystem health include a large mine (El Cerrejón, mentioned above), overgrazing by domestic livestock, and stark poverty facing the native people and more recent immigrants. But there are positive factors as well. There are progressive laws in Colombia related to ecological restoration. Moreover, since 2012, Colombia has a National Restoration and Rehabilitation Plans (pdf), as well as a Law of Remediation, which imposes large environmental offset payments from large-scale development projects (like hydroelectric dams) to underwrite conservation and restoration work. Moreover, the national park system, within its network of 56 protected areas, harbors populations of almost half of the 102 indigenous peoples in the country, and in the case of Macuira, this is clearly not just a paper park idea.

Still, the national park (25,000 ha in size; officially designated in 1977), operates with a skeleton staff attempting to carry out an ambitious management plan (pdf) despite an insufficient budget. Staff and volunteers provide short tours to day-visitors, and maintain some fenced-off livestock exclosure plots, where they are studying natural regeneration. Daily interaction with the Wayuu living in the park appear to be harmonious, and indeed there is a clear sense that part of the Park’s mission is to restore and protect the Wayuu people’s natural and cultural heritage. Recently, the Instituto Humboldt, Colombia’s stellar national research institute, has established permanent plots in the Macuira range as part of a series of 17 plots including all the tropical dry forest types in Colombia. In the Macuira, this work is done in collaboration with botanists from the Universidad de Antioquia, in Medellin. Furthermore, researchers at Kew, the Smithsonian Institute, and many conservation NGOs are all developing collaborations with the Colombian government to explore and help the country move forward with green development.  The Missouri Botanical Garden also has long-standing MoUs for joint research with 3 different institutions in Colombia, with bright prospects for deepening cooperation in the future.

Like many indigenous peoples around the world, the Wayuu are at a crossroads. Their language and some of their traditions are still alive and well, but others have already faded. There are few legal sources of income in the harsh desert, the ancestral Wayuu land. How will they manage in the future? What can they do to adapt?  Some, like our guide, José Luis, are trying to change mentalities, but they clearly need more help.  As throughout Colombia, there is clear and urgent need to build on the alpha-level studies already underway, and move onto applied ecology, agroforestry and land management programs, including community-based restoration programs and ecotourism in conjunction with the national parks.

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Our Wayuu guide José Luis Pushaina Epiayu (on the right) and Macuira park ranger Ricardo Brito Baez-Uriana (on the left), talking about birds with a local Wayuu family.

 

What does the Black-faced Antthrush tell us about tropical forest restoration?

Anna Spiers (University of Colorado Boulder) describes a recent field experiment done with Emma Singer (Hamlin College) and Leighton Reid (CCSD) during an Organization for Tropical Studies Field Ecology Course in Costa Rica.

Bird diversity and forest restoration are synergistic. Birds facilitate forest regeneration through seed dispersal, pest control, and pollination. Forest restoration replenishes lost bird habitat by providing food, protection from predators, and suitable territory for breeding and nesting. Monitoring bird communities in a regenerating forest is an effective strategy to gauge the success of restoration.

While some birds are flexible regarding the quality of their habitat, others require a narrower set of conditions to survive. One such bird is the Black-faced Antthrush (Formicarius analis), a medium-sized, ground-dwelling insect-eater, easily distinguished by its plaintive song and chicken-like strut. The bird spends its days flipping over leaves and sticks with its bill to expose tasty ants, beetles, and other arthropods (and sometimes small vertebrates). A member of a bird family highly threatened by forest fragmentation (Formicariidae), the Black-faced Antthrush is known to disappear from small forest fragments and to struggle crossing even narrow strips of open space. Finding such sensitive birds in a regenerating forest is a positive signal that forest restoration is increasing habitat for forest-dependent species.

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Black-faced Antthrush (Formicarius analis) strutting across the rainforest floor. Image: Luke Seitz/Macaulay Library at the Cornell Lab of Ornithology (ML54054261).

Earlier this month, we did an experiment to find out how different forest restoration strategies affect the Black-faced Antthrush. Specifically, we tested whether the bird exhibited a stronger territorial response in tree plantations, naturally-regenerating secondary forests, or areas where patches of trees (tree islands) had been planted to stimulate forest recovery. We expected to find that birds would be more defensive of areas where trees had been planted, given that these areas had a more closed canopy and more leaf litter for the birds to pick through for arthropods.

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Leighton holds up a speaker to conduct a bird call playback. Unsurprisingly, there was no response in this scrubby, abandoned pasture (one of the control points in our experiment). Image: Martha Bonilla-Moheno.

To test the bird’s territorial response, we amplified a locally-recorded sound file of the bird’s vocalization and recorded its response. We noted how long it took for the bird to respond, how many notes it sang in response, and how close it approached the speaker. For this species, a short call with 4 notes is a “hello”, but a long call with upwards of 12 notes is a warning to let the other birds know that this territory is taken.

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Our study area at Las Cruces Biological Station in southern Costa Rica. Each of the two restoration sites contained a tree plantation, a natural regeneration area, and a “tree island” area where patches of trees were planted to kick-start forest recovery. Image: Google Earth 2018.

Antthrushes defended restoration areas where trees were planted

As we expected, Black-faced Antthrushes responded more quickly and more forcefully when we taunted them with calls broadcast from tree plantations and tree island plantings – an indication that they were expending more energy to defend these areas. However, we only found this at one of the two restoration sites. The other site was a veritable antthrush desert with not a single response during any of our trials. Leighton’s collaborator Juan Abel Rosales often finds Black-faced Antthrushes at both sites, but this second site is near a road and dogs occasionally wander into the regenerating forest, possibly causing birds to temporarily abandon this area.

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Black-faced Antthrushes responded quickly and with many tooting notes when we played their song to them from tree islands, plantation, and mature forest, but they responded not at all in abandoned pastures or in natural regeneration. The data representing restoration treatments are from one site only – at the other site we recorded no birds during any trials.

Tree islands and plantation had a couple of habitat features that natural regeneration lacked. First, the understory was more open, providing ground-dwelling birds with greater visiblity. Second, planted areas also had deeper leaf litter, and leaf litter is essential for a bird that makes a living flipping leaves to find its dinner.

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Understory comparison between natural regeneration (left) and a tree plantation (right). Both have been recovering for 15 years. Natural regeneration vegetation is thick and still grassy from pasture days. A closing canopy in the tree plantation produced a thinner, more visible understory, with lots of nice leaf litter, full of delicious arthropods.

So what does the Black-faced Antthrush tell us about forest restoration?

 It may be telling us two things. First, restored forests growing up alongside remnant ones can be valuable habitat worth defending. When birds spend time calling, that is time that they do not spend foraging, and they can pay a price with their energy budget. Second, tree planting may create habitat for these birds faster than natural forest regeneration – although natural regeneration is highly variable from site to site, and we only found a pattern at one site right next to an old-growth forest. Promisingly, we did not see a difference between tree islands and the tree plantation, which suggests that we could plant fewer trees and still see the return of a forest-dependent bird species within about 15 years.

For more information about the Islas Project (with the tree islands) see previous NHER posts here, here, and here. Thanks to Bert Harris for some of the ideas that we used in this project!

 

 

Native tree seedlings grow best near existing forest and beneath shade in highland Madagascar

A team of MBG scientists describes a recent experiment to grow native trees in a degraded part of Madagascar’s central highlands.

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The dissected landscape of the Tampoketsa de Ankazobe in central Madagascar. Imagery: Google Earth (2018).

Seen from space, parts of Madagascar’s high plateau look like a wizened, grayish-pink brain drying in the sun. Thin, dark lines demarcate nooks and crannies – nearly the only places where bits of forest remain.

Formerly, the forests here covered more territory. Just how much territory is debated; ancient grasslands are also present in highland Madagascar. But in this area, about three hours northwest of the capital, many forests have been cleared, burned, and converted to new grassland within living memory.

To restore forests to their recent extent would benefit a range of species, including Schizolaena tampoketsana (a threatened, micro-endemic tree) and an undescribed species of fat-tailed dwarf lemur. However, restoration has been easier said than done so far. Natural forest regeneration is slow to non-existent, even near remnant forests where fire is excluded. Planted tree seedlings grow only millimeters each year, if they survive at all. Adding fertilizer seems to inhibit seedling growth. Inoculating seedlings with mycorrhizal fungi seems promising, but we are not yet sure if this will make a difference in the field.

Following a field trip in November 2016, we decided to test a couple of other tactics for growing native trees on this weathered plateau. First, we tested planting trees near existing forest. Being near the forest could help young seedlings by shading them from the hot sun or by sharing beneficial microorganisms. Second, we put shade structures over some of tree seedlings to test how much the bright, hot sunlight prevented tree growth.

We tested four tree species: Baronia taratana (Anacardiaceae), Nuxia capitata (Stilbaceae), Uapaca densifolia (Phyllanthaceae), and Eugenia pluricymosa (Myrtaceae).

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An experiment with native tree seedlings at Ankafobe, a small forest fragment on the highlands northwest of Madgascar’s capital, Antananarivo. Photo: Chris Birkinshaw.

Three of the tree species survived and grew more when we planted them next to the forest. The fourth, Nuxia capitata, was a super species and grew relatively well wherever it was planted.

Two of the four species also survived more often beneath shade structures. But interestingly, this shade effect did not completely account for the effect of proximity to forest. That suggests that shade is important for protecting young seedlings from the hot sun, but something else is going on too. Perhaps trees growing next to the forest get a boost of water, since remnant forests sit at the valley bottom where water collects. If this is true, then tree seedlings might do well in any valley bottom, not just ones with remnant forest in them.

Our study site, called Ankafobe, is only a small area, so it would be a stretch to generalize our observations to the entire region. However, we are not the only ones to have done such a test in this ecosystem. In 2000, Ingar Pareliussen led a study with the same basic elements as ours at a site ten kilometers away, at Ambohitantely. Like us, Pareliussen’s team found that seedlings planted near the edge of a remnant forest grew better than those planted further away. In contrast, shade structures did not improve seedling survival. In fact, one species grew worse in the shade.

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One vision for landscape-scale forest restoration on the Tampoketsa de Ankazobe. We used an edge-detection algorithm in Inkscape to highlight forest edges and valley bottoms, the places where trees grew best in our study. Imagery: Google Earth (2018).

Together, our two studies begin to suggest the outlines of a vision for landscape-scale forest restoration on the high Tampoketsa de Ankazobe. If native tree seedlings perform better along forest edges, it follows that a cost-effective strategy would be to focus on planting those areas first, leaving the higher, drier areas alone. Planting along edges would also be a conservative strategy given our hazy understanding of past landscapes. Some grasslands in highland Madagascar seem to be very old, and planting trees in such places could destroy habitat for grassland species, which are threatened in their own right.

For more information about this experiment, you can read our open access paper in Plant Diversity. We have also published several other blog posts about Ankafobe.

The ephemeral forests of southern Costa Rica

Damaged ecosystems don’t recover overnight, but sometimes that’s all the time that they get. CCSD scientist Leighton Reid describes new research about tropical secondary forests in southern Costa Rica, including how long these young forests last, what’s at stake, and how we can keep them around longer.

Regrowing tropical forests on marginal farm lands is one of the main ways that humans can prevent runaway climate change. With ample moisture and long growing seasons, tropical trees often can grow quickly and pull large amounts of carbon out of the atmosphere, storing it in their wood and keeping it from trapping heat. At the same time, young forests provide habitat for plants and animals and improve water quality for humans, among many other benefits.

But even in a moist, tropical climate, trees don’t grow instantly. Typically, it takes many decades for a recovering forest to stock up all of the carbon that it can hold. And it can take even longer for some plants (like orchids) and animals (like antbirds) to return. If a forest starts to grow back, but then someone cuts it down again, these time-dependent benefits never accrue.

In other words, the hopes and expectations that many people have for young tropical forests depend on young tropical forests growing old. So do they? Our new study suggests not.

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The Coto Brus Valley and Talamanca Mountains in southern Costa Rica. Photo by J. Leighton Reid.

To find out how long secondary forests persist, I teamed up with Matthew Fagan, a landscape ecologist at the University of Maryland Baltimore County, and Rakan Zahawi, director of the Lyon Arboretum, as well as two students, James Lucas at Washington University and Joshua Slaughter at UMBC.

We studied a set of historical, aerial photos from southern Costa Rica, which covered the time period from 1947-2014. Previously, Zahawi and colleagues had classified which areas in each photo were forest and which areas were farms or other non-forest land uses. By comparing the maps they made for each year, we were able to see where and when new forests appeared and how long they remained as forest before they were converted to some other land use (mostly farms).

The young forests did not last long. Half of the new forests disappeared before they were 20-years old. And 85% were cut down before they were 54-years old. Larger forests and forests near rivers lasted longer.

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An isolated forest fragment surrounded by cattle pastures in southern Costa Rica. Photo by J. Leighton Reid.

First, the bad news. Twenty years is not even close to the amount of time it takes for a young forest to become as diverse as an old-growth forest. For example, vascular epiphytes like orchids and bromeliads take more than 100 years to fully recover in young forests.

Carbon storage will also take a hit. If forests elsewhere in Latin America are as ephemeral as forests in southern Costa Rica, then carbon stocking over the next thirty years may be reduced by an order of magnitude.

Ephemeral forests could just be a problem in Costa Rica, but another study shows that secondary forests in eastern Peru have even shorter lifespans. There, secondary forests are cleared at a rate of 3-23% per year. Compared to that, the 2-3% per year rate of loss in southern Costa Rica is considerably better. And that’s not a good thing. Clearly we need more research on secondary forest persistence from other places.

There is some good news, though. Even though many new forests were short-lived, the ones that survived were predictable. And if we can predict where new forests will survive, we should also be able to help them survive longer. Larger forests and forests close to rivers were cut down less often than small forests and forests far from rivers. This suggests that restoring large, riparian forests could be a smart investment.

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Forests and cattle pastures in southern Costa Rica. Photo by J. Leighton Reid.

Governments and other organizations can also help forests persist by creating incentives for long-term forest management, providing resources to enable long-term management, and ensuring that local people will be able to enjoy the benefits that old forests provide.

We hope that this work will lead to stronger restoration commitments. Right now, dozens of countries are setting big targets for forest restoration. For example, in 2012 Costa Rica committed to restore a million hectares of degraded land by 2020 (an area about one fifth the size of the country). There is a great opportunity for Costa Rica and other ambitious countries to plan for long-term forest restoration.

If we can begin to restore a million hectares of forest by 2020, why not plan to restore a million hectares of 100-year old forest by 2120?

Melissa's Meadow, Las Cruces Biological Station, Costa Rica

A trail through secondary forest at the Las Cruces Biological Station in southern Costa Rica. Photo by J. Leighton Reid.

For more information on this research, you can read our open-access paper in Conservation Letters or watch a video of Leighton Reid presenting to the Association for Tropical Biology and Conservation back in June. Additional papers on restored ecosystem persistence are available here and here. This work is a product of the PARTNERS (People and Reforestation in the Tropics: a Network for Research, Education, and Synthesis) Working Group on Spatial Prioritization. Funding was provided by grant DEB-1313788 from the U.S. National Science Foundation’s Coupled Human and Natural Systems Program.

Rules of thumb for tropical forest restoration

Sometimes farmlands quickly regrow tropical forests on their own, but other times they don’t. Dr. Karen Holl, a professor at the University of California Santa Cruz, gives some rules of thumb for when we can save money on tropical forest restoration by letting nature do the work, and when we may need to invest in tree planting.

Ambitious targets are being set to restore tropical forest because of their importance in storing carbon, regulating water cycles, conserving biodiversity, and supporting the wellbeing of people who live in tropical countries. For example, the 20 × 20 Initiative aims to restore 20 million hectares of tropical forest in Latin America by 2020. This represents an area slightly smaller than the country of Ecuador. One big question is: How are we going to restore forests at this scale with limited funds?

One of the cheapest ways to restore forest is to let nature do the work and leave forests to recover on their own. This works in some sites where forests regenerate quickly. In other cases, usually sites that have been used intensively for agriculture, the land may be covered by tall grasses (up to 3 meters, or 10 feet high) for years. Our past research shows that even within a small region, the rate of natural forest recovery varies greatly.

SideBySide

Natural forest recovery is highly variable in southern Costa Rica, even after a decade of recovery. Left: slow recovery on a former farm, still dominated by non-native grasses, with an open canopy and little tree recruitment. Right: speedy recovery on a former farm, with virtually no grass cover, a closed canopy, and diverse tree recruitment. Photos by Andy Kulikowski.

So, how do we predict which sites will recover quickly and which ones need some help in the form of clearing pasture grasses and planting trees? If we could develop some rules of thumb it would help land managers to more efficiently allocate scarce restoration funds.

To answer this question, we drew on our long-term study on tropical forest restoration in southern Costa Rica. We have research plots at 13 different sites where we removed the land from agriculture and let the forest recover on its own. Each year we measure grass cover, tree canopy cover, and how many and what species of new tree seedling establish in the plots. We have also quantified the forest cover surrounding the plots, the nutrients in the soil, and how long cows had grazed the sites in the past.

We found that two easy-to-measure variables explained on average two-thirds of variation in forest recovery 7 years later; those were the amount of grass cover and tree canopy cover measured after only 1.5 years. Plots that had more canopy cover and lower grass cover early on had a closed tree canopy and lots of forest tree seedlings from many species after nearly a decade. We were surprised that the amount of surrounding forest cover and soil nutrients did not explain much of the variation in forest recovery.

GraphAbs

Rules of thumb for predicting tropical forest regeneration on farmlands. Forests grow back quicker when there is not too much grass, a little bit of shade, and many tree seedlings already present. Illustrations by Michelle Pastor.

Of course, our results need to be tested in other recovering tropical forests. But, if they hold true, this is good news! It means that land owners and managers just need to wait a year or two and then measure the tree canopy and grass cover. If some trees have established and are starting to shade out the grasses, land managers can use the low cost method of leaving the site to recover naturally. If the site is mostly a monoculture of dense grass, then the site is a good candidate to plant native trees. Planting trees takes more resources since it is necessary to clear around the native tree seedlings for a couple of years until they grow taller than the grasses. At least now there are some general guidelines to help chose where to invest the extra effort.

For more information, see our new paper in Applied Vegetation Science. This work was supported by the National Science Foundation.

How to grow instant fig trees to restore rain forests in Costa Rica

CCSD scientist Leighton Reid and Lyon Arboretum director Rakan Zahawi write about giant fig tree cuttings: how to make them and why some grow better than others.

Choosing the right species to include in a restoration project is a hard choice, but in the economy of nature, some species earn a bigger ROI than others. For example, Pacific sea otters maintain kelp forests by eating sea urchins, and wolves in Yellowstone National Park allow aspen groves to regenerate by scaring away tree-munching elk. These vital creatures are called “keystone species” because they hold ecosystems together, much like the keystone in an arch.

KeystoneSpecies

A keystone and three keystone species. (A) This small keystone holds up an arch in the Shoenberg Temperate House at Missouri Botanical Garden. (B) Sea otters are keystone predators in kelp forests. Photo by Marshal Hedin CC-BY 2.0. (C) Gray Wolves are keystone terrestrial predators. Photo by Gary Kramer USFWS CC-BY-NC 2.0. (D) A keystone fig tree feeding a Knobbed Hornbill in Sulawesi, Indonesia. Photo by T. R. Shankar Raman CC BY-SA 3.0.

Plants can be keystone species too. Around the world there are about 800 species of fig trees, and they hold tropical forests together by providing food for a wide array of animals. On any given day, the busiest tree in a rain forest is likely to be a fig tree with fruits. Monkeys, birds, bats, and others gather at fig trees to eat, and in the process, they deposit seeds of other plant species that they have been carrying in their guts. This chain of events, repeated day after day, often turns the area beneath a fig tree into a hotspot of plant diversity.

A few years ago, we had an idea to plant keystone fig trees in young forests in Costa Rica. We wanted the figs to grow as fast as they could, so instead of planting seedlings, we planted cuttings – big ones. With help from our local collaborator, Juan Abel Rosales, we cut dozens of twelve foot-long branches from eight species of fig trees. We stripped off all of their leaves to keep them from drying out, and then we planted our figs trees in shallow holes.

FicWhat

Rakan Zahawi (delighted!) poses with a three year-old fig stake.

To our delight, many of the fig trees grew!

The ones that did the best came from a special group, the subgenus Urostigma. Many figs in this group have a unique life strategy. They begin their lives in the top of a tree when their tiny seeds are deposited on a branch by a bird or some other animal. As they grow in the treetop, they send long roots down to the ground, and these roots harden and fuse together, forming a lattice-like trunk. Over time, these figs kill their host trees by taking most of the water, nutrients, and light. They also keep the host tree from growing outwards, giving them the nickname “strangler figs”. Maybe the ability to transform a flimsy, dangling root into a solid trunk is related to these figs being able to grow from cuttings.

To find out how well our planted fig cuttings might survive over the long-term, we also tracked down some fig cuttings that we had planted in 2004. We were happy to learn that out of the trees that survived for their first three years of life, all of them were still thriving a decade later.

Full disclosure: planting large cuttings is not a new idea.  Farmers in many parts of the tropics plant trees this way to create ‘living fences’ – with all of the normal fixings like gates and barbed wire, but with a row of living trees instead of dead posts. The advantages for farmers are many – their fences don’t rot and fall apart (that happens quickly in the tropics); the trees provide shade for cattle; they have a constant source of new fence posts (by cutting off a limb); and in some cases they can feed the young shoots to livestock.

Big cuttings have big benefits for restoration too. Not only are planted trees already several feet tall, you also get to skip the pricey nursery phase, and, most excitingly, cuttings have a tendency to fruit quickly.

Some of our young fig trees are now making fruit, but we will have to wait a bit longer to see whether they start attracting more big animals and whether those animals carry more tree seeds into our young forests. For now, we can say that others who are interested in growing keystone figs for forest restoration may have the best luck by working with the stranglers.

For more information, please take a look at our open access paper on this project in Perspectives in Ecology and Conservation and prior blog posts here, here, and here.

FigProduction

How to grow an instant fig tree. (A) Remove a long, thin branch segment from an adult tree. The red arrow shows a cut branch. (B) Strip the cuttings of their leaves to keep them from drying out, then carefully transport cuttings so as not to damage cortical tissue. Here, cuttings are padded by a foam mattress. (C) Remove the bark from a ring on the cutting to promote root growth. Here, a ring is being cut about 20 cm (8 in) above the base so that it will be just below the soil surface when planted. (D) Dig a shallow hole and plant the cutting. Be sure that the cutting is firmly planted to prevent it from toppling, but take care not to compact the soil too much around its roots. Photos by Rakan Zahawi.