Tree islands for tropical forest restoration: the outlook is rosy after 10 years

Planting tree islands has many of the benefits of larger plantations, but entails significantly less cost. Karen Holl (University of California, Santa Cruz), Leighton Reid (Missouri Botanical Garden), and Zak Zahawi (American University of Beirut) describe recent findings on tree seedling recruitment in a long-term experiment in southern Costa Rica.

Over the past few years there have been a growing number of commitments at the global, national and regional scale to restore forests because of their importance to conserve biodiversity, sequester carbon, reduce erosion, and provide goods and services to people. For example, Initiative 20×20, led by the International Union for the Conservation of Nature, aims to restore 20 million hectares of tropical forest by 2020, an area roughly equivalent to the size of Uruguay or Nebraska.

A common strategy to restore forests is to plant trees. But, the big question is: where will the money come from to plant billions of trees when there are so many pressing needs? As restoration ecologists, we started thinking about how we could most efficiently allocate resources to get the best bang for the buck and restore the largest area of forest.

Trade-offs in forest restoration strategies. Planting fewer trees leaves more to chance and can require more time, but tree plantations are more expensive and leave a bigger ecological footprint. Our study tests an intermediate option, and after 10 years it appears to provide a good balance. Figure modified from Corbin & Holl (2012).

Starting over 10 years ago, we set up a large-scale tropical forest restoration experiment in southern Costa Rica to test two ideas.

First, we tried planting tree “islands”. The idea is to plant groups of trees that attract birds and bats, which disperse most tropical forest tree seeds. The tree canopy also shades out light-demanding grasses that can outcompete tree seedlings. In one experimental treatment, we planted tree islands that covered about 20% of 50 × 50 m plot of former cattle pasture. We compared that to plots where no trees were planted (natural recovery) and to the more intensive (and more typical) restoration strategy of planting trees in rows throughout the plot (plantation).

Second, we asked: is it only possible to restore forest near remnant forests or can you restore forest anywhere in the landscape? This is important information to help guide forest restoration efforts. To do this we set up our entire experiment at 13 sites, some of which were mostly surrounded by agricultural land and some of which were adjacent to the largest remaining forests in the region.

Then we monitored establishment of new tree seedlings in our research plots over a decade. We compared the number of seedlings, number of species, and types of species in the restoration plots with those found in the nearby forest to evaluate how well the forest is recovering.

The tree island planting method not only saves money on buying, planting, and maintaining seedlings, but it also results in a more heterogeneous distribution of trees, so it looks more like a natural forest.

Profuse tree seedling and sapling recruitment in the understory between two tree islands in southern Costa Rica.

We counted over 6000 tree seedlings, 88% of which have seeds that are dispersed by animals. On average there were many more tree seedlings in the tree island and plantation treatments than in the natural recovery plots. These results suggests that some tree planting helps the forest to recover faster, but that it is not necessary to plant the whole area with trees. The tree island planting method not only saves money on buying, planting, and maintaining seedlings, but it also results in a more heterogeneous distribution of trees, so it looks more like a natural forest.

Even though there were many tree seedlings in the island and plantation plots, on average there were less seedlings of tree species that have big seeds (>0.5 cm/0.2 inches across) compared to mature, reference forests. It seems that the larger-seeded species that are common in mature forests are much slower to colonize restored sites, likely because they are eaten and dispersed by a small number of larger animals, such trogons and agoutis. Many of those dispersers are less likely to visit early successional forest.

Small frugivores, small seeds. Most of the birds we see in these experimental plots are small-gaped omnivores (e.g., Yellow-bellied Elaenia, Elaenia flavogaster, left), but it usually takes large-gaped species to disperse larger seeds¹. The figure at right shows the maximum fruit size that a bird species with a given gape size was able to consume in a cloud forest in central Costa Rica (modified from Wheelwright (1985)). In our experiment, small seeds were ubiquitous, but large seeds were mostly absent.

We were surprised that the amount of forest cover around the experimental plots had a weak effect on the number of seedlings establishing. In other words, isolated plots had just as many tree seedlings as plots right next to old-growth forests. We think that this is likely due to the fact that there are many trees in the agricultural landscape surrounding our plots; these trees include remnant trees, living fence rows, and riparian corridors. Trees in the landscape can serve an important role in both providing sources of seeds and stepping stones for the movement of seed-dispersing fauna. We anticipate that having forest nearby will be more important in future years as these forests build up greater diversity of rare, large-seeded species. Nonetheless, our results suggest that there are good prospects for restoring forests in many locations in this landscape.

Our key finding is that planting tree islands can be a cost-effective way to restore tropical forests at our study site in Costa Rica, but we hasten to note that the strategy should be tested in other locations, particularly areas with fewer forest elements in the surrounding countryside. Our study also demonstrates that tropical forests can recover some species quickly but it will take many decades, if ever, for forests to fully recover. So, preserving existing rain forests is critical to conserve biodiversity and the services they provide to people.

Diverse tree cover in an agricultural landscape in southern Costa Rica. Remnant trees in pastures, trees along fence rows, and riparian forests provide important sources of flora and fauna to speed up forest recovery.

¹See Melo et al. (2009) for an example to the contrary: small-gaped animals dispersing fairly large fruits and seeds.

This work was supported by a grant from the National Science Foundation.

Fig Stakes: Shoreline Restoration for a Costa más Rica

Andres Santana is the graduate program coordinator at the Organization for Tropical Studies. During a recent fieldtrip in southern Costa Rica, he and CCSD restoration ecologist Leighton Reid compared notes on using fig stakes for ecological restoration.

Tropical beaches are many things to many people. To plants, beaches are hot, sandy, and salty – complicating their restoration.

Costa Rica has 1228 km (763 mi) of coast line – including 1016 km on the Pacific side and 212 km on the Caribbean. Along Costa Rica’s northern Pacific coast, the beach forms the natural edge of the dry forest. Farther south the adjacent forest is more humid. Giant trees, 40 m or more in height, grow right up to the high tide mark, particularly along the Caribbean.

But as with so many tropical ecosystems, Costa Rica’s coastal forests have been subject to human impacts. Many shoreline forests were cleared for cattle ranching, and exotic grasses were introduced as forage. Some of these grasses are fierce competitors and prevent tree seedlings from establishing, even long after the pastures have been abandoned.

Playa Hermosa, before (left) and after (right) planting 2-m long cuttings of a coastal fig species (Ficus goldmannii).

In 2009, a small non-profit organization, Costas Verdes, was formed to restore coastal forests along degraded shorelines, particularly wildlife refuges. The restoration work was initially challenging; tree seedlings were hard to establish along the coast because of the harsh environment – high temperatures and salinity and lack of freshwater were among the most significant obstacles. Not to mention the invasive cattle forage grasses.

Coastal restoration at Playa Hermosa

Playa Hermosa, a surfing destination on the Central Pacific coast, was among the most heavily deforested project sites. This area, part of a wetland and river estuary, was declared a national wildlife refuge in 1998. By 2009, very little forest had naturally regenerated. This led Costas Verdes to implement a restoration project at this beach. Planting plots were established where invasive grass was removed. In other areas, grasses left intact, as a comparison. It quickly became evident that tree seedlings were outcompeted by the grass. Those in the cleared plots grew better, but they still faced the other coastal habitat challenges.

Some native trees are resistant to hot substrates and high salinity, but these species were not available in tree nurseries, most of which focused on ornamental species. This meant that seedlings needed to come from locally collected and germinated seeds. We realized that this would take time to get going. Tree seedlings under 50 cm rarely survive, even if they have the proper coastal adaptations.

To accelerate the restoration, we decided to use tree cuttings rather than growing seedlings from seed. A colleague suggested Ficus goldmannii as a candidate species, so in 2011 we conducted a planting trial. We planted 225 2-m long cuttings. Of these, 195 (87%) survived their first year. By the second year all 195 survivors had become established and were quickly providing canopy cover and lowering the temperature of the sand.

An established fig stake with a dense canopy. Note the weak, patchy grass below it.

Once fig stakes created some canopy cover, we brought in other tree species – mostly from the coastal tree nursery that we created. Shade from the fig canopy also began to inhibit the invasive grasses, which require high sunlight to photosynthesize efficiently. Reduced competition with these grasses allowed other tree seedling species to survive.

In this instance Ficus cuttings turned out to be useful in promoting restoration. We have since used cuttings for other plots with similar success.

Coastal trees and shrubs growing below established fig cuttings at Playa Hermosa.

Reforesting with Figs

 Benjamin E. Smith is a Ph.D. student at George Washington University. He recently completed a field ecology course with the Organization for Tropical Studies in Costa Rica, where he worked with CCSD scientist Leighton Reid. When he’s not coring fig trees in Costa Rica, Benjamin studies plant-herbivore interactions in American chestnut.

It was my recent privilege to spend a week at Las Cruces Biological Station in Costa Rica where I learned about some amazing properties of fig trees.

The genus Ficus contains over 800 species, which can be found in the tropical to warm temperate regions throughout the world. Where they occur, figs are vital components of their local ecosystems because they provide high quality fruits for many animals. Animals attracted by the delicious figs often carry other plants’ seeds in their digestive tracks and subsequently deposit them below the fruiting fig tree. This can lead to patches of forest with especially high plant diversity.

Two individuals of Ficus obtusifolia demonstrating the strangler lifestyle (left) and the free-standing lifestyle (right). The individual on the left has overtaken one host tree and is reaching out to claim another. The individual on the right was planted (either by humans or birds) in a fence row.

Some fig species have the ability to resprout roots, branches, and leaves from broken limbs – an adaptation that would be useful in an ecosystem with frequent disturbances, like hurricanes or landslides. Rural people have been utilizing this incredible feat of nature to create living fences for hundreds of years; they simply cut branches from a tree and plant them. Plant a large enough branch, and you’ve got an instant tree.

Instant fruiting trees could be a practical tool for ecological restoration, and there is currently an experiment underway to test this idea. But not all fig species can resprout from cuttings, so in order for this tool to be useful outside of southern Costa Rica, it would be helpful to know which species will resprout and which will not.

A healthy cutting of Ficus colubrinae. This instant tree was planted in May 2015.

Does wood density predict resprouting in figs?

We sought a way to determine whether a particular fig species would be able to resprout from a limb cutting before actually cutting apart large trees. This would mean only trees whose cuttings will survive would be used and trees that can’t resprout could be left undamaged.

We believed that wood density would be a good measure to figure this out. Wood density can tell you a lot about a tree’s life history strategy. Is it a hard tree that will resist snapping in a stiff breeze? Or is it a softer tree that might break, but then resprout?

To test this, we took core samples from seven fig species and headed to the lab. After a couple days of measurements, we had our data.

(A) OTS student Orlando Acevedo Charry extracts a core from a Ficus colubrinae. (B) Cores were cut into small pieces. We measured the mass of the water that each segment displaced to determine the wood’s green volume. (C) Next, samples were placed in a drying oven at 106° C for 24 hours. Finally, we measured the mass of the dried samples and divided by the green volume to determine wood density.

The fig species we tested turned out to have pretty similar wood densities. Also, the slight variations in wood density did not correlate with trees’ resprouting abilities. This initially came as a big disappointment, but after taking a second look at our data we started to see a trend that may actually be much cooler.

Wood density was a poor predictor of resprouting capacity (measured by tallying fig cuttings that were planted in April-May 2015; Left), but strangler figs in the subgenus Urostigma performed much better than two free-standing species in subgenus Pharmacosycea.

Fig species come in a variety of forms. Some are rather conventional free-standing trees that grow from the ground up, but others start as seedlings high in the canopy of another tree and send roots down to the ground, gradually strangling their host. Still others are shrubs, climbers, and epiphytes. We found that stakes cut from strangling figs, the ones that initially rely on a host tree, were much more likely to resprout than stakes cut from free-standing fig species. If this holds true, no measurements will be needed in the future. People around the world may be able to tell if a tree will likely sprout from a cutting just by the way it grows.

Monitoring epiphyte colonization in Costa Rican forest restoration

Leighton Reid and Miguel Chaves are investigating how tropical forest restoration influences plant diversity. Leighton is a postdoctoral fellow in the Center for Conservation and Sustainable Development. Miguel is a doctoral student at University of Missouri Saint Louis.

Epiphytes are plants that live non-parasitically on other plants. That is, they grow on the trunk or branches of another plant (often a tree) without extracting nutrients from it, as mistletoes do. In Missouri, one example is the resurrection fern (Pleopeltis polypodioides), an epiphyte famous for its ability to re-green after lengthy desiccation.

In tropical forests, epiphytes are much more diverse. Science writers commonly use the word “festooned” to describe the profuse growth of aroids, bromeliads, ferns, and especially orchids on tropical trees. In certain places, epiphytes can make up as much as 50% of a forest’s vascular plant species.

We were curious about how ecological restoration influences epiphyte communities, so over the summer Miguel Chaves worked with local conservationist Juan Abel Rosales and botanist Federico Oviedo to survey the vascular epiphyte composition and abundance on 1086 trees growing in thirteen restoration sites in southern Costa Rica. They found about one hundred species, several of which are depicted below.

This fall, we are analyzing these data to learn about how tree planting influences epiphyte community assembly compared to natural forest regeneration. In particular, we hope to shed light on two questions:

(1) To what degree does tree planting facilitate epiphyte recovery?

(2) At what spatial scale does local forest restoration interact with landscape context to influence epiphyte recolonization?

The base of this poro tree (Erythrina poeppigiana) has sufficient ferns to warrant the descriptor “festooned”. Ferns visible in this photograph include: Niphidium crassifolium, Serpocaulon fraxinifolium, Serpocaulon dissimile and Polypodium dulce.

A showy orchid (Dichaea cryptarrhena) hangs from a mossy bed below two bromeliads.

An inflorescence of Drymonia macrantha (Gesneriaceae).

Miguel and Juan Abel at work next to a particularly good-looking bromeliad (Guzmania zahnii). Photo by Karen Holl.

Plants that use other plants for support but also have roots in the ground are called “hemiepiphytes”.

These aroids (Monstera sp and Syngonium hastiferum) are secondary hemiepiphytes, that is, plants that begin life on the forest floor and climb upwards towards the light.

Many of the epiphytes that Miguel and Juan Abel observed were flowerless seedlings, like this Gongora armeniaca (Orchidaceae).

Transplanted bromeliads improve microclimate and facilitate arthropods in restored forests

Estefania Fernandez is a masters student at the University of Montpellier, France. She spent the past six months working with scientists in the Center for Conservation and Sustainable Development on a tropical forest restoration experiment in southern Costa Rica.

Costa Rica is one of the world’s most biodiverse countries, hosting 4% of flowering plant species in an area representing only 0.03% of the Earth’s terrestrial surface. With a large diversity of ecosystems, ranging from mangroves to cloud forests, Costa Rica hosts a unique family of (almost exclusively) Neotropical plants: the Bromeliaceae, commonly called bromeliads. With their colorful inflorescences and strikingly patterned leaves, numerous bromeliads are cultivated around the world for their ornamental value. Less is known, however, about their ecology in tropical ecosystems, particularly in regenerating forests.

Werauhia gladioliflora rosette, showing its overlapping leaves.

Many of the so-called “tank bromeliads” are epiphytes, meaning that they grow non-parasitically on other plants. These bromeliads have ample rosettes of overlapping leaves, capable of holding considerable amounts of water. These water tanks keep them hydrated, and plant detritus that accumulates in these structures also provides bromeliads with nutrients. Arthropods take refuge in bromeliad rosettes, and consequently these plants attract mammals and birds seeking prey. Mutualistic ants build their nests in bromeliad rhizospheres, or root zones, and frogs lay eggs in the tanks. When sufficiently numerous in tree canopies, bromeliads can stabilize local temperature and humidity.

Water stored inside a W. gladioliflora tank. (Photo courtesy of Dave Janas)

Despite these important ecological roles, vascular epiphytes like bromeliads are often scarce in regenerating tropical forests. Their recovery could be slowed by limited seed dispersal or by a lack of suitable recruitment sites. One way to overcome dispersal limitation is to transplant individuals. In our study area in southern Costa Rica, transplanting bromeliads is relatively simple because they are easily found on fallen tree branches in the old growth forest reserve at Las Cruces Biological Station. We hypothesized that transplanting bromeliads from the old growth forest into 10-year old forest restoration sites would buffer local temperatures and increase arthropod abundance and diversity compared to bare, control branches.

Measuring local temperature in a transplanted Aechmea dactylina.

To test our hypothesis, we transplanted 120 bromeliads into three restoration sites in southern Costa Rica. The restoration sites are part of the Islas Project, an NSF-funded restoration experiment led by Drs. Karen Holl and Rakan Zahawi. Bromeliads were sterilized and attached to tree branches in the restoration sites with twine. Each day, we measured the microsite temperature on branches with and without transplanted bromeliads, as well as ambient temperature in the nearby air. To characterize arthropod colonization, we extracted and identified arthropods (to order) from transplanted bromeliads after two and three weeks.

We found that transplanted bromeliads decreased local temperatures on tree branches, creating a less stressful microclimate for other organisms. Bromeliads also facilitated arthropods; transplanted bromeliads were quickly colonized, especially by ants. We also observed small frogs inside of some bromeliad tanks, but none on the bare branches where we did not transplant bromeliads.

We found this frog (Craugastor stejnegerianus) in a small Catopsis sessiliflora tank. (Photo courtesy of Dave Janas)

Our observations suggest that bromeliad transplantation can buffer microclimates and create useful structures for invertebrates. If so, this method could improve restoration outcomes for canopy flora and fauna. Given that this experiment was conducted over a single field season, it is still an open question whether transplanted bromeliads will survive over longer time periods. It will also be important to learn whether transplanted bromeliads will facilitate colonization by other epiphytic plants. We did find some evidence of this as ferns were already growing in several bromeliads’ rhizospheres after two months.

Drones can help monitor forest restoration

Leighton Reid is a postdoctoral fellow in the Center for Conservation and Sustainable Development.

Hexacopter flying over a restoration site. The red, digital camera is visible between the landing bars.

Monitoring restoration projects is important to demonstrate progress and learn what works and what doesn’t, but it can be time consuming and expensive. As such, restoration practitioners around the world are looking to automate tasks like monitoring, and one way this can be done is with unmanned aerial vehicles, or drones.

Over the past two years I’ve worked with a research team in southern Costa Rica to test how well drones can monitor tropical forest restoration. We used hexacopter drones: helicopter-like contraptions with six rotors. Each drone had a consumer-grade digital camera attached to the bottom. We flew the drones over thirteen restoration sites and then used Ecosynth computer software to stitch the images together and create three-dimensional models of the vegetation structure.

Drones accurately estimated forest structure

Drone-based measurements of canopy height closely matched our hard-won field measurements (but with less sweat and insect bites). The drone-based system also detected canopy gaps, predicted fruit-eating bird movements, and estimated above ground biomass. The ability to accurately assess above ground biomass is particularly important; it suggests that drones could be used to monitor carbon accumulation in regenerating forests.

Editors’ choice – a must read

Our research on drones and forest restoration was published this week in the journal Biological Conservation. The editors selected it as the must-read choice of the month, saying:

“The rapidly expanding use of unmanned vehicles to monitor vegetation and other aspects of biodiversity is an exciting development in conservation biology. This article also demonstrates that bird abundance can be estimated using data gathered by UAVs.”

The paper is freely available for download through August 27, 2015 at the publisher’s website.

Researchers Jonathan Dandois and Dana Nadwodny (University of Maryland Baltimore County) launch a drone at a site in Costa Rica [Photo courtesy of Karen Holl].

Talking Ecological Restoration on the Osa Peninsula, Costa Rica

Over the weekend, thirty scientists and land managers met in a rainforest clearing on Costa Rica’s Osa Peninsula. We discussed tropical, ecological restoration. Among the attendees were some of the luminaries of tropical biology and conservation. Expertise ranged from forestry to power grid infrastructure to loan financing to mangrove snails. Here are a few thoughts that I took from the gathering.

Red-lored Parrot (Amazona autumnalis) in a royal palm (Raphia taedigera)

Tropical forest restoration isn’t just tree planting. Planting trees is a standard restoration practice, but it can become impractical at the enormous spatial scales needed to preserve hundreds of thousands of organisms and their interactions. Moreover, forests are made of more than just trees; fruit-eating animals (like spider monkeys) and meat-eating animals (like jaguars) are vital for maintaining ecologically intact tropical forests. Even flies can do the grunt work of restoration.

Above: Spider monkeys (Ateles geoffroyi) drinking nectar from Ochroma pyramidale flowers. Spider monkeys also disperse tree seeds throughout the forest. One scientist at the workshop quipped that a lowland, Neotropical rainforest without spider monkeys has serious problems.

Restoration doesn’t happen without people. Knowing your neighbors can afford special opportunities. The longer that people have lived in a landscape, the richer their potential as informants and collaborators. Restoration gains ground when it provides things that people want, like clean water, wild nature, and jobs. Working with people in government could be the surest way to achieve restoration permanence.

Watch out for the trap of the ivory tower. Designing restoration experiments is intellectually satisfying, but to make a difference at a regional or global level, these ideas also need to be scaled up and implemented as land management practices. Ideally, restoration research is place-based; that is, it’s designed to meet the unique conservation challenges of its particular landscape using local resources.

-Leighton Reid

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