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.

Advertisements

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.

expdesignnher

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.

om-i3

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.

wheelwright_nher

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.

landscape

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.

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.

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 launch a drone at a site in Costa Rica [Photo courtesy of Karen Holl].

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].