Planting trees recovers 70 years’ worth of dead wood carbon pools in less than two decades

By Estefania P. Fernandez Barrancos, a PhD candidate in Biology at the University of Missouri – St. Louis and a fellow of the Whitney R. Harris World Ecology Center. Her most recent research paper in Forest Ecology and Management is freely available through March 9^th.

When most people walk through a forest the last thing they probably look at is dead vegetation, and unless you are an avid mushroom harvester you probably don’t even notice dead logs. However, dead wood stores an important amount of carbon. An amount important enough that if dead wood disappeared it could promote more changes to our already rapidly changing climate.

Mushrooms on a dead log. Photo: JL Reid.

Dead wood is also a crucial habitat for many organisms such as fungi, insects, and birds. Many insects and fungi use dead wood as a source of food and nutrients, and several species of birds are only able to nest in dead logs.

A Resplendent Quetzal (Pharomachrus mocinno) exiting its nest inside a standing dead log to go harvest food for its fledglings. Photo: Estefania Fernandez.

Anthropogenic disturbances, such as logging and deforestation, can significantly decrease the amounts of dead wood present on the forest floor, sometimes leading to losses of up to 98% of dead wood. The implications of dead wood loss are potentially warmer temperatures due to the release of carbon contained in dead wood as well as the loss of habitat that is critical to many forest organisms. Tropical ecosystems contain some of the most biodiverse habitats on Earth, yet they are among the ecosystems that suffer the most from anthropogenic disturbance. For example, most forests in the county of Coto Brus in Southern Costa Rica, our study area, were transformed into cattle pasture or coffee plantations in the 1950s-1980s. Today, the landscape consists of a mosaic of cattle pasture, coffee plantations, and small forest remnants.

Deforestation to create farms and cattle pastures has decreased the amount of dead wood in southern Costa Rica. Photo credit: JL Reid.

Forest restoration is the process of assisting the recovery of an ecosystem that has been damaged or destroyed (SER International Standards) and it has a high potential to reverse the problem of dead wood loss through different strategies. In the Tropics, the most common restoration strategies are passive and active restoration. Passive restoration consists of allowing an ecosystem to recover with minimal to no human input.  In contrast, active restoration consists of assisting the ecosystem in its recovery through actions such as tree planting.

Old-growth forest (A) and and two restoration treatments: tree plantations (B) and natural regeneration (C). Old-growth forests are ≥100 years old. Plantations and natural regeneration were 16-17 years old at the time of the study. Photos:  Juan Abel Rosales & Estefania Fernandez.

Recently, I studied the pattern of dead wood re-accumulation through time after disturbance in southern Costa Rica as well as the effectiveness of passive and active restoration at recovering dead wood as it is found in undisturbed forests. To evaluate dead wood accumulation through time, my team and I surveyed dead wood volumes inside 35 forest patches of increasing ages (from 3 to over 100 years old) that were former coffee plantations. We evaluated the effectiveness of active vs. passive restoration at recovering dead wood by surveying dead wood volumes inside 17-year old passive and active restoration plots and inside nearby old-growth forests. Our passive restoration treatment was represented by natural regeneration plots around which fences were established to exclude cattle and where vegetation was allowed to re-establish naturally. Our active restoration treatment was represented by restoration plantations, where seedlings of two native (Terminalia amazonia and Vochysia guatemalensis) and two naturalized (Inga edulis and Erythrina poeppegiana) tree species were planted 17 years ago to facilitate the re-establishment of vegetation. Our reference ecosystem included nearby old-growth forests over 100 years old.

Juan Abel Rosales measures the diameter of dead logs in order to estimate their volume in an old-growth forest in Southern Costa Rica. Photo: Estefania Fernandez.

To measure the diameter of dead, rotting logs, we measured the distance between two tent poles set vertically along the logs’ edges. Photo: Estefania Fernandez.

Jeisson Figueroa Sandí establishes a transect to evaluate dead wood inside a forest fragment. Photo: Estefania Fernandez.

We found that dead wood recovers following a logistic shape through time in our study area: volumes are low initially, increase rapidly, and then plateau. The low volumes of dead wood at the beginning of succession could be explained by the fact that most of the wood remains are typically harvested by local inhabitants after lands are abandoned in our study area. As pioneer trees recolonize abandoned coffee plantations and subsequently die, they produce dead wood. As the forest grows older, there is a mix of short-lived pioneer trees and long-lived trees which contribute to large amounts of dead wood on the forest floor through branchfall and their own deaths.

Dead wood volumes as function of forest age in a chronosequence of secondary forests in southern Costa Rica. Blue dots represent the raw data (i.e. course woody debris, or CWD, volumes per hectare). The red line represents the predicted values from a generalized linear model plotted using a smoothing function. Eight outliers that were included for the analysis where CWD volume per transect was ≥125 m³ha^-1 were removed for better visualization. CWD volumes in plantations (purple dot), natural regeneration (yellow triangle) and five nearby old-growth forests (green dot) are also represented. Mean CWD volumes per hectare for each restoration plot (n=5) and corresponding 95% confidence intervals are shown.

We also found that restoration plantations contain 41% of dead wood amounts found in old-growth forests, whereas natural regeneration only contained 1.7% of dead wood volumes found in old-growth forests. The extremely low recovery of dead wood in natural regeneration might be explained by the fact that our natural regeneration plots were dominated by exotic grasses which typically hamper tree colonization. If there are no trees growing in the plots, there cannot be dead wood either. This is an important finding, because it shows that restoration plantations area a faster and more efficient way to recover dead wood in this fragmented, pasture-dominated landscape, even though this restoration strategy might be more time consuming and expensive due to the costs and time of planting seedlings.

Overall, our study unveils an important forest process, showing that dead wood carbon pools recover following a dynamic logistic pattern through time in this Neotropical forest region. Knowing that dead wood is 50% carbon, our findings allow us to predict carbon stocks in Neotropical forests more accurately. Our study also shows that restoration plantations accelerate the recovery of dead wood carbon pools in this Neotropical ecosystem, and potentially promote the preservation of dead wood-associated biodiversity.

For more information, see our recent paper in Forest Ecology and Management, which is freely available online through March 8^th, 2022.

Do we really need to plant a trillion trees? Tree islands are an ecologically and economically sound strategy to facilitate tropical forest recovery

Karen Holl (UC Santa Cruz) and Leighton Reid (Virginia Tech) describe lessons learned from a 15-year study of tropical forest restoration in southern Costa Rica. Their new paper is published in the Journal of Applied Ecology.

It seems that everybody from business people to politicians to even Youtubers is proposing that we should plant millions, billions, or even trillions of trees. They cite a host of reasons, such as storing carbon, conserving biodiversity, and providing income. These efforts should be done carefully and with a long-term commitment to ensure that the trees survive and to prevent unintended negative consequences, such as destroying native grasslands, reducing water supply in arid areas, or diverting attention from efforts to reduce greenhouse gas emissions.

Another important question is whether we really need to plant that many trees to restore forest. In a new paper in the Journal of Applied Ecology, we summarize some the lessons we have learned about a different approach.

Volunteer plants tree seedlings in one of our plantations in southern Costa Rica. Photo: Karen Holl

Over 15 years ago, we set up an experiment in southern Costa Rica to test whether planting small patches or “islands” of trees could speed up forest recovery for a lower cost than typical tree plantations. The idea is to plant small 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. As a result, over time these tree islands spread as they grow and facilitate the establishment of a lot more trees.

Compared to tree plantations, the tree island approach has two major benefits. First, it better simulates the patchiness of natural forest recovery. Second, it costs much less than planting rows and rows of trees.

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 15 years it appears to provide a good balance. Figure modified from Corbin & Holl (2012).

In our experiment, we planted tree islands that covered about 20% of a 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). We repeated this set-up at 15 sites in 2004-2006.

Over the past 15 years, we have monitored the recovery of vegetation, litterfall, nutrient cycling, epiphytes, birds, bats, arthropods, and more. Our data reveal a few key lessons about how to restore tropical forests more ecologically and economically.

First, our data show that planting tree islands is as effective as bigger tree plantations, despite cutting costs by around two-thirds. Compared to plantations, tree islands have similar recovery of nutrient cycling, tree seedling recruitment, and visitation by fruit-eating animals. Both tree islands and plantations speed up tropical forest recovery compared to letting the forest recover on its own. After 15 years, cover of trees and shrubs in the island planting plots has increased from 20% to over 90%.

Drawing of our three treatments showing a few trees establishing in the natural regeneration plots, the tree island merging canopies merging in the island plots, and the rows of trees in the plantation. Artist: Michelle Pastor.

Second, we have found that larger tree islands are more effective than smaller islands in enhancing the establishment of fauna and flora, as larger tree islands attract more birds and shade out competitive grasses.

Third, while tree islands cost less than plantations, some landowners won’t use the tree island approach because the land looks “messier” than orderly tree plantations. Some people prefer to plant lots of trees that are valuable for timber or fruit, rather than having the diverse suite of species that are typical of a tropical forest. So, the tree island planting strategy will be more suitable in cases where the goal is to restore forest.

Natural recruitment of trees seedling in the understory of a canopy of planted trees.

Our results and those of others show that the tree island planting approach holds promise as a cost-effective forest restoration strategy in cases where there are seed sources nearby to colonize and animals to disperse them, and where the spread of tree islands is not likely to be slowed by fire or invasive species. But we need more long-term studies to judge whether tree islands will be effective in other tropical forest ecosystems and to test other questions, like how the particular tree species used affect forest recovery, or what is the best distance to leave between tree islands.

More broadly, our study shows that tropical forests can recover some species quickly but it will take many decades, or longer, for forests to fully recover. So, preserving existing rain forests is critical to conserve biodiversity and the services that intact forests provide to people.

Yes, carefully-planned tree planting can help accelerate tropical forest recovery. But, in many cases we don’t need to plant trees everywhere. Rather we should use restoration strategies that encourage trees to plant themselves.

To learn more about our research, read our new article in the Journal of Applied Ecology, visit our websites (Holl Lab, Reid Lab), or watch a 7-min. video below.

Karen Holl describes the tree planting restoration approach and our long-term experiment in southern Costa Rica.

Los investigadores principales describen el método de applied nucleation y nuestro experimento a largo plazo en el sur de Costa Rica.