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.