Heredia Province, Costa Rica, March 2015
I am tramping in dark woods, though it’s a sunny day here in Costa Rica. A man’s voice calls out “ocho,” “diez,” and a string of Spanish words. I know the speaker is Bernal, a local forester, though I can barely see him through the dense vines and the stems of young trees. He is calling measurements of red-marked trees to another forester, Jeanette. She stands braced on both feet to write his numbers on her clipboard.
Not far away rises the fabled dense forest of Braulio Carrillo National Park, a jewel of the Central American cordillera and world attraction. From sea level the park ascends to 9,700 feet, its folded slopes covered in tropical forest and clouds.
The ragged hillside we’re on is also typical of the region – and of Earth’s tropics. Here, ancient, primary forests were cut for human use; this slope became a rice farm. In 1986, a scientist, Bryan Finegan, arranged for this and three other plots to be let alone – no farming, no cattle-grazing – to record what Nature would do next. i
What tree species filled in? Which ones lost out? Almost 20 years later, which trees are winning the ongoing ecological game known as “forest succession?” The numbers and species of trees and their width and height can answer many questions: How much carbon is stored in these trees? What ecosystem services are provided, such as water quality, erosion control and wildlife?
Why did I come here? Because annual measurement of trees on this and eleven other one-hectare plots, ongoing for two decades, raises lively questions for tropical land management and global climate policy. ii
As Earth’s tropical rainforest continues to be destroyed at a horrendous rate, the area of former primary forest land expands. In 2010 the FAO counted more than half of all forest land in the tropics as secondary forest. iii
These expanding patches that humans have touched or ruined hold a tantalizing clue because trees could grow there. Rapidly growing trees in young forests soak up more atmospheric carbon faster than old-forest land or urban areas.
So experts are asking: If significant areas of available land were regrown as forest, could they absorb enough carbon dioxide to slow and maybe reverse the rise in atmospheric carbon dioxide concentrations? iv
I met the trio of experienced tree-counters - Bernal Paniagua, Jeanette Paniagua and Enrique Salicette at La Selva Biological Station, whose jungle campus is one of the world’s premier tropical research stations. We rode for about an hour on rutted clay roads, past rocky pastures laced with stands of trees. The trio pointed out trees and birds by their Latin and common names. They stopped when Enrique spotted a new flower, so he could pluck and add it to La Selva’s tropical plant collection
I was the guest of the Organization of Tropical Studies (OTS), a research organization in Costa Rica and the United States.v I had extraordinary help from the originator and lead scientist of this project, Robin L. Chazdon of the Department of Ecology and Evolutionary Biology at the University of Connecticut, though she was conducting research in Brazil at the time. Besides a flood of scientific papers, her 2014 book Second Growth: The Promise of Tropical Forest Regeneration in an Age of Deforestation vi has won wide new interest in the topic.
An exciting story is emerging at the one-hectare plot I visited, called Arrozal, and the others in Chazdon’s “Bosques” project. The new story is being written also by a wider long-term study of other recovering forests in Latin America known as NeoSelvas. One finding is that second growth forests in the early stage accumulate biomass, e.g. carbon, faster than predicted by standard models, but biomass varies dynamically over time.viiThus is emerging the first detailed picture of these forest ecosystems through time. Still, scientists can’t say whether, they will resemble the original primary forest in another hundred plus-plus-plus years.
Bernal, Enrique and Jeannette seem excited to be finding so many new and healthy trees. The day I visited, Jeanette added to the database 25 trees that had grown to the required girth of 5 centimeters. She recorded just three that had died. (No one cycle of data-taking can tell the whole story; forest study plots have the equivalent of bad hair days.)
[Update: The standard method to compute once and future forest traits is chronosequencing; the data-taker records three traits of the trees on a plot, taking a snapshot of the present forest; then these traits are projected backward and forward in time, on the assumption that they stay the same.
[But this model is called into question by a compilation of the most extensive long-term data set ever assembled for Neotropical (Latin American) forests. In the June 30 Proceedings of the National Academy of Sciences, Natalia Norden and co-authors including Chazdon modeled a wide range of traits of regenerating forest in seven plots in Brazil, Mexico, Nicaragua and Costa Rica, including the one at Arrozal.
[They found that the standard method predicted just part of these forests’ traits. Which trees “succeeded” on a plot changed over time and from plot to plot. A big factor turned out to be what grows outside of a plot. Example: a regenerating forest plot is likely to grow more tree species sooner when old forest remnants are nearby.] viii
Fieldwork has found a promising variety of tree species on the Arrozal plot, though it has been left to regrow for less than two decades. Chazdon said its species richness is helped by the embrace of Braulio Carillo park. And along the red rutted roads among the pastures here, I saw many patches of 75-foot old trees from which birds and other wildlife can spread seed.
The larger story here has a hero: Nature regrowing itself. The supporting cast includes those who let this degraded land alone for decades for Nature to go to work.
Money will be a key plot twist in what happens to secondary forests like these. Many landowners in the tropics sell or rent their poor-seeming land for cash crop production. From the highways near here, I noticed pineapple plantations, which are said to be expanding.
But a better understanding of secondary forest traits could raise the present value of land available for regrowth. National governments such as Costa Rica’s could grant more benefits to owners who agree to leave land entirely alone or plant trees strategically. Governments could count the recalculation of these forests’ carbon storage toward national and international climate goals.
The results of this research can help more local people hang on to, live with, and enjoy their forests. More like Bernal could learn to call birds, chatter with monkeys and get excited about flowers they haven’t seen before.
If this work revalues tropical landscapes by showing it can store more carbon than previously thought, viii all of us should let forth a few whoops.