If you ask me what I study, I’ll usually tell you that I study ecology – the science of interactions between organisms and the environment. That’s enough for some people. A lot of people, actually. If you ask for details, I’ll say that I study restoration ecology – the science of assisting of the recovery of damaged ecosystems. I’ll also tell you that I do field work in Costa Rica, and that the objective of my research there is to learn how to turn beat-up cow pastures back into rainforest. That much detail is enough for almost everyone.
But, if you are one of the few who is really persistent or has already formulated a few opinions about the best strategies for turning cow pastures in rainforests, I’ll sometimes mention that I study Applied Nucleation. If you’ve found this blog, you may fit into one of those categories.
This is what we want to turn back into rainforest.
Before I tell you about Applied Nucleation though, I need to tell you a little bit about ecological succession.
Imagine yourself inside of a mature rainforest. There are tall trees all around you, and each one is a different species. There are lianas (woody vines) the size of your thigh hanging from the canopy. The understory where you are standing is dark and humid, and you could easily walk off of the trail because there is very little vegetation down on the ground. Maybe there are monkeys eating figs up in the treetops. Maybe there’s a Scarlet Macaw croaking overhead.
This is what it looks like inside a mature rainforest.
Now, imagine yourself in the same forest after a Category Four hurricane.
The trees have all been broken off or uprooted. It is hot without the shade from the canopy. You should put on some SPF because the tropical sun is intense. The forest has become a brush pile, and there is no way to walk through it without a machete and a chainsaw. No monkeys, no macaws.
Almost immediately, the forest will begin to regenerate. Gradually and predictably, it will become more and more like the rainforest that was there before the storm. This process is called ecological succession. The first trees to come back will be light-loving pioneer species like Cecropia. These spindly trees produce bajillions of tiny seeds, which manage to find their way (via the digestive tracks of birds and mammals) to every conceivable place that a Cecropia tree might be able to grow. As the brush pile diminishes and the canopy closes in, more species of trees, herbs, lianas, and animals will return to the site.
A horizontal view of ecological succession
So the recovery of species composition following a disturbance is reasonably predictable. There is also a well-documented pattern for how forests recovery spatially.
Imagine looking down at the recovering forest from a helicopter. At first, right after the storm, it’s just a mess of downed trees, but if you fly over again after a couple of years, there will be a few isolated tree canopies. Under those sparse canopies, the climate will be a little bit cooler and a little bit moister than in the surrounding area. Those canopies will also attract seed-carrying birds that will defecate from the branches, adding to the pool of tree seeds down below. Over time, those few scattered tree canopies will act as nurseries, and other trees will germinate and grow underneath and around the edges – expanding the size of the canopy patch or nucleus. Eventually, those patches will meet one another and coalesce, closing the canopy and making a continuous forest over what was just recently more like a brushy savanna. This pattern of patch formation and expansion is called the nucleation model of succession.
A helicopter view of nucleating ecological succession.
Now you’ve got the Nucleation. Here I’ll tell you about the Applied.
Hurricanes, despite their hugely destructive power, are a relatively benign disturbance in intact ecosystems. Trees blow over, but there are still seeds in the soil that can germinate and grow up to become the next generation of trees. Fires, floods, and landslides fall into the same category. Other disturbances are more insidious.
When forests are cut down by humans, for example, we generally don’t let them regrow, at least not right away. First we like to grow some crops. Make some money. In southern Costa Rica, where I do my fieldwork, the crop of choice was coffee. In the 1950s-70s, much of the Coto Brus Valley was deforested for coffee production. But in the 1990s and early 2000s, coffee prices bottomed out – a result of failed negotiations in the International Coffee Organization. Farmers were forced to try something else, and for many of them, something else was cattle.
A Coto Brus farmer and his cows. Twenty years ago, this was a coffee plantation. (Photo by Jim Reid)
For various reasons, various people now want to turn some of those cow pastures back into rainforest. Some farmers want to regrow forest in order to protect springs, prevent erosion, and preserve a little piece of the natural world to pass on to their grandchildren. Others want to restore rainforest to keep endangered species from going extinct and to mitigate the worst effects of global climate change by sequestering atmospheric carbon inside of wood.
This is easier said than done. After years of intensive agriculture, tropical cow pastures generally have few or no tree seeds left in the soil. And birds and mammals are not likely to bring new seeds either; pastures are hot, dangerous, and virtually devoid of food resources. Simply removing cows from the land may not be enough. In some cases, you could fly your helicopter over an abandoned pasture every year for decades and see nothing but an itchy carpet of exotic pasture grasses.
To get one of these pastures back on its successional track towards a mature rainforest, the most obvious thing to do is plant some trees. You can plant a lot of trees, as in a tree plantation, but this is expensive. It’s also a lot of work, and it tends to have a long-lasting influence on the overall diversity of the new forest. (Remember standing in the mature rainforest eleven paragraphs ago? Remember how every tree was a different species?)
Instead, my research group is exploring the possibility of planting just a few trees in small patches. The premise of the strategy is that the small patches will create favorable conditions for new trees to arrive and grow. Ultimately, we think that the patches will get bigger and bigger until they meet and form a new, highly diverse rainforest. We call this strategy Applied Nucleation, after the nucleation model of succession.
If you’ve made it to the end of this post, you might as well keep reading. Check out our lab website for more information!
Here is one of our study sites from Costa Rica. The plot on the right is the one that represents the applied nucleation approach.