Tropical forests are often prized for their incredible biodiversity. But the true value of tropical forests
also rests in their ability to act as environmental buffers. Tropical forests are extremely useful carbon sinks, or stores of carbon. Because of the high abundance of photosynthesizers such as trees, forests ultimately remove more carbon dioxide from the atmosphere than they release, in a process known as carbon sequestration. Understanding tropical forests’ role as carbon sinks is crucial as we develop solutions to global anthropogenic climate change. However, human ecological disruptions such as overlogging and slash-and-burn agricultural methods have disrupted this natural cycle. As a result, important nutrients such as nitrogen and phosphorus have become limited in these forests. Nutrient limitation could create a cascade of negative consequences, slowing forest growth and preventing the ecosystem from sequestering carbon.
However, what if we tried to supplement these ecosystems with artificial nutrients? Over a period of four years, Yale Assistant Professor of Ecology and Evolutionary Biology Michelle Wong and a team of forest ecologists explored the impact of human disturbance on carbon sequestration in tropical forests in Panama. The team sought to explore how artificial nutrient supplementation might affect the functioning of different forest types as carbon sinks. In January, they published the results of their study in Nature Communications.
Wong’s team began by investigating the relationship between a forest’s age and its growth in response to fertilization. Ecologists sometimes use the term “succession” to describe how different kinds of organisms occupy the forest at different stages in its lifespan. At first, only small shrubs and grasses grow, before being succeeded by saplings and trees. “Succession age is the process in which organisms organize themselves after some type of disturbance event,” Wong said. Wong’s team analyzed 88,000 tree specimens across four Panamanian forests, aged zero, ten, thirty, and six hundred years. After nitrogen fertilization, the team observed a strong increase in biomass accumulation, or growth rate. Additionally, the researchers observed that younger forests appeared to respond more intensely to this intervention. As succession proceeds and forests mature, supplemental nitrogen has less impact on the accumulation of aboveground biomass, especially in environments already disturbed by human activity.
Wong explained that this correlation results from the dynamics between young and mature forests. Older forests have a more developed canopy, which prevents them from accessing light and slows their growth. With less growth, mature forests typically maintain a very steady state of nutrient cycling relative to younger forests, which is difficult to disrupt, even with fertilization. “In really young forests, right after a disturbance event, they have a ton of light and are competing rapidly for it, so they are growing really fast, relative to how small they are,” Wong said. In a young forest, where trees are still clambering for skylight, nutrients from the soil are funneled into rapid growth. With artificial fertilization, the greater biomass accumulation in young forests indicates more rapid carbon sequestration, demonstrating that younger forests could play a more significant role in climate change mitigation than mature forests.
The team’s findings offer insight into the forest’s role as a carbon sink and a tool in understanding climate change mitigation. However, the researchers consider this result only the first step in understanding nutrient limitation, and have suggested repeating the experiment with greater geographic variation. “We want to get to a place where we can restore forests by thinking of ecosystem function more holistically, and even planting some nitrogen-fixing species,” Wong said. Wong’s current research explores plant-soil-microbe interactions across various soil types. Understanding how trees use nutrients efficiently will enhance our understanding of tropical forest ecology and help us navigate the changing climate of our planet.