Tropical forests store a 3rd of the world's carbon of their wood and soil. However, their future as a carbon sink stays uncertain. Scientists have long wondered whether nutrient-poor tropical soils will limit the power of mature and recovering forests to grow.
A study published June 6 offers a hopeful answer, showing that forests have resilience strategies that help them overcome the challenge of nutrient depletion.
“We don't need to worry too much about it,” concluded senior creator Sarah Betterman, a tropical forest ecologist on the Cary Institute of Ecosystem Studies. “Because of these resilience strategies, trees may be able to support carbon sinks in the future, even in the face of nutrient depletion. Our findings support tropical forest restoration and long-term climate solutions. support the conservation potential of intact forests.”
An unparalleled experience
Increased levels of carbon dioxide within the atmosphere may promote the expansion of tropical forests by facilitating photosynthesis. However, scientists fear that an absence of certain nutrients – especially phosphorus – will limit forest growth, shrinking potential carbon sinks. Soils within the tropics are generally low in phosphorus because of weathering, and increasing rates of decomposition in addition to climbing CO2 levels are expected to make soil nutrients much more scarce.
In the biggest experiment of its kind, the team checked out how forests of various ages adjust the 2 nutrient acquisition strategies used to access phosphorus: one strategy relies on an enzyme called phosphatase; Some are secreted by tree roots, and others benefit from mycorrhizal. Fungi These fungi live within the soil and might partner with trees to trace and release nutrients within the soil. Both strategies include a big carbon and nitrogen cost to the tree.
The scientists, led by Michelle Wong, a former postdoc on the Carey Institute and currently an assistant professor at Yale University, desired to know the way forests of various ages reply to changing soil nitrogen and phosphorus levels of their nutrient availability. How to regulate strategy
“While belowground processes are critical to ecosystem function, they are much less understood than aboveground processes because they are more difficult to study,” Wong said.
Located within the lowland tropical rainforest of Panama, the sector experiment covered a large area. Its 76 plots, spread across 16 square kilometers of mountainous terrain, range from recently abandoned pastures to 600-year-old stands of forest. While some plots were left alone, others were fertilized with nitrogen, phosphorus, or each.
For one yr, the team measured the activity of phosphatases and mycorrhizal fungi within the plots, to find out the resilience of each strategies and whether forests spend money on these strategies in a different way as they age and nutrient levels change. Boundaries change.
Solving the climate problem
Forests of various ages responded in a different way to nutrient additions, indicating that “trees are actively responding to their nutrient environment,” Wong said.
In young forests, where nitrogen is essentially the most limiting nutrient, adding phosphorus didn't change phosphatase activity, but adding nitrogen did. With enough nitrogen, trees can then spend money on strategies to accumulate more phosphorus.
In old forests, phosphatase activity increased in response to phosphorus fertilization, implying that nitrogen limitation is eliminated because the forest matures after which phosphorus becomes limiting. Betterman identified an inclination for nitrogen limitation in young forests to diminish over time in his previous work.
Phosphatases seem like a highly flexible nutrient acquisition strategy, increasing by half in response to nitrogen, and decreasing by half in response to phosphorus inside each forest age class. Responses to mycorrhizal colonization were, by comparison, less consistent and predictable.
While the outcomes are encouraging, Betterman cautions that “we don't yet know if resilience is sufficient to provide all the nutrients that forests will need in the future.” Phosphatase, for instance, relies on breaking down a type of phosphorus which will develop into more scarce in the longer term, so its utility could also be limited. Still, “there may be a buffering capacity to reduce nutrient limitation, at least for a while,” Bitterman said.
Wong added that the power to regulate strategies could mean that forests have “more flexibility to recover from land-use change or maintain productivity in a carbon-rich world.”
To inform higher forest restoration efforts
For forest managers and organizations leading reforestation efforts, the findings offer some practical advice: “We need to consider nutrient limitations when we're reforesting,” Betterman said. “One way is to make sure we're using a diversity of trees with different phosphorus acquisition strategies. We also need to make sure we're using trees that have different phosphorus levels at each site. are consistent.”
Currently, most reforestation efforts don't implement this level of maintenance — it's more about getting plants into the bottom quickly, and using whatever species can be found. Even so, Batterman feels optimistic about using forests as a natural climate solution.
“We can implement it quickly, it's low cost, and it has many co-benefits, such as protecting watersheds, increasing biodiversity, and protecting species that are important to local people. But it It needs to be done right. That's the point where science can guide the process and ensure that carbon is there for a long time to come.”