Equatorial Atmospheric Variability Modulates Carbon and Energy Fluxes in a Wallacean Tropical

Deep within the humid, mist-shrouded mountains of Sulawesi, Indonesia, a forest that seems to exist in a perpetual state of wetness is teaching scientists something unexpected about how nature breathes. For decades, the prevailing wisdom held that tropical forests are primarily limited by water: when the rains stop or the air becomes too dry, trees close their pores to save moisture, and their ability to pull carbon dioxide from the atmosphere slows to a halt. This idea has shaped how we understand the global carbon cycle, especially during severe droughts like those brought by El Niño, which are often assumed to turn these lush ecosystems from carbon sinks into carbon sources. However, a new investigation into a specific high-altitude rainforest suggests that the story is far more complex. In this wet, mountainous environment, the trees are not waiting for rain to work; they are waiting for light. The forest's ability to absorb carbon is not dictated by the thirst of its roots, but by the clouds hanging above its leaves.

Researchers from IPB University and Indonesia's meteorological agency set out to test this idea using a tower equipped with sensitive instruments in the Lore Lindu National Park. This site, known as the Bariri Flux Site, sits at an elevation of 1,416 meters, where the air is cool and the forest is frequently wrapped in clouds. The team analyzed five years of continuous data, from 2013 to 2017, a period that included one of the most intense El Niño events on record. They were looking for the specific triggers that cause the forest to speed up or slow down its carbon absorption. To do this, they had to look beyond the usual seasonal changes and examine the rapid, day-to-day shifts in the atmosphere. They focused on three types of atmospheric waves that travel across the equator: the Madden-Julian Oscillation, which brings periods of heavy rain and cloud cover; Kelvin Waves, which move eastward; and Equatorial Rossby Waves, which move westward. These waves act like pulses of weather, bringing sudden changes in cloud cover and sunlight that last only a few days.

The first major discovery was a surprising disconnect between water and carbon. In many parts of the world, when the soil dries out, trees stop eating carbon to conserve water. But in this mountain forest, the soil remained wet almost the entire time, even during the severe 2015–2016 El Niño drought. Measurements showed that the water content in the top layers of the soil never dropped low enough to stress the trees. Despite this abundance of water, the forest's carbon intake still fluctuated wildly from day to day. When the researchers compared these fluctuations to the weather data, they found that the trees were not reacting to the water in the ground. Instead, their activity was tightly locked to the amount of sunlight reaching the canopy. On days when thick clouds blocked the sun, the forest's carbon absorption dropped, regardless of how wet the soil was. On days when the clouds cleared, the forest surged, even if the air was dry. This revealed that in this specific environment, the forest is limited by energy, not by water. The trees are not thirsty; they are simply waiting for the sun to break through the mist.

The study then looked at how these rapid atmospheric waves influenced the forest. When the Madden-Julian Oscillation or Equatorial Rossby Waves brought their wet, cloudy phases over the mountain, the incoming sunlight vanished, and the forest's carbon intake dropped immediately. This was expected, as clouds block the energy the trees need to photosynthesize. However, the behavior of the Kelvin Waves told a different and more nuanced story, one that depended entirely on the background climate. Under normal conditions or during a La Niña phase, the passage of a Kelvin Wave brought clouds and a corresponding drop in carbon absorption, just like the other waves. But during the peak of the 2015–2016 El Niño drought, the forest reacted in the opposite way. When a Kelvin Wave passed through during this extreme drought, the forest actually absorbed more carbon than usual.

This counterintuitive result suggests that the forest was suffering from a different kind of stress during the El Niño. The intense heat and dry air of the drought had likely pushed the trees to the edge of their limits, causing them to close their pores to avoid drying out completely. When the Kelvin Wave arrived, it brought a temporary shield of clouds. This cloud cover did two things: it blocked the scorching direct sunlight that was driving the trees toward heat stress, and it increased the amount of scattered, diffuse light that can penetrate deep into the forest canopy. This temporary relief allowed the trees to reopen their pores and photosynthesize efficiently, turning a wave of weather that usually suppresses growth into a moment of recovery. The forest was not just surviving the drought; it was using these brief, cloudy interruptions to breathe again.

These findings challenge the simple view that tropical forests always respond to drought in the same way. The study shows that the response of an ecosystem to a weather event cannot be understood in isolation; it depends on the broader climate context. A cloud that suppresses growth during a normal year can actually help a forest survive an extreme drought. The research also highlights the importance of looking at the atmosphere on a daily scale rather than just monthly or yearly averages. The rapid, high-frequency pulses of weather are not just background noise; they are the primary drivers of how these forests function day-to-day. By ignoring these short-term fluctuations, scientists may have been missing the very mechanisms that allow these ecosystems to persist through climate extremes.

Ultimately, the work from Lore Lindu suggests that our models of the global carbon cycle need to be updated to account for these high-frequency atmospheric interactions. The forest is not a passive victim of the climate; it is a dynamic system that responds to the immediate availability of light and the relief from atmospheric stress. While the soil remained wet throughout the study, the true bottleneck for the forest was the sky above. The trees were not waiting for the rain to fall; they were waiting for the clouds to part, or in the case of the El Niño drought, waiting for the clouds to return to protect them from the sun. This delicate balance between light and heat, played out on a timescale of days, determines whether this vital mountain forest continues to pull carbon from the air or releases it back.