Forest research links carbon uptake and water use

What is climate change ecology?

Climate change ecology studies the interactions between climate change and natural ecosystems. This is a two-way process. On one hand, climate change influences ecosystems, leading to questions such as whether rising temperatures and reduced rainfall can make rainforests collapse. On the other hand, ecosystems can impact the climate through modulating carbon and water cycles on Earth. For example, ecosystems can mitigate global warming by taking CO₂ out of the atmosphere through photosynthesis. Numerous environmental interactions and feedbacks cascade into a rather complex relationship between terrestrial ecosystems and climate change.

When we talk about ecosystems, we often think about trees and leaves. However, the advances in earth observation techniques in the past 20 years, such as eddy covariance, leaf gas exchange, and earth system modeling, have helped us see way beyond that. It also presents a great opportunity for us to push the boundaries of knowledge and better understand ecosystem dynamics.

Could you share more details of your recent study on the link between droughts and forests’ ability to absorb carbon dioxide?

Ecosystems take up different amounts of CO₂ from the atmosphere yearly and this leads to variations in the atmospheric CO₂ concentration (e.g., when land ecosystems take up more CO₂, the CO₂ concentration level in the atmosphere grows slower). Scientists have been discussing the reason for the variations in ecosystem CO₂ uptake and atmospheric CO₂ concentration level, and have provided rather contrasting explanations—some suggest temperature anomalies are the main reason, some suggest soil moisture, others point to historical land-use changes.

In this study, we looked into the magnitude change of the variation over the past 60 years and found a dominant role tropical extreme droughts played in modulating the atmospheric CO₂ variation. This new discovery advances our understanding of the global carbon cycle, and highlights how tropical droughts, previously neglected and poorly considered in climate and carbon models, have in fact modulated the long-term variation in atmospheric CO₂. As extreme droughts happen, ecosystems experience downregulation of photosynthesis, as well as tree mortality, fire, regeneration, and deadwood decomposition. These processes would make the ecosystem CO₂ uptake more variable in the long term. In particular, we found that droughts in tropical Asia ecosystems have outsized impacts on the long-term variation of the global carbon cycle, which will motivate our future studies to examine drought impacts on tropical Asia ecosystems, including those in Singapore.

You have a particular interest in the dynamics of the terrestrial carbon cycle and global photosynthesis, topics which seem to be attracting growing global interest. Could you share a bit more about them and how they play an important role in mitigating climate change?

Climate change is induced by human emissions of heat-trapping greenhouse gases such as carbon dioxide. Since 1958, the global atmospheric CO₂ level has increased by roughly 30% and global temperatures, by one degree Celsius [1.8 degrees Fahrenheit]. A key function of ecosystems is carbon sequestration. Through photosynthesis, ecosystems take up CO₂ in the air and lock them into the biomass, therefore providing a nature-based solution to offset human emissions of CO₂.

However, ecosystems not only take up CO₂ through photosynthesis, but also release it through respiration and disturbances (e.g., fires and land use changes). As such, we need to have a good understanding of all individual carbon fluxes in the terrestrial carbon cycle to correctly quantify the carbon sequestration ability of the terrestrial ecosystem.

Currently, the terrestrial ecosystems can offset roughly 25% of human CO₂ emissions every year, forming a so-called land carbon sink. The overarching goal of my research is to understand how this land carbon sink will change with climate change, and what is the best practice to maintain and enhance this carbon sink for climate change mitigation. My team’s and my current work is mostly focused on photosynthesis, as it is the start point of the terrestrial carbon cycle and determines other carbon fluxes, and as we still face substantial uncertainties in estimating photosynthesis for large regions.

What research projects are you currently working on?

My team and I are currently working on two projects aimed at advancing our understanding of the terrestrial carbon cycle in tropical Asia.

In one, we examine the structural and physiological traits of trees in Southeast Asia. These traits such as leaf nutrient content, chlorophyll, and leaf amount are the functioning blocks of ecosystems. With a good understanding of these traits, we would be able to quantify the carbon sequestration ability of tropical Asia ecosystems.

In the second project, we are investigating the joint impacts of climate change and land use on the carbon cycle in Southeast Asia. The main rationale is that Southeast Asia is a global hotspot of deforestation and it loses tons of carbon every year. We hope to uncover how much carbon could have been saved if we conserve these forests in a changing climate, to guide regional carbon restoration efforts.

In addition, we hope to develop a state-of-the-art carbon model specifically for tropical Asia for timely and accurate prediction of the regional carbon cycle. We are also looking to explore the long-term impacts of climate change on ecosystem functions and understand how much ecosystems have adjusted to climate change. We aim to establish advanced infrastructures, such as eddy covariance towers and solar-induced fluorescence sensors, in Singapore to monitor the land-atmosphere CO₂ exchange of local ecosystems, and use the measurements to develop our ecological theories and support our studies on the terrestrial carbon cycle in tropical Asia.