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The main focus of research in my lab is to better understand how climate change will affect photosynthesis and transpiration in the land biosphere. These are fundamental processes that determine the carbon-water status of land ecosystems. They also determine the state of the global carbon cycle: For example, a decreasing rate of photosynthesis would reduce the uptake of carbon by the land biosphere, and in turn increase the amount of human-made carbon emissions that accumulate in the atmosphere -- where they can further amplify the Earth's greenhouse warming. Any changes in the rate of land carbon uptake are therefore critically important, but we do not have reliable estimates of regional or global uptake rates since photosynthesis cannot be directly measured at scales greater than a few leaves.
My lab applies two recently developed observational tools, carbonyl sulfide (COS) and solar-induced chlorophyll fluorescence (SIF), that provide independent estimates of carbon uptake during photosynthesis. COS is an atmospheric trace gas at a level roughly a millionth that of CO2. SIF is an optical signal emitted by plant leaves that can be observed from space with satellite remote sensing. Combining the two methods will help to tease apart climate change effects on carbon and water dynamics that are closely coupled in land ecosystems, and contribute to more accurate predictions of future changes in land carbon cycling.
We are particularly interested in tropical, mediterranean, boreal, and arctic systems as they are highly vulnerable to extreme events such as heat waves, and gradual environmental shifts due to climate change. Our work typically combines theory and model development with field measurements, with some recent examples listed below.
tropical rainforest
We monitor carbon cycling in the tropical rainforest at La Selva, Costa Rica. Tropical forests account for more than half of global land carbon uptake, but their remote locations and extreme conditions make it difficult to collect data. We measure gas exchange (CO2, H2O, COS, CO) and optical signals (SIF and other vegetation indices) using tower-based instruments, eddy covariance, automated chambers, and portable instruments to collect data at a range of scales, from individual leaves to the whole ecosystem. Our goal is to better understand how variations and long-term shifts in environmental conditions will affect carbon uptake in this globally important biome.
mediterranean semi-arid shrubland
At Stunt Ranch UC Reserve in the Santa Monica mountains, we evaluate how various shrub and tree species native to California respond to extreme events, in particular heat waves, combined with seasonal summer drought.
example: Sun et al (2024) Restricted internal diffusion weakens transpiration–photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange. Plant Cell & Environment 47, 1813-1833, https://doi.org/10.1111/pce.14840
boreal forests
Our work at field sites in Finland, Alaska, and Canada uses a combination of methods (eddy covariance, automated chambers, COS, SIF) to quantify various aspects of carbon uptake in boreal forests, such as the role of stomatal conductance or timing of the spring onset of photosynthesis.
example: Kooijmans et al (2019) Influences of light and humidity on carbonyl sulfide-based estimates of photosynthesis. PNAS 116, 2470-2475. https://doi.org/10.1073/pnas.1807600116
arctic tundra
At a multi-factorial climate change experiment near Pituffik (formerly Thule Air Base), Greenland, we compared how warming with and without added water affects the growth and functioning of tundra vegetation (Salix arctica, a tiny creeping willow). The warming strongly increased the rates of carbon uptake and water use efficiency, but only when combined with additional water, and the carbon emitted from the permafrost soil was 30,000 years old!
example: Lupascu et al (2014) High Arctic wetting reduces permafrost carbon feedbacks to climate warming. Nature Climate Change 4, 51–55. https://doi.org/10.1038/nclimate2058
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