Winters in the northern hemisphere are brutal. The harsh conditions drive some species to hibernate; bears reduce their metabolic state to conserve energy until spring. Forests also endure winter by conserving energy; they shut down photosynthesis, the process by which a green pigment called chlorophyll captures sunlight and carbon dioxide (CO2) to produce the chemical energy that fuels the plants.
The total production of chemical energy resulting from photosynthesis is called Gross Primary Production (GPP). GPP in evergreen forests tells scientists how much CO2 these vast and remote systems are breathing in.
Because photosynthesis pulls CO2 out of the atmosphere, understanding forestactivity is crucial for tracking global carbon levels. For decades, scientists have used satellites to monitor the changes in greenness of deciduous forests to track GPP.
In the fall and winter, deciduous leaves turn brown and drop when they’re dormant. In the spring and summer, the chlorophyll returns as green leaves and photosynthesis ramps up. However, evergreen trees retain their chlorophyll-filled green needles year round, preventing scientists from detecting the onset and decline of photosynthesis on a large scale.