Forests are typically thought of as carbon sinks, absorbing more greenhouse gases from the atmosphere than they release. Trees and plants suck in carbon dioxide during photosynthesis and release oxygen back into the air. However, they also exchange methane with the atmosphere, primarily through microbes in tree trunks and in soil. Methane exchange in trees is more complicated than that of carbon dioxide: A tree's age, the season, and where on a tree a measurement is taken can influence whether the tree is determined to be a net source or sink of methane.