Forests are generally carbon dioxide sinks when they are high in diversity, density or area. However, they can also be carbon sources if diversity, density or area decreases due to deforestation, selective logging, climate change, wildfires or diseases. One study in 2020 found that 32 tracked Brazilian non-Amazon seasonal tropical forests declined from a carbon sink to a carbon source in 2013 and concludes that "policies are needed to mitigate the emission of greenhouse gases and to restore and protect tropical seasonal forests". In 2019 forests took up a third less carbon than they did in the 1990s, due to higher temperatures, droughts and deforestation. The typical tropical forest may become a carbon source by the 2060s. 
An assessment of European forests found early signs of carbon sink saturation, after decades of increasing strength. The Intergovernmental Panel on Climate Change (IPCC) concluded that a combination of measures aimed at increasing forest carbon stocks, andsustainable timber offtake will generate the largest carbon sequestration benefit.
Life expectancy of forests varies throughout the world, influenced by tree species, site conditions and natural disturbance patterns. In some forests, carbon may be stored for centuries, while in other forests, carbon is released with frequent stand replacing fires. Forests that are harvested prior to stand replacing events allow for the retention of carbon in manufactured forest products such as lumber. However, only a portion of the carbon removed from logged forests ends up as durable goods and buildings. The remainder ends up as sawmill by-products such as pulp, paper and pallets, which often end with incineration (resulting in carbon release into the atmosphere) at the end of their lifecycle. For instance, of the 1,692 megatonnes of carbon harvested from forests in Oregon and Washington from 1900 to 1992, only 23% is in long-term storage in forest products.
The Food and Agriculure Organization (FAO) reported that: "The total carbon stock in forests decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020". However, another study finds that the leaf area index has increased globally since 1981, which was responsible for 12.4% of the accumulated terrestrial carbon sink from 1981 to 2016. The CO fertilization effect, on the other hand, was responsible for 47% of the sink, while climate change reduced the sink by 28.6%. In Canada's boreal forests as much as 80% of the total carbon is stored in the soils as dead organic matter.
Carbon offset programs are planting millions of fast-growing trees per year to reforest tropical lands, for as little as $0.10 per tree. Over their typical 40-year lifetime, one million of these trees can sequester up to one million tons of carbon dioxide.