This question is from 2014 with answers from 2015. Just to add the point of view of some research that has been done since.
In essence, new calculations show that NCS (natural climate solutions: a combination of land management, forestation, etc):
...can provide over
one-third of the cost-effective climate mitigation needed between now
and 2030 to stabilize warming to below 2 °C.
Source: Griscom et al (2017), Natural climate solutions, PNAS, https://doi.org/10.1073/pnas.1710465114
Turns out that trees, other than sequestering carbon, also emit volatile organic compounds (VOC) that act as greenhouse gases. Therefore, the answer isn't as simple as planting trees.
Quoting some recent articles on the subject:
Many scientists applaud the push for expanding forests, but some urge caution. They argue that forests have many more-complex and uncertain climate impacts than policymakers, environmentalists and even some scientists acknowledge. Although trees cool the globe by taking up carbon through photosynthesis, they also emit a complex potpourri of chemicals, some of which warm the planet. The dark leaves of trees can also raise temperatures by absorbing sunlight. Several analyses in the past few years suggest that these warming effects from forests could partially or fully offset their cooling ability.
Source: Gabriel Popkin (2019), How much can forests fight climate change?, Nature, http://doi.org/10.1038/d41586-019-00122-z
That doesn’t mean that all forests cool the planet, however. Researchers have known for decades that tree leaves absorb more sunlight than do other types of land cover, such as fields or bare ground. Forests can reduce Earth’s surface albedo, meaning that the planet reflects less incoming sunlight back into space, leading to warming. This effect is especially pronounced at higher latitudes and in mountainous or dry regions, where slower-growing coniferous trees with dark leaves cover light-coloured ground or snow that would otherwise reflect sunlight. Most scientists agree, however, that tropical forests are clear climate coolers: trees there grow relatively fast and transpire massive amounts of water that forms clouds, two effects that help to cool the climate.
Source: Gabriel Popkin (2019)
Atmospheric chemist Nadine Unger, then at Yale University in New Haven, Connecticut, conducted one of the first global studies examining one part of this exchange: the influence of volatile organic compounds, or VOCs, emitted by trees. These include isoprene, a small hydrocarbon that can warm the globe in several ways. It can react with nitrogen oxides in the air to form ozone — a potent climate-warming gas when it resides in the lower atmosphere. Isoprene can also lengthen the lifetime of atmospheric methane — another greenhouse gas. Yet isoprene can have a cooling influence, too, by helping to produce aerosol particles that block incoming sunlight.
Source: Gabriel Popkin (2019)
Plants take up carbon dioxide and release volatile organic compounds
(VOCs), in a similar way to how other organisms breathe in oxygen and
exhale CO2. These VOCs are oxidized in the atmosphere and then
contribute substantially to the burden of tiny particles suspended in
the air, which are known as aerosols. Aerosols produced from VOCs are
known as secondary organic aerosols (SOAs), and affect both air
quality and Earth’s climate. The total rate of SOA production was
thought to be the sum of the individual rates associated with the
oxidation of each VOC. But writing in Nature, McFiggans et al.1 show
that a more accurate description is needed to improve the
representation of SOAs in computational models of air quality and
Fangqun Yu (2019), Atmospheric reaction networks affecting climate are more complex than was thought, Nature, https://doi.org/10.1038/d41586-019-00263-1
Sources of the articles:
one; two; three