A Tree is defined as a Plant that is usually very high with a main stem or trunk that's wooden and branches. Other plants that are large like trees and have a similar shape to trees are sometimes also called trees. This includes bamboo and Bananas. 
Trees and climate change
Trees can help a bit to reduce Global warming.
As trees grow, they help stop climate change by removing carbon dioxide from the air, storing carbon in the trees and soil, and releasing oxygen into the atmosphere. Trees provide many benefits to us, every day. They offer cooling shade, block cold winter winds, attract birds and wildlife, purify our air, prevent soil erosion, clean our water, and add grace and beauty to our homes and communities. When Rainforests and other Forests are cut down or burnt this worsens global warming.
The reason that logging is so bad for the climate is that when trees are felled they release the carbon they are storing into the atmosphere, where it mingles with greenhouse gases from other sources and contributes to global warming accordingly. The upshot is that we should be doing as much to prevent deforestation as we are to increase fuel efficiency and reduce automobile usage.
We should never forget that burning Fossil fuel matters at least as much as deforestation.
Some years back, petroleum companies started funding environmental concerns. It may seem strange that this would be the case, but it should be remembered that many of the materials we use can be produced either from naturally growing plants, or from fossil fuels. Much effort has been devoted to quantifying the "global warming" effects of deforestation. While it's true that both biological and geological carbon compounds can add atmospheric carbon, a couple facts should be remembered. Living things grow and die. In the case of a tree, it grows by biological processes of taking elements from the soil and air, and with the use of energy combining them into semi-stable organic compounds. The tree keeps adding these compounds to it's body throughout it's life. Throughout it's lifetime it is subject to attack by animals, fungi, and natural disasters like fire. Like all living things, the tree eventually loses it's battle and dies. Ultimately, the tree decomposes. During it's decomposition, or it's natural destruction by fire, all the atmospheric carbon it had sequestered is reemitted. Deforestation has some bad effects. Trees that would have lived for centuries get burnt down now. New trees don't grow up to replace them. If the forests are allowed to stay sizable amounts of carbon are locked up there. As trees die and decay others grow to replace them. Can we rely on publications like the Scientific American to give accurate Scientific information? See, Deforestation and Its Extreme Effect on Global Warming. We can't even be sure with science publications.
Deforestation, and land use are very serious topics. They are both going to be increasingly important as accumulated environmental damage and increasing population have their cumulative effects. [This is] another one of the areas where our thinking has been manipulated. There are very good reasons to maintain natural forests. There are also very good reasons to believe our thinking has been manipulated. Remember Frank Herbert's premise that the way to manipulate thinking is to manipulate information? Media manipulation is serious and ongoing. So is scientific manipulation. Grants can direct study, and study can generate publication. Just follow the money. Who's giving the grants? What do they want to know, or want us to know? For that matter, the harder question is what do they not want us to know? It's just that the attack on silvaculture has been extreme, and has reduced forest production, and forest health tremendously. It's become an emotionally charged political movement. It's also become an important deflection. Forest products are being replaced by petroleum products, and silvaculture's potential contribution to sustainability is ignored. We feel as if we've done our part, by forcing someone else to change their practices, yet we ignore our own part in the overall problem. 
A Tree is also "Something that sits in the ground and remains in the same spot for hundreds of years but manages to jump out in front of you on your way home from the pub."