In the U.S., the last Friday of April is Arbor Day. The holiday is celebrated in 32 countries at different times of the year, and it's a great excuse to plant some new trees. Everyone loves trees. They're great for climbing, lounging under, swinging from. They provide shade, beautiful leaves in autumn, and they absorb CO2 and create oxygen. The more trees, the better, right? Not always. While trees provide many great benefits, some species require a lot of water to grow. The wrong kind of tree planted in the wrong place can trim the amount of water reaching the water table and increase its salinity. Diversity is also important. Monoculture forests are susceptible to disease than natural and mixed-species forests.