When adding a tree you will be automatically prompted to enter the species. You can search for species by their common or scientific name. To calculate the ecological and environmental impact of trees and to have the most accurate map, you'll want to add species information to all trees.
If you're unsure of the species, examine the different parts of the tree for clues. Characteristics like leaf shape, leaf fall color, bark, twigs, fruit, flowers and form all help to identify species. First, observe whether the tree has bear cones and needle-like leaves typical of conifers and evergreens. Many trees with bear cones and leaves that hug twig and are scale-like fall into the conifer family. Broadleaf trees, which include the deciduous family, are characterized by thin, flat leaves that shed annually.
There are many great online resources that will help you identify species on the go. For North American trees, start by checking out the Arbor Day Foundation's Tree Identification Feed Guide here.