A Venn diagram would be the most effective graphic organizer for comparing and contrasting information about the types of trees that grow in different parts of the country. The Venn diagram allows you to visually represent the similarities and differences between two or more groups, making it easy to see which trees are common to both regions and which are unique to each area.
A T-chart could also be useful for listing characteristics, but it wouldn't visually show the relationships between the types of trees as effectively as a Venn diagram.