The Arbor Day Tree Identification Guide app (US$4.99) does exactly what its name suggests. It offers clear, beautifully illustrated step-by-step interaction.
You start by choosing a section of North America to explore -- either primarily western or eastern regions. You then choose the kind of leaves (needles, flat or scaly), and you're off.
I found the app to be lovely and easy to use. It's an excellent match for smart middle schoolers. Grades 3-5 may enjoy the identification portion, but the vocabulary in the "big reveal" will really push their reading abilities.
The app offers a useful glossary of terms, as well as a full list of supported trees -- so you can browse the botany library without having to perform any specific searches.
The app is a little treasure, one that should appeal to any family that hikes or enjoys geocaching in the wild.
Want to learn more about Arbor Day? Visit the foundation's web site: http://arborday.org.