I came across a website for a Massachusetts artist, Jesse Green, a.k.a. "The Machine," who creates sculptures from trees, using a chainsaw. This type of art is familiar to anyone who lives in New England as you often will see a squirrel or bear in a front yard or holding up a mailbox. You can listen to an interview with Green here.
Another artist, Andy Goldsworthy, uses stone, water, and other natural materials to create works of art that often don't last for longer than a day. You can view his work, and learn the philosophy behind his art in Rivers and Tides: Andy Goldsworthy, Working with Time [DVD 730.92 RIV]. You can learn more about Goldsworthy here.
Another form of ephemeral art made of natural materials is the sandcastle. For some, this may be the wrong time of year to be thinking about sandcastles, but for others, a crisp sunny day on a deserted beach might be the perfect time for a sandcastle artist to work. Read more about the art in Sandcastles: Great projects: From Mermaids to Monuments by Patti Mitchell [736.9 MIT].
On a smaller scale, you can create works of art with materials found right in your own backyard. For kids we have several books including Kid Style Nature Crafts: 50 Terrific Things to Make with Nature's Materials by Gwen Diehn [J 745.5 DIE] and for adults we have The Complete Book of Nature Craft Techniques: From Baskets and Bows to Vinegars and Wreaths, Everything You Need to Know to Craft with Natural Materials by Deborah Morgenthal [745.5 MOR].