Installations are a type of concept art that is in three dimensions and usually interactive, sometimes relying on the participation of the viewer to create the art. They can be audio, visual, intangible (an idea) or physical. Trees, which already have so much meaning in many societies, lend themselves well to installations. They have been used in many ways, from hanging a complete tree, including root system, from the ceiling, to simply decorating a tree where it stands so as not to damage it. Use trees and tree products in your own work to make a statement, but avoid destroying a tree if you can. In fact, using a tree destroyed naturally, such as by fire, lightning or flooding, or one destroyed by human interference, such as by logging, can help to make your piece more meaningful.
Art supplies are wherever you find them and should not be only products you can buy in stores. You can make your own brushes with sticks and horse hair, or make dyes and paints from flowers, barks and even insects. Bones and feathers can be used to make mobiles, while butterfly wings (using wings from dead bugs to avoid potentially killing endangered species) have been used to make collages and decorative jewelry. Even rocks can be spray painted and glued together to make sculptures. Some fungi, such as tree conks (shelf mushrooms), can be used as natural canvases to make 3D paintings.
A certain sensitivity is absolutely essential if you plan to use animal parts. Some animals are dangerous and may need to be killed to get the part you want; others may be endangered, in which case you must avoid killing them. Use safety precautions when gathering your supplies and look for better alternatives. For example, if you know a hunter, ask him for the pelt and bones of an animal he has hunted, to ensure that the parts you don't need won't go to waste. Instead of killing mice, look for owl pellets, which can have the bones of a number of small animals. Some types of insects are endangered (many butterfly species, caterpillars) or dangerous (ticks, dengue mosquitos, Africanized bees). Therefore it is best to harvest your supplies from dead or dying insects that you come across, instead of hunting them.
Using natural supplies and figuring out what to say with them can be overwhelming if you've never done it before, but the possibilities are truly limitless. Even if all you want is to paint nature, going out into the woods and collecting reference samples of what you want to paint will help you create a better image. Try making a mobile out of dead butterflies and dragonflies by spraying them with sealant and then mounting them from a twig using clear fishing wire. Dye reeds using plants and then create baskets.