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Leaf Fossils
Leaves can be fossilized on a tree's surfaces. Common leaves found as fossils include maple, sycamore, ginkgo, willow, poplar, oak and ferns, which consist of a central ridge with veins arching away from the ridge, much like modern fern leaves. Most leaf fossils are not very big -- only about 2 to 4 inches in length -- but some rare fossil leaf discoveries have yielded specimens more than 6 feet long.
Petrified Wood
Trees and bark themselves can fossilize over thousands of years. When a tree or piece of bark fossilizes, it is known as petrified wood. Petrification happens when the mineral silica slowly replaces the cells of tree bark, until the entire piece of bark or tree becomes fossilized. According to fossil website Fossil Plant (fossilplant.com), after petrification the entire cell makeup of a petrified tree is still structurally identical to how it was before, except it is solid rock. Fossil Plant states that entire forests can become fossilized under the right conditions.
Reptile Fossils
Reptile fossils on trees or tree bark are rare, but they do exist. One such reptile fossil, known as Hylonomus lyelli, was discovered by Nova Scotian geologist John William Dawson in 1859 in a tree trunk in Joggins, Nova Scotia. The Hylonomus lyelli fossil is famous because it is believed to be "the only fossil discovered of the earliest known reptile," according to the Vancouver Sun, and dates to about 312 million years ago.
Seed Fossils
Seeds, specifically the cone or seed-bearing portion of a plant, can often be found as fossils on trees and bark. According to Fossil Plant, gymnosperms -- plants such as ginkgos and conifers whose seeds grow unprotected on the outside, cone portion of the plant -- fossilize more easily than other seeds. Usually cone or seed fossils are incomplete and in small pieces, but Fossil Plant reports that discoverers in certain localities have found cone fossils that are so complete they look like they've just "fallen from the tree."