File:Araucarioxylon arizonicum (fossil wood) (Chinle Formation, Upper Triassic; south of Adamana, Arizona, USA) 4 (26667482818).jpg

Quartz-permineralized fossil wood from the Triassic of Arizona, USA. (public display, Field Museum of Natural History, Chicago, Illinois, USA)

Plants are multicellular, photosynthetic eucaryotes. The oldest known land plant body fossils are Silurian in age. Fossil root traces of land plants are known back in the Ordovician. The Devonian was the key time interval during which land plants flourished and Earth experienced its first “greening” of the land. The earliest land plants were small and simple and probably remained close to bodies of water. By the Late Devonian, land plants had evolved large, tree-sized bodies and the first-ever forests appeared.

The fossil shown above is "petrified wood", which is a horrible term for what is technically called permineralized wood. Biogenic materials such as wood or bone have a fair amount of small-scale porosity. After burial, the porosity of wood or bone can get partially or completely filled up with minerals as groundwater or diagenetic fluids percolate through. The end result is a harder, denser material that retains the original three-dimensionality (or close to it). The wood or bone has become “petrified”. Well, no - it’s become permineralized. The most common permineralization mineral is quartz (SiO2). Sometimes, fossil wood or bone has been permineralized with radioactive minerals such as black uraninite (UO2) or yellowish carnotite (K2(UO2)2(VO4)2·3H2O). Recently, fossil bones permineralized with cinnabar have been identified (García-Alix et al., 2013, Lethaia 46: 1-6).

From museum signage:

The fossil forest of Arizona is situated south of Adamana, in Apache County. Actually, there are four of these "forests", or areas in which the silicified logs are sufficiently abundant to be called a forest. The logs belong to a cone-bearing tree -- Araucarioxylon arizonicum, allied to the present Norfolk Island pine. The wood has been replaced by silica; the brilliant red and other colors are due chiefly to deposits of iron and manganese. The logs are not in place; they have dropped down from a higher horizon in which they were originally entombed.

Stratigraphy: Chinle Formation, Upper Triassic