Beginning October, the aspen, the willows, the cottonwood trees take their golden colors, enlighten the landscapes, make them just beautiful before the days of snow. . . . . . .I “discovered” my first aspen (of my life, or I did not notice before. . . ) trees a few kilometers north of the southern entrance on the roadside. Nothing else to do than to park somewhere and have a short hike in the woods, and take photographs! The golden leaves, sometimes crimson, purple, were a wonderful discovery to me; and the smooth wind making the leaves dance on the branches, the very light music of the wind in the leaves. . . . Enjoy very simple gifts of nature, take time to look at the trees, listen to the wind. . . . . There are many aspen near the creeks, small rivers and lakes; walking under there trees is easy and only for that, it is worth to take some time on a tour in the Yellowstone. On the last picture is a (soft!) example of what I realized only later: the cars everywhere on the roadside, and people jumping out of their cars to take pictures and drive ahead.
The driver guide told us that there were three main kinds of tree (all conifers). The most common is the Lodgepole pine (Pinus contorta). There are also Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) but they are in the minority. The trees never rot, so they stay standing but dead until something causes them to fall down, and then they just lie there. We could see this from the plane coming in
Other than those three trees and various grasses, there are the various paintbrush type flowers (which we either didn't see or didn't photograph), and the Yellow Monkey-Flower (Mimulus guttatus) which we did see and photograph.
Leaving aside the question of whether bacteria are animals, plants or something else, the colors that one sees at the edges of the hot springs where they run off are caused by mats of bacteria. These bacteria can live in water that is almost boiling although the deeper water is clear.