A common way to make trees is to intersect two or more flat faces, each with the tree texture, which also contains transparency. From above these have a + or * shape, depending on how many faces you use:
(click on each image to toggle between large and small versions)
This gives the tree a 3d appearance without using too many polygons. However, simply intersecting those faces can lead to a mirror effect from some angles:
This gives the tree an unnaturally symmetrical appearance. It may be hard to notice when racing, but when watching replays or taking screenshots, the effect can be quite unsatisfactory. There are a variety of ways of preventing the symmetrical effect while keeping polycounts low. More will be added to the following list when or if they occur and can be explained properly:
Method 1 - bend the planes into 4 L shapes