The image on the right is from the Vergilius Romanus, a Byzantine illustrated manuscript of the Roman poet's works dating from the late fifth century AD. The trees in whose shade Tityrus plays his flute have reasonably "realistic" trunks, but they are on a smaller scale than the human figure and the animals. The foliage meanwhile is represented by single leaves that are too large in scale and stylized to the point that they look like clover-leaves. The modern concept of realism is irrelevant here: this representational technique is a symbolic notation of the basic components of a tree, and the viewer's mind supplies what the eye cannot. Click on the image to enlarge it.
Painted some time in the 1930s, Park in Lipetsk by Aleksandra Rubleva (1908-1943) shows a conventional rendering of foliage by areas of color to which minimal amounts of shading and texture have been added, and on the tall poplar on the left, some stippling to suggest individual leaves. The eye and mind work together to extend the impression of foliage to all the areas of color that are associated with the trunks and branches of the trees. This generalizing process is actually fundamental to our perception of everyday reality, and we take it so much for granted that it requires a conscious effort to see it as a process at all. To consciously use the psychological tricks of perception to portray complex objects and surfaces takes an even higher level of awareness, or (as is more usually the case) a set of learned techniques.
This is particularly apparent in the rendering of trees in European oil painting from the fifteenth century to the nineteenth. 
An example from each century shows the evolution from symbolic representation of trees to the highly impressionistic twentieth-century representation above. You can click on any of these to see the paintings from which they are drawn.
On the branch on the right, each leaf is separately represented, and it is unnaturally straight and regular in shape. A look at the whole picture reveals that it's not in fact a branch, but a complete tree from the background to Lorenzo di Credi's Portrait of a Lady, painted in the late 15th century. Only the most naive viewer would conclude from this painting that in medieval Italy there were trees that grew to the height of saplings, straight as a ruler, very nearly symmetrical, and with relatively sparse foliage. Realism in the modern sense is simply not the intent here. Paradoxically, each leaf and twig is "realistically" represented, but we are invited to let the result symbolize a tree as we know them experientially (and as Lorenzo did in his time). By comparison, Rubleva's twentieth-century tree is in the simple sense completely "unrealistic", but the resulting impression closely mimics the actual perception of a tree.
By a hundred years later, the convention for rendering trees was no longer medieval: the tree on the left, from the background to Titian's Venus and a Lute Player (c. 1562-65), already gives the overall shape of a tree, even if rather schematically, and suggests the foliage with pattern and texture. This technique remains suprisingly consistent over the succeeding centuries.
In the 1660s, the Dutch landscapist Meyndert Hobbema's A Woodland Road (right) produces a very realistic foliage effect by dense and consistent stippling. This effectively approximates for the eye the minutely subdivided outline of a tree, and greatly enhances the realistic illusion overall.
Rembrandt's Cottage Among Trees (left), also from the mid-17th century, is an example of foliage in a pen drawing. Working with lines rather than brush-strokes forces another kind of convention: a mass of hooks and loops replaces brush-strokes in producing the effect of foliage.
On the left, Thomas Gainsborough's Mountain Landscape with Peasants Crossing a Bridge, painted in 1783-1784, employs a dynamic, cursive patterning that suggests not just foliage but its movement. The effect is even a little suggestive of Turner.
On the right, a Russian landscape from the late 19th century, Isaak Levitan's Meadow at the Edge of a Forest, renders foliage with a technique that is intermediate between the types illustrated here by Hobbema and Gainsborough: it is more impressionistic than Hobbema's dense texture, but not as freehand as Gainsborough's effect.
A repertoire of conventional techniques for the depiction of nature, developed since the Renaissance and spread through studios and art schools, brought a relatively uniform "realism" to the pictorial surface of European landscapes. But it is worth remembering that the two representational extremes, the pictorial symbol and the illusionary impression, and every gradation between them, have always been available at any time. Another manuscript from fifth-century Byzantium (right) illustrates a scene from The Iliad with trees that are rendered impressionistically rather than symbolically, while at the close of the nineteenth century many of Europe's Modernists returned experimentally to older and "simpler" pictorial conventions. This is why in landscape paintings the characteristics of a particular culture are more likely to be in the choice and arrangement of otherwise similar conventional details. For example, it may not be possible to tell much about the nationality of an artist from the way trees are painted, or even from a superficial suggestion of locality, but their compositional role in the painting, and the more subtle details of their treatment, may point more clearly to a particular national culture.
