Since we can't see all the leaves from a distance, we draw groups, clumps of leaves.
Here are some methods to draw trees, for example, comics, manga, or just a drawing.
The main thing to know is that we can't draw all the leaves of the tree, so we will draw clumps of leaves. The second thing is that there are many different kinds of trees, deciduous, coniferous, etc... So you have to know what kind of tree you want to draw, if you're in a realistic universe, fir trees are more in the northern hemisphere, in altitude, in the mountains, some kinds of pines by the sea, baoabs more in the southern hemisphere, etc... If you draw a specific tree, the best thing to do is to copy it from nature or from a picture and observe it carefully, texture of the bark, width of the trunk, height of the tree, proportions, division of branches, types of leaves, etc... But we can also create a universe in which trees appear and invent some. To do this, we can summarize that a tree is composed of roots, a trunk, branches and leaves.
The steps
The roots
First, we draw the roots of the tree disappearing into the ground. Sometimes the roots are more apparent, sometimes they are less so, but you can always see something that implants itself in the earth and is covered.
The trunk
Next, the trunk of the tree is drawn as an elongated block upwards, usually a little thinner at the top.
The branches
The trunk then divides into branches. This division varies according to the type of tree. Sometimes there are more branches, sometimes less, sometimes the trunk divides itself, sometimes there are only branches that grow on the trunk that remains in a block up to the top as for fir trees. If we draw a tree in winter, we will not draw leaves, but we will draw the small branches that are on the main branches. These small branches are usually covered by the leaves, but they can also be seen on certain types of trees such as pines or trees in early spring, when the leaves are still small.
The leaves
This is the last part. But some people also draw in another way, the drawing is free, each one its techniques. You can also start by drawing the leaves and then some branches between, then the trunk. You will have to draw some kind of "clouds", rather rounded, bumpy, depending on the type of leaves. To draw these clumps of leaves, you will have to take into account the perspective, if the clumps of leaves are in the front, they will be bigger and if they are in the back, they will be smaller and hidden behind those in the front.
Shading
Drawing the shadows allows you to better understand the shapes of the tree, that the trunk is not a tire track but a cylindrical volume, that the tufts of leaves are not spots but balls.
Ensuite, on peut ajouter des détails et préciser à l'infini... 
Closer up
If you draw a tree from closer, you will only see a part of it, in this case you can specify a little more. For the texture of the bark, we will generally add bumpy lines, holes, bumps, or, for a smoother trunk, some straighter lines. For the leaves, there are an infinite number of leaf shapes, but most often, a leaf is composed of a branched stem around which a thin platform develops. This platform is often rounded, thinner at the base, wider in the middle and thinner again at the end, sometimes pointed, but there are many other kinds of leaves, sometimes in "star", more or less separated around the branches of the stem, sometimes with several leaves on a stem ... etc... Conifers have thorns, which are drawn in another way, in tufts of fine needles.
In drawing, each one has his own style and way of doing things, so there is nothing better than to practice and to come and do it accompanied by other enthusiasts by taking comic book/manga drawing classes. Apolline offers courses in Lausanne, Morges, Montreux, Bienne and Delémont. And to deepen your knowledge even more, you can take part in the Comic/Manga drawing vacation camps which take place all over the French-speaking part of Switzerland during the school vacations!