One simple way to draw a cartoon tree is to start by drawing a circle that intersects over the top of a rectangle. Turn the circle into leaves by drawing a cloud-like formation using the circle as a guide. Where the circle meets the rectangle, draw a "y" shape in the rectangle. Add blades of grass at the bottom of the rectangle by drawing multiple spikes.
To draw a more detailed cartoon tree, first start by drawing two vertical curved lines that end at approximately the same height and curve in the same direction, either left or right, but curve slightly differently from one another. Draw the two lines far enough away from each other to imitate the trunk of a tree. Connect the two arcs with a slightly curved line on the bottom. This makes the trunk of the tree. Add two thin branches to the top, connecting these two branches to the trunk of the tree to make a "Y" shape. Under the branch on the right, draw a small, thin branch near the top of the trunk. On the left side, bellow the original branch, draw another slightly larger branch in the shape of a knife blade.
Draw a small cloud-like shape over the end of the smallest branch, and draw a bigger cloud-like shape that covers the tops of the three bigger branches and ends just above the trunk. Erase any lines of the cloud-like formation and the branches that intersect, making some of the branches disappear behind the leaves. Color in the tree if desired.
For additional details, add three or four vertical curved lines of various length that start at the bottom of the trunk for texture, and add additional bubbled arcs to the leaves.