Here's a simple way to draw a tree using pixel art.

First, draw a simple tree trunk. Then paint it with wood color. 

Draw tree leaves using a few small circles. Draw the bottom circle so that it covers the top circle. 

Paint one small circle green.

Paint it with a lighter green and a darker green, creating three layers.

Fill in all the smaller circles in the same way, and draw a small part in the middle where you can see a bit of a tree branch.

 

Simply draw the darker parts of the tree trunk. 

 

The drawing is complete.

I'll draw a simple background to check if it matches well. 

How to Draw a Tree Easily with Pixel Art