I figured out how to draw trees so I thought I would share

Step 1: make a sketch so that your tree will stand up straight.

Step 2: pick the Dip Pen Bleed brush or something with a similar texture. Pick a base brown color, this will be the color that is neither light nor dark

Step 3: draw squiggly lines going up, don't forget your roots!

Step 4: make a new layer underneath and choose a the lightest color for your tree

Step 5: repeat step 3

Step 6: make another layer underneath all the other ones and choose the darkest color for your tree

Step 7: fill in all the gaps with your darkest color

Step 8: add branches if you like

Step 9: delete the sketch layer and you're done!

Step 10: step back and appreciate the proud tree you've just created :D

I hope this was helpful!