Question on "welding" 2 objects

Another way to accomplish this is to use an Opacity mask. Select your object containing the brown stems and view your transparency panel. In there, there's the option to make a mask. That'll hide EVERYTHING - but you can draw white shapes on that black background (make sure you're "focused" with your mouse on the mask (right side) itself). Anything WHITE will be visible (so you can choose to expose one or the other. Anything black will remain transparent.
How to edit artwork using transparency and blending modes in Illustrator

Perhaps the easiest way is to use your pen tool to draw a shape that encompasses only the part of the artwork that you want to see, then set the fill and stroke on the shape to "none".

Next select both the shape you drew and the leafy image. Make sure the shape you drew is above the leafy image. Now create a clipping mask (Select Object menu...Clipping Mask...Make).

Can you upload a larger image? This one is too small to see what is going on with it. Showing your layers panel expanded would also be helpful.