Tree & roots are a fractal transport system, trapping carbon (black), releasing oxygen (red). When carbon dioxide (CO2 black and red) touches leaves it is used in oxygenic photosynthesis. Oxygen (O2, red) emanates from the leaves. Carbon (black travels downwards and is sequestered in roots and soil. Of course much carbon is used in making plant sugars. Some carbon also travels down with the leaves that turn yellow and fall, but much of that carbon is later released as CO2.
Reduces CO2, so helps mitigate climate change. I take joy in oversimplifying, as a first step. I used exactly the same code as for my trees (standard fractal tree), except here the tree starts off in 2 directions. Then added stages for changing CO2 to O2 (into air) and C traveling downwards. Scratch's limit of 300 clones is a serious constraint, so that I have to stamp and kill off all clones used in the tree and leaves before cloning the CO2, and the black carbon traveling down with the roots. Code for tree and roots repeats branching, each time shrinking. Results in a self-similar pattern known as "fractal."
