Reforesting is a complicated task, it is nothing like

for (int i = 0; i<= 100000000; i++) { Tree t = new Tree(); t.plant(); }

You need specific types of trees for specific microclimates, otherwise they won't thrive and might die. You need a healthy species mixture, otherwise you are creating monocultures vulnerable to mass pest infestation. You need the right amount of rain. You should not reforest good arable land, because we are almost 8 billion now and everyone wants to eat.

Israelis are very good at reforestation, but it took them over 100 years to recreate some modest forests there.