I started to simply collect seeds along hike paths and public spaces, where they wouldn't have the opportunity to grow, and either throw them or plant them farther where vegetation is more poor, dry, and decreasing year after year

That can be oak, hackberry, pine, fig trees seeds or cuttings. I clear up the ground around, uprooting a few weeds, and leaving them at the top with dead branches under, I think it helps keeping moisture, and hope it'll grow. I'll monitor the results

Also I eat a lot of fruits (bought at supermarket, produced in France or Italy), I keep their pits and seeds (peaches, apricots, clementines, mandarines, oranges, lemon, peppers, and other green garbage, and bury them in those places (instead of throwing them away in the garbage bin). They might not be able to grow, but at least it should be interesting for the soil life and it saves up garbage bins weight, etc..