If you want to capture all forests in Portugal this makes sense. I assume there are missing a lot of forests?
Keep in mind that OSM is no register for personal data.

Unfortunately only aerial imagery before the fire season is provided by Bing, ESRI etc. for mapping with OSM. Maybe you have better sources?

Have a a look at http://wiki.openstreetmap.org/wiki/OSM_Tasking_Manager
Maybe you get support by the portuguese Tasking Manager instance.

Good luck!