When you finish planting a forest, sometimes it's not just a forest. I.e. it has a resource (e.g. iron, coal) on the tile as well. At least that's how I remember it.
I proposed today a version which generates only forest resources (camps, plantations, lumbermills)
example (with more resources for VP)
If you plant a forest
[922421.015] Reforestation: RESOURCE_DEER
[922421.015] Reforestation: RESOURCE_FUR
[922421.015] Reforestation: RESOURCE_DYE
[922421.015] Reforestation: RESOURCE_SILK
[922421.015] Reforestation: RESOURCE_TRUFFLES
[922421.015] Reforestation: RESOURCE_LAVENDER
[922421.015] Reforestation: RESOURCE_POPPY
[922421.015] Reforestation: RESOURCE_HARDWOOD
if you plant a jungle
[922421.015] Reforestation: RESOURCE_BANANA
[922421.015] Reforestation: RESOURCE_DYE
[922421.015] Reforestation: RESOURCE_SPICES
[922421.015] Reforestation: RESOURCE_TRUFFLES
[922421.015] Reforestation: RESOURCE_COCOA
[922421.015] Reforestation: RESOURCE_NUTMEG
[922421.015] Reforestation: RESOURCE_CLOVES
[922421.015] Reforestation: RESOURCE_PEPPER
[922421.015] Reforestation: RESOURCE_COCONUT
[922421.015] Reforestation: RESOURCE_HARDWOOD
[922421.015] Reforestation: RESOURCE_RUBBER