Note that these organizations, too, are advocacy organizations with a specific bias.
Tree Care Industry Association - Industry group / trade association for companies
Forestry USA (a general forestry portal) under "Networking" has a list of Forestry and Forest Products Industry Associations
Forestry USA is a privately-funded "portal" - a collection of organized links - related to forestry and forest products in the US. Many useful links, but there is little information about their interests.
Online and downloadable resources from the ISA (International Society of Arboriculture) has links to
Also link to the Forestry Images database which has Creative Commons licensed photos of types of trees, related insects, other pests and diseases, wildlife, people, etc. Can be used for your presentations or papers.