TreesAreGood.org is an educational website providing reliable information about trees and is managed by the International Society of Arboriculture (another good resource). 

Texas A&M Forest Service conserves and protects the resources and lands of the Lone Star State as well as provides a wealth of knowledge and assistance to land owners, residents, and municipalities on tree knowledge, insects & disease, wildfire, and management of timber production.

Cross Timbers Urban Forestry Council is a nonprofit organization dedicated to this ecological region.  It brings together public and private sector professionals as well as citizens to promote education in arboriculture and urban forestry and support to community urban forestry programs in the region.

The Ancient Cross Timbers Consortium: Hosted by the University of Arkansas, this consortium hosts a wealth of knowledge and research on the overall cross timbers region.

Texas Invasives.org is a Texas-sized partnership to manage non-native invasive plants and pests in Texas.