I may take on the first tree. Texturing it to look like it fits in TF2 will be fun.

I may just model it, set up the uvw map with ambient occlusion and let another person take over the texturing part.
The shield looks like something that could be pulled off in an hour or two easily. Texturing using a normal map could cover the ridges and semi-protruding designs on the shields.
I've never modeled leaves before really; however, it seems like the leaves would simply be a "plane" with a diffuse map, as well as an opacity map.
Question to OP
Do you want just one variation of the tree, or several? How many places will this tree be present in your map?
Question to TF2 Modelers
I have one question for any modelers who model for TF2. Does TF2 like stuff made in Mudbox/Zbrush? High-detail models? The tree could have some bumps and stuff applied to it acting like skin bark or something.