Our visit to the nature trail at the Texas Cooperative Wildlife Collection area featured three dominate trees of the Post Oak Savannah:
 
 

Quercus stellata - the Post Oak - with leaves usually widest at the middle. deeply lobed, a stellate (star-shaped) trichomes

Quercus nigra - the Water Oak - with leaves usually widest at the tip (spatulate). barely lobed, and few trichomes (glabrous).

Ulmus alata - the Winged Elm - with leaves not bilaterally symmetrical at the base (oblique) and - often - 'winged' shoots resulting from extensions of the bark. (more photos)

Ilex vomitoria - the Yaupon - dioecious (photo is of a pistillate plant), evergreen shrubs that make up most of the understory of local oak woodlands.  See more photos of Ilex species, including  Ilex paraguariensis, the South American species that produces Yerba Maté.  The fruit is a drupe that is important to local and migrating songbirds

Vaccinium arboreum - the Farkelberry or Tree Blueberry - with leaves usually entire (no toothing or lobing) and leathery.  Its flowers are bisexual (perfect) and, like its relatives in the ethnoflora (blueberry, cranberry), its fruit is an edible berry.

The third common understory shrub,  Crataegus crus-galli, produces little pomes and these mark the local 'hawthorne' as a relative of the apples and pears (Rosaceae, Maloideae).  Its leaf margins are saw-toothed (serrate) - see images of Crataegus species.