The soil here was drier and was disturbed less by things like floods. The canopy also cut off substantially more light than down by the river, making it harder for shade-intolerant species to grow. Compare the different needs of the different plants in the two previous questions. Some plants like the alder and cottonwood like lots of water and lots of light. Others can tolerate drier conditions and less light.
Generalists have broad ecological tolerances and can live in many different places.
Things like Scotch broom are generalists. Specialists have very narrow ecological tolerances and may only live under very specific conditions. Things like the spotted owl are specialists. This distinction is useful because it helps one think about what that species does, where that species will be found, and what conditions are necessary for that species to survive. It's a good label for many plants and animals.