From a biological point of view, the "brightness" of a tree is most likely a product of the structure and chemical composition of its foliage, which will vary between species, as well as the intensity of the incident light.
"Brighter" trees usually means that the leaves reflect more light - the leaf size, pigmentation (colouration) and thickness of its shiny waxy covering - all contribute there. Hotter drier climates usually means lighter colours and smaller leaves to reduce the absorbed light and heat; and thicker wax coverings to retain more moisture. So it is possible to have trees that are very much brighter compared to those you would see in higher and colder latitudes such as Pennsylvania.