Some populations of pua kala have fewer spines, for example. It probably has much to do with the environment of Hawaii, much of which is highly favorable to continuous growth because of mild temperatures and sufficient rainfall. Annual plants are well-adapted to the temperate climates, where part of the year is very favorable to plant growth and the colder part is not. In the environment favourable to continuous plant growth, a short annual life-span is a handicap. High rainfall tends to wash out nutrients and less rainfall found in many drier areas of Hawai'i limits growth.
Native plants lack chemical and physical defenses.
These associated traits found in Hawaiian plants are parallel with traits often found in island animals. The lack of protections for both plants and animals on islands relate to lack of predators. It requires extra photosynthetic energy for plants to produce thorns or protective chemicals. If a plant doesn't produce them and also is not assaulted by insects or animals, it can use that energy to grow larger and produce more offspring. Those genetic traits for such protection may well drop out of the gene pool over time.