Final answer:
The least important trait for pioneer species in primary succession is the ability to outcompete others for sunlight, as they usually grow in environments with low plant competition.
Step-by-step explanation:
The trait that is LEAST important for a pioneer terrestrial plant species in primary succession is the ability to outcompete neighbor plants for sunlight. During the early stages of primary succession, competition for sunlight is minimal as few other plants are present.
Pioneer species are typically characterized by their rapid dispersal, ability to live in habitats with little or no soil, and thrive in open areas with full sunlight. As these pioneer species like lichens and mosses establish themselves, they gradually modify the environment, making it suitable for subsequent plant communities that include grasses, shrubs, and eventually, if conditions allow, a forest.