I'd go with weeds. While trees could be in the secondary succession, it is more likely than not that small plants and such would be easier to grow first. Upon the Primary succession, lichen, ferns, weeds and other small plants like that sprout, and eventually evolve into trees and other large plants as the nutrient content of the soil increases. You can do more research on this if you doubt my answer or email me for more information. Hope this helps.