Life form is a significant life history trait for weeds
(whether native, exotic, or exotic invasives), but the results
do not support the life form prediction. If life form
correlates with generation time and shorter generation time
increases population growth rates, there should be more
perennial weedy herbs, fewer perennial weedy subshrubs
and shrubs, and even fewer weedy trees. The results
indicate more grasses, vines, and trees and fewer shrubs
and subshrubs. These results imply that life form itself,
rather than generation time, is causing the observed
differences. The increase in annual and perennial weedy
grasses may be caused by the selective advantage of
generalized wind pollination (see above) or the relationship
between grasses, wildfires, and altered fire regimes.
The increase in annual and perennial herbaceous vines and
perennial woody vines may be caused by the ability of
these species to overtop and out compete existing
vegetation (Baker’s (1965) choking growth trait). This
result is consistent with Daehler’s (1998) increase in
natural area invaders in families with climbers (vines). The
increase in trees is consistent with Williamson and Fitter’s
(1996) and Crawley et al.’s (1996) result for invasives and
exotics in the UK, but Daehler (1998) found no relationship
between natural area invaders and families with trees.