Light variability typical of deciduous forests enhances the performance of non-native plants

Alien plant invasions in forests can severely threaten native biodiversity
and ecosystem functioning. Canopy closure in deciduous
forests can cause variability in light levels,
but light levels in forest edges and evergreen forests are
constantly high and low, respectively. We tested how light conditions and
light variability affect the
invasiveness of alien plants by
growing five pairs of invasive and noninvasive alien
species under conditions of high,
variable (with high light in the initial phase and low
light in the latter phase of the experiment), and low light
intensities. The total duration of the experiment was 200
days. The alien plants were grown singly or
competed with a native deciduous or evergreen tree
seedling. Overall, invasive plants produced marginally more
biomass than noninvasive plants. From high to low (averaged
across variable and low) light conditions, biomass of invasive plants
decreased less than that of noninvasive species (-26.7%
versus -37.7%), indicating that invaders may have a greater capacity to
invade forests than noninvasive plants. From low to variable light
conditions, biomass of invasive plants increased more than that of
noninvasive species (63.7% versus 48.9%), indicating that the advantage of
invaders over noninvasive species is most
pronounced under variable light conditions. Competition with the
deciduous tree reduced biomass more for the noninvasive plants
than for the invasive plants, indicating that interspecific competition
may further explain why some alien species can invade while
others cannot. Our results indicate that high light
early in the season benefited more for the invaders, and reduced
irradiance from the middle of the experiment limited growth of
the noninvasive plants more than that of the invasive plants. Together,
our results suggest that variable light levels, simulating
those in deciduous forests or caused
by disturbances, may promote the invasiveness
of alien plants.