Shade tolerance, seedling banks, and cyclone release: Trajectories of woody invasion in disturbed tropical forests

Background/Question/Methods

Intense storms (cyclones, typhoons and hurricanes) are relatively infrequent but cause massive and widespread modifications to the physical environment of forests. An increase in resource availability, primarily light and space, is the most obvious effect. The disturbance affords a large spatial and temporal window of opportunity for invasion and empirical research demonstrates growth and recruitment rates of invasive species increase following such events, and they spread readily. While some weed species seem to be transient, others persist over the long-term, altering forest structure and composition. What are the functional traits and attributes of these persistent invaders and what conditions facilitate trajectories resulting in persistent invasion? I synthesize evidence of weed invasion in forests following intense storms and describe key characteristics and strategies of persistent invaders as well as the environmental conditions in which they are successful.

Results/Conclusions

Invasive species alter the successional trajectory of forest following intense storms by usurping space and reducing recruitment success of native species. While shade-intolerant herbaceous invaders are dominant initially, they are relatively transient with high mortality as the canopy beings to close and light levels at the forest floor decrease. Woody invaders, particularly shade-tolerant small trees with fleshy fruits, are persistent and often demonstrate lower mortality than native species and higher growth rates. Accelerated invasion via release of shade-tolerant woody seedlings from the understorey is a response observed for many persistent invaders. Scrambling species (vines and lianas) with shade-tolerant seedling recruitment also demonstrate rapid spread, often inhibiting recruitment of native species and creating the phenomena of persistent ‘strangled gaps’. The combination of traits exhibited by successful woody invaders allows them to persist during inter-storm intervals, mature and contribute to the next wave of invasions in the event of another disturbance. The level of structural damage to the forest, and subsequent light levels in the forest understorey, strongly influences the abundance and trajectories of invasive species following intense storms. However, there is some evidence that litter and debris loads impede recruitment of invasive species, as well as small-seeded native species. Increased heat in the understorey may also differentially influence the establishment success of some species. Fragmented forests are likely to be exposed to some of the more severe consequences of intense storms as the structural damage to them is often greater due to edge effects, and they are more susceptible to bombardment of propagules of weed species from the matrix vegetation.