Gap phase dynamics has been hypothesized to play an important role in the maintenance of high diversity in the tropics. When one or a few trees die, an opening in the canopy occurs, resulting in increases in light levels and other plant resources. Seedlings and saplings grow rapidly in gaps, competing for these resources; only one canopy tree ultimately will be able to occupy the space relinquished by the original gap-forming tree.
For gap phase dynamics to contribute to species diversity, one must
assume that there are differences in resources (light, nutrients, etc.)
associated with different parts of the gap (i.e. gap edge or
centre), and that different species are adapted to these differences. Such a pattern is referred to as "gap partitioning".
Larger gaps are expected to contain greater resource heterogeneity
than smaller gaps, and thus should show higher diversity of
regenerating trees. This hypothesis was recently tested by Stephen
Hubbell and co-workers using data from a 50-ha mapped forest plot on
Barro Colorado Island in Panama. Although gap sites were found to have
greater species diversity of saplings, this was due entirely to higher
stem density in the gap sites. The number of species encountered per
stem did not differ between gap and nongap sites. Thus, recent evidence
suggests that gap phase dynamics may not be the major mechanism for
maintaining diversity in tropical forests. There is, however, clear
evidence for important niche differences in tropical trees related to
soil types and forest hydrology.
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