Canopy gaps do not help establish pioneer tree species in a South Florida dry forest

Background/Question/Methods

Canopy gaps are usually thought of as resetting the successional sequence, acting as the basic unit of turnover in forest communities.  They allow pioneer species to grow in mature forests, due to increased light availability.  There appears to be a consensus about this gap replacement model in rainforests, but in seasonally-dry tropical forests, there is less agreement about their role in the successional process.  In the driest tropical forests, gap phase dynamics do not appear to exist.  In this study, canopy gaps were identified with a LiDAR digital canopy model in a Key Largo, Florida, hardwood forest.  Sapling structure and composition were recorded in the gaps.  Weighted averaging calibration was applied to relative abundances of trees in stands of known age to determine the successional age optimum for each tree species, and weighted averaging regression was used to calculate inferred stand ages for each gap’s saplings.  The study aimed to determine the distribution of gaps between forest age classes, and compare the inferred successional ages of trees growing in gaps.

Results/Conclusions

The inferred stand ages were greater in young forest gaps than in surrounding, unimpacted forest, suggesting that succession was actually advanced.  Contrary to expectation, the presence of early successional species was not promoted in the gaps.  Soil and water conditions in the gaps may favor mature rather than pioneer species.  Due to an increase in soil thickness and water storage associated with gap creation, the development of late-successional, evergreen tree species with greater year-round water requirements appear to be favored.  The young forest had three times as much area in gap phase as medium or old forest combined, suggesting that  the gaps are an intermediate step in the successional process, not the end of the sequence.  In young forests, these gaps may represent individual areas of rapid succession.  Establishment of pioneer species may not take place without major disturbances such as fire or intensive agriculture that remove the entire canopy and consume or erode soils.