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Keywords:

  • Gap dynamics;
  • Gap size niche differentiation;
  • Intermediate disturbance hypothesis;
  • Recruitment limitation;
  • Seedling recruitment;
  • Subtropical forest;
  • Tree diversity

Abstract:

Question: Is tree regeneration in canopy gaps characterized by chance or predictable establishment.

Location: Coastal scarp forests, Umzimvubu district, Eastern Cape Province, South Africa.

Methods: Estimation of richness of gap-filling species across canopy gaps of different size. Data are compared with regeneration under the canopy. Probability of self-replacement of gap forming species is calculated.

Results: Forest area under natural gap phase was 7.8%, caused mostly by windthrow (54%). The abundance and average size of gaps (87.8 m2) suggests that species diversity may be maintained by gap dynamics. However, only four of 53 gap-filler species displayed gap size specialization and these were pioneer species. An additional 13 species were more common in larger gaps but there was no gradient in composition of gap-filler species across gap size (p= 0.61). Probabilities of self-replacement in a gap were low (< 0.3) and common canopy species were equally abundant in gaps and the understorey. Species composition in gaps showed no pattern of variation, i.e. was unpredictable, which suggests absence of a successional sequence within tree-fall gaps. There was also only a slight increase in species richness in gaps at intermediate levels of disturbance.

Conclusions: Coastal scarp forest appears not to comprise tightly co-evolved, niche-differentiated tree species. Unpredictable species composition in gaps may be a chance effect of recruitment limitation of species from the species pool. Chance establishment slows competitive exclusion and may maintain tree diversity in these forests. These data suggest that current levels (≤ 3 gaps per ha) of selective tree harvesting may not cause a reduction in species richness in this forest.