The differential effects of bracken (Pteridium aquilinum (L.) Kuhn) on germination and seedling performance of tree species in the African tropics

Bracken fern Pteridium aquilinum (L.) Kuhn stands act as an ecological filter that influences regeneration of other plants. Woody species are often slow to re-colonise bracken-dominated areas within forest landscapes likely reflecting, at least in part, effects on tree seeds and seedlings. We experimentally examined tree seed germination, seedling survival, and seedling growth under field conditions in the Bwindi Impenetrable National Park, Uganda. The six treatments were (1) undisturbed ferns control ‘‘Bracken’’, (2) fern fronds cut and removed but litter left intact ‘‘Cut’’, (3) ferns and coarse litter removed “Cut and litter removed”, (4) fern fronds pinned down ‘‘Bent’’, (5) bracken and soil removed and replaced with forest soil “Forest soil” and (6) bracken and rhizomes removed but soil returned “Bracken soil”. We implemented the experiment in five locations with a total of 120 1 m × 1 m plots, 28,800 seeds of 14 woody species and 1920 seedlings of eight woody species. ‘Bracken soil’ treatment (5.7 ± 1.2%) and the ‘Cut and litter removed’ treatment (5.4 ± 1.6%) had the lowest germination. All late-successional species germinated better under undisturbed bracken while six of nine pioneers germinated better in the bracken-free plots. Seedling survival was greatest overall in the ‘Forest soil’ treatment (50.3 ± 4.9%) followed by the ‘Bracken’ treatment (48.1 ± 2.1%) while the ‘Cut’ treatment had the lowest survival (27.5 ± 2.8%). Three of four pioneer species survived best in bracken-free plots while two of four late-successional species survived best in full bracken. Cumulative growth in seedling height (cm ± 1 SE) for all test species combined was highest in the ‘Bracken soil’ treatment (7.5 ± 1.3) followed by the ‘Forest soil’ treatment (7.0 ± 0.8) while the ‘Cut’ treatment had the lowest height increment (1.9 ± 0.7). All test species grew better in bracken-free plots than in full bracken though none of the main effects were significant (P < 0.05). Taken together, these results imply that bracken hinders establishment of pioneer species but favours germination of late-successional species. This ecological filter is determined by shade and by soil-mediated processes and contributes to arrested succession as most seed inputs are from pioneers.