The role of bracken (Pteridium aquilinum) in forest dynamics = [De rol van adelaarsvaren (Pteridium aquilinum) in de bosdynamiek]

Summary

Bracken fern ( Pteridium aquilinum ) causes stagnation in forest succession in many parts of the world. The mechanisms by which bracken affects the establishment and growth of plant species are studied, focusing on the regeneration of tree species in forest habitats.

Bracken is well adapted to the forest environment. Data show that productivity is maintained under low light levels. When the tree canopy is opened up or removed, above-ground production increases sharply to produce dense swards. Under most tree species ( Pinus , Quercus , Larix , Betula ), the translucent canopies allow for the long term persistence of bracken clones. On the dry sandy soils of The Netherlands, beech ( Fagus sylvatica ) is the only tree species that can completely inhibit bracken growth. The relationship between bracken and ancient forests is mainly historical, and due to the lack of sporal regeneration and low survival of bracken outside the tree canopy cover.

Under favourable conditions, bracken builds a physical structure that strongly reduces light levels available to plants growing under its canopy and smothers plants under collapsing and dead fronds. A field experiment indicated that below-ground competition by bracken is negligible. Species diversity of the forest understorey was negatively correlated with frond biomass.

The slowly decaying litter builds a thick ectorganic soil layer that impedes establishment of light-seeded species. An experiment demonstrated how, besides bulk density and thickness, also the horizontal layering of the organic particles contributes to reduced species establishment on deep ectorganic soil layers. Despite numerous claims that the bracken plant produces substances that are toxic to other plants, experimental evidence did not confirm this allelopathic nature of bracken. A review of the literature suggested that the potential phytotoxicity of bracken is due to the release of compounds that protect the plant against herbivory.

Field experiments showed that voles and mice make preferential use of the protective bracken cover. The increased abundance of rodents resulted in a high seed predation under bracken. The hoarding behaviour of wood mice ( Apodemus sylvaticus ) caused a directed dispersal of seeds towards the bracken structure. Bracken impinges on all causes of successional change in the vegetation. Tree establishment is blocked unless the clones are physically disturbed. Some implications for forest management are discussed.

Key-words: allelopathy, bracken, competition, litter, Pteridium aquilinum, seed dispersal, seed predation, stagnation, succession.