NEWS
Figure 3 : Mismanagement of grasslands often leads to encroachment by non-herbaceous or ‘ woody ’ species . One such species – Bracken fern ( Pteridium aquilinum ) – can rapidly establish in dense stands . ( Photo : Dave Thompson )
Figure 4 : Beneath the continuous frond canopy of the bracken , the environment lacks a crucial plant resource – sunlight , and the soils are toxic from bracken exudates . Few , if any , indigenous species are able to persist here . ( Photo : Dave Thompson )
established , it is difficult to eradicate because of its persistent underground rhizomes and large reserves of carbohydrates which favour vegetative spread , a tolerance of ( indeed the species reportedly benefits from ) physical disturbance , including fire , and toxicity to herbivores which means the species is little affected by animals .
Figure 5 : A small and isolated grassland fragment , which is typical of grasslands in Limpopo and more broadly , faces a big challenge . Encroachment by bracken , seen as grey in this aerial drone image , threatens local biodiversity . ( Image : Rion Lerm )
Studies from around the world have linked the establishment of bracken across various habitats with the concurrent decrease in local species richness due to competitive exclusion by P . aquilinum . The high density of the bracken frond canopy , accumulation of subcanopy litter and allelopathic exudates from the plant tissues combine to create a shaded , resource-limited and toxic environment that suppresses the germination , growth and survival of indigenous flora ( Figure 4 ). In short , the establishment of bracken in dense stands is an ecological disaster , diminishing local biodiversity and ceasing the provision of associated ecosystem services .
Figure 6 : Frond growth following the winter dormancy will be removed from experimental bracken-eradication areas . Subsequent regrowth will be removed periodically throughout the growing season in an attempt to deplete the below-ground reserves . ( Photo : Dave Thompson )
The grasslands of Limpopo – being hyperdiverse , highly fragmented and under threat , are a microcosm for the status of grasslands globally . Now , a tiny grassland fragment – just over three hectares in size , has come into the spotlight in the global fight against bracken ( Figure 5 ).
Starting in 2020 , a collaborative effort between the SAEON Ndlovu Node , the national Department of Environment , Forestry and Fisheries and the non-profit organisation Friends of the Haenertsburg Grasslands , embarked on documenting the diversity of the grassland flora in bracken-encroached and adjacent bracken-free areas ( Figure 7 ). The baseline data collected now ( Figure 8 ) is invaluable for several reasons : 1 ) to generate a list of species which persist despite the presence of the bracken ; 2 ) to identify traits associated with the persistence of indigenous flora ; 3 ) to pinpoint the percentage cover value for bracken beyond which local species richness declines ; and 4 ) to act as the benchmark against which to measure the efficacy of control measures .
When the new bracken growth emerges after the winter dormancy ( Figure 6 ), a rigorous and ongoing campaign of frond cutting will be implemented in parts of this so-much-more-thana-grassland , paving the way towards documenting the process , rate , trajectory and magnitude of local biodiversity recovery .
It is a botanical biodiversity story of David and Goliath . And we all know how that story ended .
23 Grassroots Vol 20 No 4 December 2020