Initial heavy losses arose as a result of the rapid deforestation of the uplands following establishment of a permanent British colony on St Helena in 1659. The clearances were primarily to make way for pasture, but as one of the larger trees on the island with a trunk of reasonable girth, Black Cabbage would have been particularly sought after as a source of firewood and timber. A further wave of destruction followed in the late 19th and early 20th Centuries, when large tracts of unproductive land on the steep slopes of the Central Ridge were converted to plantations of New Zealand Flax. The flax industry collapsed in the 1960s, but vast areas are still blanketed by the original monocultures. The net result of these waves of destruction has been to confine the remaining cloud forest to isolated fragments with little opportunity for expansion, limited genetic exchange and heavy constraints on normal ecological processes such as cycles of succession and recolonization.
Although the remaining cloud forest fragments are now protected and relatively stable in extent, they continue to suffer from deterioration. Present day threats derive mainly from interactions with non-native species. There is certainly some direct competition from invasive trees such as Bilberry (Solanum mauritianum Scop.) and Quinine (Cinchona officinalis L.), which are already abundant on Diana’s Peak Ridge. The Flax continues to encroach, particularly at High Peak. Sprawling growths of Small Fuchsia (Fuchsia coccinea Curt.) also form dense tangles which can creep over living branches, although these are more of a threat amongst Tree Fern thicket. Probably a more serious threat is posed by ground cover herbs which compete for open soil, and thus remove the already limiting micro-habitats suitable for germination. The native cloud forest community is largely lacking in low-growing, patch-forming species, so the impact of encroachment by pasture grasses such as Cow Grass (Paspalum scrobiculatum L.), Running Sedge (Kyllinga brevifolia Rottb.) and the invasive Feather Moss (Pseudoscleropodium purum) into freshly-created clearings is probably much underestimated.
A final threat comes from disease. In both 2008 and 2014, a sudden wave of die-back killed significant numbers of large, mature trees in the Diana’s Peak area. The cause of this die-back is unknown and there were few external symptoms. In both instances, the effect was manifested during warm, dry conditions following a period of sustained, heavy rain. Infection by an oomycete of the genus Phytophthora (de Bary) is one possibility, but thus far no pathological tests have been possible. There are sufficient trees remaining that the impact of the disease was not particularly noticeable by the following year. Despite this, continued waves of loss at a similar frequency (removing many prime, mature specimens from the population), coupled with a decline in the rate of recruitment, could lead to a long-term crash.
The threats to the species affect not just the population of Black Cabbage Tree in isolation, but also the associated habitat which it creates. The latter is particularly sensitive because a cluster of trees growing in close proximity is normally necessary to provide sufficient space to sustain an open understorey. There is relatively little evidence to assess the change in distribution of this habitat, but there is a reasonable likelihood that it has already suffered severe declines. Today there are very few pockets remaining on Diana’s Peak Ridge. A few concentrations remain around High Peak, but only a single aggregation of approximately 25 trees at ‘The Dell’ can be called a fragment of ‘woodland’. As a consequence, community associates such as Large Kidney Fern (Dryopteris cognata (C.Presl) Kuntze), Dwarf Tongue-Fern (Grammitis ebenina (Maxon) Tardieu), the Spiky Yellow Woodlouse and Rainbow Leaf-Beetle (Vernonia wollastoniana White) are severely threatened with extinction.
The scarcity of Black Cabbage woodland is largely due to the fact that most of the surviving trees are now individually dispersed, often densely crowded within a matrix of Tree Fern thicket or invasive vegetation. This may partly reflect patterns of tree planting in the 1990s. However, even where clusters exist, they may fail to suppress the established, denser understorey. Creation of open spaces below the trees is certainly not a simple case of succession, since Tree Ferns will tolerate the heavy canopy shade and even encroach under it. Research is much needed to understand the dynamics of cloud forest habitat cycles, how the clearings form and what maintains them.