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Commidendrum robustum 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Magnoliopsida Asterales Compositae

Scientific Name: Commidendrum robustum
Species Authority: (Roxb.) DC.
Common Name(s):
English Gumwood
Synonym(s):
Aster roxburghii Hook.f.
Commidendrum robustum (Roxb.) DC subsp. robustum
Conyza robusta Roxb.
Taxonomic Notes: Commidendrum forms a monophyletic endemic genus with no very close affinities (Noyes and Reiseberg 1999).

The Gumwood (Commidendrum robustum (Roxb.) DC.) was split into two subspecies by Cronk (2000):
  • subsp. robustum; all extant plants belong to this taxon.
  • subsp. gummiferum Cronk; this taxon was provisionally ascribed to confusing 19th Century specimens of uncertain identity, which had been formerly classified as C. gummiferum (Roxb.) DC.

Some of the specimens labelled as C. gummiferum lie within the range of variation exhibited by C. robustum subsp. robustum. The remainder certainly belong to a distinct taxon, but their classification as a subspecies is difficult to reconcile with the fundamental differences in flower and inflorescence structure from subsp. robustum. In fact, they are somewhat closer to the False Gumwood (C. spurium (G.Forst) DC.). For this reason, the current treatment restores C. gummiferum as a species, though uncertainties persist over its true position, and there is a strong possibility that it may be a hybrid between the two surviving parents.

The well-defined C. robustum is thus retained as a species concept without division at subspecific level. Subspecies gummiferum has been promoted to species level under C. gummiferum, along with the Red List assessment done by Q. Cronk in 1998.

Assessment Information [top]

Red List Category & Criteria: Critically Endangered B1ab(v)+2ab(v) ver 3.1
Year Published: 2016
Date Assessed: 2015-07-31
Assessor(s): Ellick, S. & Lambdon, P.W.
Reviewer(s): Clubbe, C.P.
Contributor(s): Cairns-Wicks, R.
Justification:

Although planted as an ornamental and landscaping tree in various parts of the island, Gumwood (Commidendrum robustum (Roxb.) DC.) is now restricted to just two truly wild localities. The range is much less than 100 km2 and the area of occupancy is under 10 km2. We also consider that the species meets two further requirements of criterion B:

(a) The population can be considered severely fragmented as the two extant sites are highly isolated from each other and juveniles are apparently unable to establish in the surrounding habitat.

(b) Direct evidence of a decline is limited as previous census data do not date back beyond the current generation of trees. However, we infer that future declines in the number of mature individuals are inevitable based on the similar age structure of the surviving population (most trees are now reasonably old), and low levels of regeneration at the existing sites. There has also been an observed loss of some scattered remnant wild trees from outlying localities over the past few decades.

In the previous Red List assessment, Cairns-Wicks (2003) classified Gumwood as Endangered B1ab(iii,iv). Direct declines have been relatively limited since this time, but data have been accumulated to demonstrate the severe problems of recruitment, which makes the conservation plight more pressing. The upgrade of the threat status to Critically Endangered ultimately results from the citing of severe fragmentation for the first time, but the new categorization seems merited on wider grounds.

Previously published Red List assessments:

Geographic Range [top]

Range Description:

The extent of occurrence (EOO), based on the area of a minimum convex polygon around known localities, is 20.4 km2. The area of occupancy (AOO), based on a 2 km × 2 km grid, is 8 km2.

The Gumwood is found in the wild at only two sites, with a larger subpopulation at Peak Dale and smaller sub-population at Deep Valley. All other specimens on the island have been planted, although there is one tree at Thompson’s Wood of uncertain provenance. The total area of habitat occupied is approximately 1.63 ha.

Countries occurrence:
Native:
Saint Helena, Ascension and Tristan da Cunha (Saint Helena (main island))
Additional data:
Estimated area of occupancy (AOO) - km2:8
Number of Locations:2
Lower elevation limit (metres):400
Upper elevation limit (metres):600
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:

A census conducted in 2013 – 14 calculated the wild Gumwood population at 679 mature trees through direct counts at each wild site, with 97.6% of the total world population contained within the Peak Dale site, and the remaining 2.4% at Deep Valley.

The previous Red List assessment estimated the population in 2003 approximately 1,000 trees, including at two localities (Longwood and Marias) which have now disappeared (Cairns-Wicks 2003). The results of the recent census therefore show a decline in the wild population over the past 11 years.

Current Population Trend:Decreasing
Additional data:
Number of mature individuals:679Continuing decline of mature individuals:Yes
Population severely fragmented:Yes
No. of subpopulations:2

Habitat and Ecology [top]

Habitat and Ecology:

A species of St Helena’s mid-altitudinal range. Several sources of evidence suggest that dense Gumwood forest once covered extensive areas of the island. Gumwood occupied moderately damp soils up to 600 m elevation, but also extended across drier plateaus and ridges down to 400 m, showing substantial resilience to dry conditions. Numerous isolated individuals persisted on cliffs and rocky outcrops well into the 1800s, though few remain today.

Gumwood flowers in sporadic bursts throughout the year, particularly in winter, with the timing probably linked to rainfall. The pendulous flowers are visited by a range of small insects but the endemic hoverfly Loveridgeana beattiei (Doesburg & Doesburg) is particularly common. The light, windblown achenes spread reasonably efficiently and germinate in large numbers during winter where suitable conditions exist, even on arid land.

Gumwood supports a number of endemic invertebrate species, several of which are now very rare or, in a few cases, perhaps extinct.

Systems:Terrestrial

Use and Trade [top]

Use and Trade:

The species has been used in the past for timber and fuel. Currently used for craft products including photo frames, with potential for more of this kind of utilization in the future.

The Gumwood was recognized in 1977 as St Helena’s national tree.

Threats [top]

Major Threat(s):

The vast tracts of Gumwood woodland largely disappeared in well under a century following the establishment of a permanent British colony on St Helena in 1659. The principle cause of the losses were probably land clearances to make way for pasture, and the felling of trees for firewood and (less suitably) for timber. At the same time, foliage was browsed by goats (Capra hircus) and seedlings uprooted by the numerous wild pigs (Sus domesticus) and cattle (Bos taurus). These large vertebrate herbivores have now largely been eradicated on the island.

There is little regeneration of the species in the wild. At the Peak Dale site this is mainly due to widespread damage to seedlings by feral cattle, apart from a small fenced section on private land and another fenced section managed by the St Helena Government’s Terrestrial Conservation Section. Regeneration is also hampered by invasion of the understorey by weeds; a continuous ground layer prevents the light, wind-blown achenes from reaching the ground. The trees are also prone to damage from rats which bark the trees for their gummy sap, and Rabbits (Oryctolagus cuniculus) which graze the Gumwood seedlings. With little natural regeneration and the majority of the mature specimens being of approximately the same age and relatively old, it is not difficult to infer a current and future continuing decline in the number of mature individuals in the population. Gumwood habitat is also under severe threat from competition by invasive species, particularly wild mango (Schinus terebinthifolia Raddi) at Peak Dale.

Cairns-Wicks (2003) also noted that approximately 10% of the total population at Peak Dale was lost in the early 1990s as a result of Jacaranda Bug (Insignorthezia insignis (Browne)). The predatory ladybird Hyperaspis pantherina Fürsch was subsequently released as a biocontrol agent and this was successful at suppressing the outbreaks. However, Jacaranda Bug is still common on the trees and periodic population surges are of some concern. It is likely that the problem still contributes to low levels of mortality in combination with other stresses. Even in the absence of sap-sucking pests, the plants produce gummy exudates which may be colonized by black sooty moulds, coating the leaves and reducing photosynthesis. It is not known whether the sooty moulds are a recent introduction or if this is a long-standing, natural phenomenon.

Conservation Actions [top]

Conservation Actions:

Both extant sites are protected under the recently designated National Conservation Area (NCA) network. Peak Dale is located the Peaks National Park, and the Deep Valley subpopulations fall within the Deep Valley Nature Reserve. Development Plans for all NCAs are currently in development and will be legal documents. The species will also be protected under new legislation: the Environmental Protection Ordinance, which is expected to come into effect in late 2016 and will supersede the current Endangered Species Ordinance.

The St Helena Nature Conservation Group (SNCG) run a monthly conservation day at Peak Dale- the ‘Gumwood Guardians’- which helps clear invasive species, carries out rat baiting and helps plant back understorey within the site. They have funding from the Joint Nature Conservation Committee (JNCC) to carry out a research project which aims to develop an effective method for the conservation of endemic St Helena Gumwoods at Peak Dale and their associated ecosystem.

There are also plans underway to fence the entire Peak Dale population with funding from multiple sources (RSPB, JNCC, SNCG, and the St Helena Government). It is hoped that the exclusion of feral cattle, in addition to on-going rat baiting, will lead to greater regeneration at this site. The St Helena Government’s Terrestrial Conservation Section also operates a seed collection and storage programme to aid future conservation work. Gumwood seed is also stored within the Royal Botanic Gardens, Kew’s Millennium Seed Bank initiative.

Large-scale plantings have been made at both Peak Dale and Deep Valley to supplement numbers, and at other sites of varying ‘wildness’ (e.g. at Knotty Ridge, Horse Ridge, Longwood and the Millennium Forest). Except where subject to reasonably intensive management, survival of the transplanted individuals has been poor, principally due to competition with invasive species. The Millennium Forest is the most important of the initiatives. This community-led restoration site near Horse Point is currently run by the St Helena National Trust, with the aim of restoring 250 hectares of Gumwood woodland near the site of the lost Great Wood. The Great Wood was a Gumwood-dominated forest that once covered large swathes of the eastern side of the island. It had largely disappeared as a result of livestock grazing and timber harvesting by the mid 1700s, but fragments survived until the early 20th Century. Since 2000, over 10,000 trees have been planted across approximately 35 hectares at the Millennium Forest site, with plantings of other endemic species such as Dwarf Ebony (Trochetiopsis ebenus Cronk) occurring more recently. Survival rates have been moderately good, and many of the trees are now flowering although few currently exceed 2.5m in height. Abundant seedling germination has been recorded in some places, but none of the new generation have currently reached maturity and as the forest is currently managed (e.g. by weeding, watering and translocation of natural seedlings) it is debatable whether the progeny can be considered “wild” for some time.

Gumwoods are also planted in school endemic gardens, and in private gardens and other sites around the island. A commercial plantation for craft timber has also been established at Thompson’s Hill.


Citation: Ellick, S. & Lambdon, P.W. 2016. Commidendrum robustum. The IUCN Red List of Threatened Species 2016: e.T37583A67370371. . Downloaded on 28 July 2017.
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