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Nesohedyotis arborea 

Scope: Global
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Taxonomy [top]

Kingdom Phylum Class Order Family
Plantae Tracheophyta Magnoliopsida Rubiales Rubiaceae

Scientific Name: Nesohedyotis arborea
Species Authority: (Roxb.) Bremek.
Common Name(s):
English St Helena Dogwood, Dogwood
Synonym(s):
Hedyotis arborea Roxb.
Taxonomic Notes: Endemic genus of the Hedyotideae, resembling East African genera Hedythyrsus and Pseudonesohedyotis (Tennant 1965), which are small shrubs and not small trees like Nesohedyotis.

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A2a; B1ab(iii,v)+2ab(iii,v); C1+2a(i); D ver 3.1
Year Published: 2016
Date Assessed: 2015-07-28
Assessor(s): Ellick, S. & Lambdon, P.W.
Reviewer(s): Clubbe, C.P.
Contributor(s): Cairns-Wicks, R.
Justification:
Dogwood (Nesohedyotis arborea) has a very restricted distribution, confined to two fragments of cloud forest which are both less than 2 km across. Although a number of nursery-grown plants have been reintroduced to natural situations, the rate of sapling recruitment remains very low and the  total population of truly wild plants has declined to less than 50 mature individuals. This represents a fall of approximately 72% over less than one generation (about 100 years), qualifying the species as Critically Endangered under criteria A2 and C1. As there is no opportunity for cross colonization between sites, and neither population is likely to be stable in the long-term, the distribution can be considered severely fragmented and thus also meets the necessary thresholds under both B1 and B2.
Previously published Red List assessments:

Geographic Range [top]

Range Description:

Endemic to St Helena Island, South Atlantic Ocean.

The extent of occurrence (EOO), based on the area of a minimum convex polygon around known localities, is 7.7 km2. The area of occupancy (AOO), based on a 2 km × 2 km grid, is 8 km2. Following IUCN Red List Guidelines, the EOO is therefore increased to 8 km2 to match the AOO.

The species is restricted to the Central Ridge above 700 m and is found in only two locations within the recently designated Peaks National Park: along Diana’s Peak Ridge and on the cliffs at High Peak. 

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):750
Upper elevation limit (metres):823
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:

A 2013 – 14 census estimated the number of truly wild mature Dogwoods to number just 47 individuals; 24 of these were located in the High Peak area (55.3% of the total population) where they occupy a very restricted expanse of suitable habitat, covering approximately 8.8 ha of cliff face hemmed between pasture and flax stands. The rest (44.7%) are very thinly scattered over a much larger area along Diana’s Peak Ridge. This represents a decline in this species of 72% over less than one generation, from approximately 132 individuals in the mid-1990s (Cairns-Wicks 2003). Counts are somewhat uncertain due to the difficulty of distinguishing Dogwood from the unrelated but superficially very similar Whitewood (Petrobium arboreum (J.R.Forst. & G.Forst.) R. Br.).


A number of cultivated specimens have been transplanted back into wild situations in the Diana’s Peak area. It is now unclear exactly which plants were introduced and perhaps the true wild population is smaller or larger than estimated. However, the best guesses indicate that there are approximately an additional 100 individuals surviving from the recovery programme, with 49 around Warren’s Valley and 45 near Cuckold’s Point (the most southerly of three summits). The major efforts at re-vegetation occurred between 1996 and 2005 (Smith and Williams 1996). Many of the specimens are now mature and have seeded well for some years, but there has been little seedling recruitment.

Current Population Trend:Decreasing
Additional data:
Number of mature individuals:47Continuing decline of mature individuals:Yes
Population severely fragmented:Yes
No. of subpopulations:2

Habitat and Ecology [top]

Habitat and Ecology:Found in damp Tree Fern thicket and cabbage tree woodland, the Dogwood is well-adapted to the cloud forest zone. Although the natural distribution is somewhat obscured by recent plantings, surviving wild trees are most commonly found in dense thickets of cabbage trees, ranging from sheltered slopes to exposed situations just below the summits. Old trees are resilient to high winds but younger saplings seem to require sheltered environments in order to establish. The species is an excellent interceptor of mist, with the leaf tips adapted to drip-tips which help to condense the mist. Dogwood also roots easily, allowing fallen trees to recover rapidly following landslips, etc. The species is functionally dioecious; male plants possess reduced stigmas and female plants possess reduced anthers, but in both cases the poorly developed sexual organs seldom if ever appear to be fertile. The flowering season is between late summer and early winter and pollination is by insects, mainly hoverflies (Percy and Cronk 1997). The seed can be dispersed via wind; however, it does not travel far from the parent plant.
Systems:Terrestrial
Continuing decline in area, extent and/or quality of habitat:Yes
Generation Length (years):100

Threats [top]

Major Threat(s): Until the planted individuals reach sexual maturity the number of mature Dogwoods remains low and fragmented. The quality of the habitat at High Peak is deteriorating potentially threatening the Dogwoods growing there.

Conservation Actions [top]

Conservation Actions:

The species is currently protected by the Endangered Species Ordinance; however, this is due to be superseded by the Environmental Protection Ordinance (currently in the final stages of drafting), which is due to be in place in 2016.

The species is also afforded protection via the recently designated National Conservation Area (NCA) network, as it falls within the Peaks National Park boundary, and will be safeguarded through NCA Development Plans (in development).

The St Helena Government’s ‘Habitats Team’ conduct forest restoration and removal of invasive species within the Diana’s Peak area. The team also collects seed from wild and planted trees and rear seedlings from this. These are then planted back into suitable habitat. It is recommended that groups of at least 20 trees are established in close proximity to increase the chances of achieving a suitable diversity of both sexes. Recent efforts to restore cabbage tree woodland have been initiated in the High Peak area, and further attempts to rehabilitate Dogwood saplings form part of the management plans.

The St Helena Government’s Terrestrial Conservation Section also operate a seed collection and storage programme to aid future conservation work.


Citation: Ellick, S. & Lambdon, P.W. 2016. Nesohedyotis arborea. The IUCN Red List of Threatened Species 2016: e.T30378A67379194. . Downloaded on 27 August 2016.
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