ST HELENA OLIVE
Common name: St Helena Olive
Scientific name:Nesiota elliptica
IUCN status: Extinct in the wild
The olive is a native tree of the tree fern thicket and cabbage tree woodland. In 1850’s Melliss recorded that there were between 12 and 15 trees growing on the northern side of Diana’s Peak. In 1977, Mr. George Benjamin discovered a single plant, possibly the last survivor from the population described by Mellis over one hundred years earlier.
The tree that was found near Diana’s Peak was about 5m with a trunk diameter of about 0.4m. The tree had collapsed at one time so that part of the trunk was laid down. Roxburgh (1813-1814) described the olive as a”pretty large, but low spreading tree” with “dark coloured, hard and very useful” wood. The leaves, 50-70mm, are thick, rounded, curling outwards at the edges, hairy underneath and borne in opposite pairs.
The flowers are small, 10mm in diameter, pink and borne in clusters. The main flowering months are between June and July but flowering often continues throughout the year. Seed capsules develop slowly and usually contain three ovales. Today when fertilization is succesful, only one seed is usuallu viable.
The olive is self-incompatible. That is pollen from flowers on the same tree is rejected. Being unable to self-pollinate is a mechanism to ensure outcrossing beacuase different trees are needed to pollinate the flowers. However, when only one tree survives in the world then this mean that the tree is bound to become extinct. Fortunately for the olive, this incompatibility is “leaky in that occasionally a polen grain is able to succesfully reach and fertilize the ovules.
NUMBER OF SURVIVING POPULATIONS
After the discovery of the last tree in the wild, cuttings were taken to try and propagate it. Only two were succesful. One was planted in the Endemic Nursery at Scotland in September 1988. The second was planted near High Peak but subsequently died.
Hundreds of attempts have been made to try and get seed from both the wild tree and the cutting in the nursery. However, because the olive is self-incompatible only three seedlings have been raised from the the cutting grown at Scotland. Two seedlings were planted at Pounceys in November 1992 and one in Scotland nursery in 1995. Two seed which were produced this year (1997) have been given to George Benjamin to sow and we are keeping our fingers crossed that they will germinate.
The tree at Diana’s Peak died in October 1994. Thus the olive tree is extinct in the wild. About 14 fungal and bacterial infections were identified from this tree before it died. It is likely that they contributed to the death of the tree although some fungi and bacteria would have only infected the tree because it was dying.
The cuttings from Scotland also suffer from fungal infections which is contracted from the wild tree. The cutting was able to grow despite these infections however, stress in the plant brought about due to restricted root development and waterlogging has allowed these fungi to become more aggressive and are now killing the cutting.
CONSERVATION RECOVERY PROGRAMMES
Attempts to halt the infectons in the cutting have so far failed. New chemical fungicides, has arrived with the RMS and it is hoped that these will work where others have failed. However, because the tree is dying from the roots up it might be too late.
Considerable effort is being put in to trying to save the cutting because it is the last tree. The tree other olives are all seedlings from this cutting and thus are more inbred. The cutting could be the key to any recovery effort for the olive. It is planned that the small seedling growing at Scotland will be moved to the Peaks. It is thought that the preferred climate and growing conditions will be tye best way of encouraging healthy growth.
When flowers ara available pollinations are made between the cutting and the offspring. However, because of the close relationship between the plants the incompatibility mechanism still acts to prevent fertilization. Hundreds of pollinations will be required to obtain any seed.
It is planned to study the flowers to better understand the incompatibility mechanism. This follows on from the work of Andrew Jackson who came to St Helena to work on the olive in 1991. It is hoped that by understanding how the self-ncompatibility mechanism works we will be able to ‘overcome it’ and increase the success rate of fertilization. This work will be carried out using money donated by the WWF as part of the endemic plants rescue programme.