THE SERPENTINE VEGETATION OF USSANGODA (HAMBANTOTA DISTRICT) AND NICKEL ACCUMULATINa PLANT SPECIES
The vegetation of serpentine soils has fascinated plant taxonomists, physiologistsand ecologists. Many serpentine plant species are used as indicator plants in geobotanicalexploration of mineral deposits and phyto-rernediation of polluted soils.Some of the world's serpentine plants have the remarkable ability to selectivelyaccumulate Ni from the soil.
Serpentine body at Ussangoda is overlaid with an ultrmafic soil, which ischaracteristically rich in Ni, Cr, Fe and Mg. The plain is called 'Rathupas thalawa'- Red soil plain - due to the deep red color of the soil. It is host to a unique typeof vegetation tolerant of toxic edaphic condition. As is characteristic ofserpentine floras in other parts of the world the vegetation is sparse and thespecies diversity is low. The plants are stunted prostate in habit and show stressfeatures. The vegetation contrasts sharply with the adjacent non-serpentinevegetation of thorny scrubland and is comprised of plant associations dominatedby Hyabanthus cnneaspermus, Evolvulus alsinoides, Fimbristylis [alcata,Eurphorbia indica, Crotolaria latebrosa and Blumea obliqua. Among the otherplants which are confined to specific areas on the plain and are of limiteddistribution are Phyllanthus simplex, Mollugo nudicaulis, Cassia mimosoides,Chlorophvton laxum, Fimbristylis acuminata, Polygala javana, Ischaemumtimorense and Striga euphrasiodes.
All the plants listed above are serpentine facultative. Of special significance is theoccurrence of two types of Evolvulus alsinoides wither with blue flowers or withwhiteflowers. Hybanthus enneaspermus is also found in two types either withpink flowers or with white flowers. The two flower types of Evolvulus showdistinct flavonoid profiles on paper chromatograms. The existence of 'flavonoidraces' has been reported from other serpentine soils in the world.
The Ni content of the species analysed ranges from 173-2173 ppm on a dryweight basis and is as follows. Hybanthus enneaspermus 2174 ppm, strigaeuphrasioides 1400 ppm, Cassia mimosoides 1140 pprn, Blumea obliqua 1054pprn, Evolvulus alsinoides 1023 ppm, and Crotolaria latebrosa 604 ppm. Thissignifies that the mechanism of Ni tolerance is either detoxification and orextrusion rather than exclusion.
While the normal Ni content in plants is reported to be 2-15 ppm, all theserpentine species which we have analyzed were found to be accumulators(>100ppm.). Six of these were hyperaccumulators (>lOOppm.). The significanceof the above findings and the urgent necessity to conserve this unique vegetationand its habitat will be discussed.