Rarity of Strobilanthes Blume and Stenosiphonium Nees

 

Strobilanthes is a large genus (350-450 species) distributed widely over south and south-east Asia. In southern India and Sri Lanka there are 74 species found mainly along the Western Ghats of southern India and in the Sri Lankan Hill Country. Stenosiphonium, a genus closely related to Strobilanthes, comprises three species and is confined to southern India and Sri Lanka.

The lack of herbarium material for many species of the two genera is striking. Wood (1995) noted that three Sri Lankan species of Strobilanthes, namely S. caudata T. Anderson, S. hypericoides J.R.I. Wood and S. nigrescens T. Anderson are only known from the type collections and that a further six species are only known from a few collections, all of which were made over 60 years ago: S. arnottiana Nees, S. deflexa T. Anderson, S. gardneriana (Nees) T. Anderson, S. punctata Nees, S. thwaitesii T. Anderson and S. zeylanica T. Anderson. Thus, nine species out of a total of 30 in Sri Lanka are rare i.e. known only from type or from few collections made more than 60 years ago.

A BRAHMS database of Strobilanthes herbarium specimens at BM, K, MH, OXF, is currently being produced by Elizabeth Moylan. From this we can provisionally extend the list of ?rare? species (i.e. those known from only a few collections) to include the following from southern India: S. amabilis, S. andersonii, S. anamallica, S. bolampattiana, S. canarica, S. ciliata, S. circarensis, S. decurrens, S. dupeni, S. gracilis, S. meeboldii, S. papillosa, S. reticulata, S. scorbiculata, S. urceolaris.

Additionally, whilst Stenosiphonium cordifolium (Vahl) Alston has been widely collected, S. wightii Bremek. and S. setosum T. Anderson are known from five and seven collections respectively. Thus together, the two genera comprise 77 species in southern India and Sri Lanka, of which 26 may be considered rare. 

Strobilanthes is often the dominant shrub in upland forests of the southern Indian and Sri Lankan hills. However, large areas of such vegetation in southern India and Sri Lanka were cleared for plantations during the last century, and in the forests of southern India, Strobilanthes were cut down to encourage the growth of young tree seedlings (Gamble 1888). The destruction of habitats, and the selective cutting of Strobilanthes may have led to some species becoming extinct.

Recently however several rare species have been rediscovered: Strobilanthes punctata Nees, rediscovered after 67 years; S. dupeni Bedd., previously only known from the type specimen collected by Beddome in 1881; S. arnottiana Nees, last collected by Beddome; S. decurrens Nees, again previously known only from Beddome collections; Stenosiphonium setosum rediscovered by botanists from TBGRI, Palode, Kerala (see Thomas et. al., 1995) 78 years after the last collection. It would appear that the destruction of habitats alone does not account for the rarity of so many species of Strobilanthes and Stenosiphonium.

Another factor that may contribute to the apparent rarity of some species is the plietesial flowering cycle which has been recorded for several Strobilanthes (Matthew 1970; Wood 1994, 1995). Plietesial plants grow for a number of years, flower gregariously, set seed and then die.

In southern India the most famous plietesial Strobilanthes is S. kunthiana (Nees) T. Anderson. Robinson (1935) recorded a twelve year flowering cycle for this species from the Nilgiri Hills between 1838 and 1934, based on the diaries of one of the earliest European families to settle in the Hills.  Matthew (1970) reported a twelve year flowering cycle for the same species in the Palni Hills. Nilgiri means blue mountains and it is suggested that the gregarious flowering of the blue flowered S. kunthiana gave the hills their name.

The flowering cycle of S. kunthiana and of a handful of other Strobilanthes species is well documented. However, for the majority of Strobilanthes and for Stenosiphonium there are no detailed records.

During early 1996, I spent two months in Sri Lanka and southern India collecting Strobilanthes and Stenosiphonium and visiting herbaria. I visited several localities where Stenosiphonium cordifolium had been collected flowering in abundance one or two years earlier, and found a small flowering population at just one of the localities. At all others there were abundant seedlings. Whilst this is certainly not conclusive evidence for a plietesial flowering cycle, it does at least suggest that after gregarious flowering of S. cordifolium the plants die, and are followed by the rapid germination of seedlings of the next generation.

I returned again to southern India in 1997. I had already collected Stenosiphonium cordifolium and S. setosum (the latter from the living collection at TBGRI), and on my return trip I was keen to collect the third species of Stenosiphonium - S. wightii. S. wightii is known only from the following collections (herbaria given in brackets:

All were collected from the Mundanthurai Forest area which has been extensively botanized since the time of Robert Wight in the early 1800?s. The paucity of specimens of this species was therefore surprising.

Five of the collections are dated - 1947, 1958, 1971, 1984, and the intervals between them are 11, 13 and 13 years respectively. The apparent rarity of Stenosiphonium wightii therefore might be because it is plietesial, with a 13 year cycle. If so then 1997, 13 years after 1984, should have been a year of mass flowering.

Soon after my arrival in India early in 1997, I visited Mr Chellandurai, Government Botanist at the Survey of Medicinal Plants Unit in the Sidha Medical College, Palyamkottai. He had visited the Mundanthurai Forest during February of 1997 and had collected Stenosiphonium wightii which was flowering gregariously over a long distance along the roadside. Stenosiphonium wightii known previously from only seven collections was growing in abundance in 1997. The apparent rarity of this plant is a reflection of it?s flowering cycle. To collect S. wightii you need to be in the right place at the right time - Mundanthurai, every 13 years.

There is however a potential flaw in this story. The interval between the 1947 and 1958 collection is only 11 years. Further, the Sidha Medical College herbarium contained another specimen from 1987 (the label of which described it as ?rare?). Neither specimen fits in with the 13 year cycle, but nor do they disprove the plietesial idea. Both Wood (1994) and Matthew (1970) have observed that in plietesial species of Strobilanthes a number of individuals in a population flower asynchronously. These are most abundant in the years immediately preceding and following a mass flowering. The 1947 specimen (two years out of synchrony), and the rare 1987 specimen (three years out) are probably asynchronous individuals. I visited the Kerala High Ranges in 1997, three years after a mass flowering of S. kunthiana. Whereas the hillside was covered in blooms in 1994, I found, after a lot of searching, a single specimen in 1997.

The S. wightii example would suggest that there are more plietesial species than have so far been recorded in the literature. If so, could data from herbarium specimens be used to predict the next mass flowering of other species? S. wightii with such a restricted distribution and sufficient specimen details allowed us to generate a hypothesis to explain its apparent rarity. Luckily, doing this in 1997, a year of mass flowering, allowed us to at least partially test the idea, and conclude that the apparent rarity of S. wightii is a reflection of its plietesial life cycle [it is of course important to collect data on abundance of flowering specimens etc. during the coming years, and any information on this would be very welcome].

Unfortunately, the S. wightii example was the exception rather than the rule. The age of collections of most other rare species means that they lack locality details and dates. Even if they have this information, determining flowering cycles can still be problematic. In the S. kunthiana example, including the rare flowering specimen from 1997 in the sequence of dates, and taking no account of the abundance of the plant during that year would totally obscure the twelve year plietesial flowering cycle of this species. For specimens to be of any use they must include precise locality details, date and abundance, and few rare species are known to this level of detail.

The destruction of the natural vegetation of the hills of southern India and Sri Lanka for plantations etc. must surely have threatened the survival of some species, and for those, it might be too late. However, other under-collected species might be plietesial, and the fact that considerable populations of several species have recently been rediscovered after many years adds some weight to this idea.

Collecting Strobilanthes and Stenosiphonium. involves being in the right place at the right time. Further botanical explorations might yet show other apparently rare species to be locally abundant, if only periodically.

If anyone has observed gregarious flowering in Strobilanthes I would be interested to receive details.

References

Gamble, J. S. 1888. The Nilgiri ?Strobilanthes?. Indian Forester 14: 153-158 .

Matthew, K. M. 1970. The Flowering of the Strobilanth. Journal of the Bombay Natural History Society 67: 502-506.

Robinson, M.E. 1935. The flowering of Strobilanthes in 1934. Journal of the Bombay Natural History Society 38: 117-122

Thomas, J., Santhosh Kumar, E.S.S., Jabbar, A. 1995. Stenosiphonium wightii Bremek. (Acanthaceae): A new record from Kerala. Journal of Economic and Taxonomic Botany 19: 483-485.

Wood, J.R.I. 1994. Notes relating to the Flora of Bhutan: 29. Acanthaceae, with special reference to Strobilanthes. Edinburgh Journal of Botany 51: 175-273

Wood, J.R.I. 1995. Notes on Strobilanthes (Acanthaceae) for the Flora of Ceylon. Kew Bulletin 50: 1-24