A Brief Note on Masting and its implication with reference to some Strobilanthes spp.

Madhu Ramnath

Introduction

For reasons that are as yet quite unclear, some plants of the same specie flower simultaneously and gregariously once in several years, a characteristic known as masting. Unlike plants that flower and fruit annually or biennially, masting is a pattern of reproduction in which an entire population of a species reproduce together. This entails a synchronicity in flowering and fruiting of a species – sometimes even in different continents – setting off a chain of events that may cause changes in the various animal populations in a forest.

There is no conclusive explanation of the masting behaviour but some theories have been put forward to account for this phenomenon. It is supposed that masting is a response to predators and the habit is most pronounced in plant populations subject to the heaviest seed predation. On the other hand, we can expect masting to be absent in species that produce animal-dispersed fruit with protected seeds: the satiation of the dispersal agents by masting would be quite poor strategy[tooltip content=”Silvertown, J.W., The Evolutionary Ecology of Mast Seeding in Trees; The Linnean Society of London, 1980.” url=”” ]Ref:1[/tooltip]. In tree populations that suffer high levels of pre-dispersal seed mortality in years of low seed production, the masting habit appears well developed. Some biologists claim that there is evidence to suggest that mast years are highly correlated to climatic conditions and that trees use environmental cues to achieve synchrony; poor seed production may be a consequence of environmental and physiological constraints[tooltip content=”ibid” url=”” ]Ref:2[/tooltip]. However, Janzen concluded that the timing of reproduction in plietesials (plants that flower en masse and die) is set by an internal physiological calendar and not by external weather cues[tooltip content=”Janzen, D.H., Why bamboos wait so long to flower; Annual Review of Ecology and Systematics, 7, 347- 391, 1986.” url=”” ]Ref:3[/tooltip]!

Effects of masting

Whatever the causes of masting in trees and shrubs may be, the effects on animal populations are vast and cannot be always predicted. A much publicized effect of the flowering of bamboo is the famine that will follow. This is usually due to the increased population of bush rats and field mice (of several forest species, such as Mus booduga, M.platythrix, Gollunda elliotii, Rattus blandfordi, Vandeleuria oleracea, etc.) following the mast seeding of leads them to the agricultural crop and causing the aforementioned famine.

Among indigenous people who also hunt and eat rodents, mast seeding of bamboo led to an additional supply of meat: in turn, agricultural bamboo (Dendrocalamus strictus), as happened in parts of central India in the years 1992-93. After the masting year(s), the increased rodent population then goes back to its habitual food sources which, not being enough to satiate the larger numbers,damage was effectively controlled!! Other impacts of an increased rodent population may be on ground-nesting birds and insects that may be predated upon by the rodents after the masting period is over. In one study[tooltip content=”Koenig, W., & Knops, J., The Mystery of Masting in Trees; American Scientist, July-August 2005.” url=”” ]Ref:4[/tooltip] of masting of (acorn) Querus spp. trees in a deciduous forest, the effect increased mice and deer population as well as ticks, leading to higher incidents of Lyme disease.

The large gaps in our understanding of the ripple effects on animal populations in any forest, due to the masting phenomenon, should make us cautious. Interventions, especially those that change land-use patterns or affect ecosystems, need to be well considered from this rather challenging point of view. Apart from the various species of bamboo that have flowered and offered us an opportunity to observe the chain of events that follow, we also have several species of Strobilanthes that exhibit the masting character. Several of these species such as S.kunthianus, S.consanguinea, S.callosa[tooltip content=”S.callosa and the following 4 species listed have now been placed in other genera; most of them have a 7-year flowering cycle.” url=”” ]Ref:5[/tooltip], S.perfoliatus, S.sessilis, S.scrobiculata and S.ixiocephala are essentially confined to the Sahyadris; S.auriculata[tooltip content=”S.auriculata is recorded from Bastar, Chhattisgarh, said to flower in a 12-year cycle. Last flowered in the winter of 2000.” url=”” ]Ref:6[/tooltip] belongs to the central Indian forest. Many of the lesser known species that flower only periodically have been little studied, for the simple reason that their flowers have been unavailable for botanical scrutiny during field visits.

STROBILANTHES[tooltip content=”These are herbs, shrubs and undershrubs with their young branches 4-angled. The leaves are opposite and unequal , toothed or subentire, petiolate and with cystoliths. The secondary nerves are arched in most species, and run obliquely nearly up to the margin and then go parallel to it. The flowers are in axillary and/or terminal spikes or panicles. Bract 1, bracteoles 2, small or sometimes absent. Calyx usually 5-partite, segments free or basally connate. Corolla tube ventricose above, lobes 5, subequal, twisted to the left in bud. Stamens 2, anthers oblong, 2-celled, muticous. Ovary 2-locular, ovules 2 in each locule. Style filiform, stigma of one linear branch, other obsolete. Capsules oblong, 2 or 4 seeded; seeds compressed, glabrous, on curved retinacula.” url=”” ]Ref:7[/tooltip]

From the Greek strobilus (a cone) and anthos (a flower), referring to the dense inflorescence.

Many species of this genus flower only periodically (and usually gregariously) and then die. However, from the available literature we find that there is not much consensus about what constitutes this genus. Hooker[tooltip content=”Hooker, J.D.; Flora of British India (1875; L. Reeve & Co. 5, Henrietta Street, Covent Garden, London)” url=”” ]Ref:8[/tooltip], apart from saying that many of the species differ much in habit, inflorescence and the size of flowers, limits the species to those with 2 ovules in each cell of the ovary. Those species with more than 2 ovules have been relegated to other genera. The total number of species is given as 181, of which 180 belong to Asia and 1 to Africa; other sources[tooltip content=”Wikipedia, Strobilanthes” url=”” ]Ref:9[/tooltip] list 250 species! Haines[tooltip content=”Haines, H.H.,: The botany of Bihar and Odisha (1924; Adlard & Son & West Newman, Ltd., UK)” url=”” ]Ref:10[/tooltip] lists 4 species for Bihar and Odisha; the flowers of only two species have been seen (S. scaber and S. auriculatus) and happen to have flowers in dense spikes; the determination of the other two species is uncertain.

According to Bremekamp[tooltip content=”Santapu, H., and Henry, A.N.; A dictionary of flowering plants in India (1973; CSIR, New Delhi)” url=”” ]Ref:11[/tooltip], the genus Strobilanthes Bl. in the restricted sense does not occur in India and the numerous species treated as such in Indian floras belong to smaller genera. Heywood[tooltip content=”Heywood, V.H., and Chant, S.R.,; Popular encyclopedia of plants (1982; Cambridge University Press)” url=”” ]Ref:12[/tooltip] claims that there are 250-300 species with flowers that are blue, violet, white or sometimes yellowish, borne in spikes, panicles or singly in the leaf axils. Fyson[tooltip content=”Fyson, P.F.,; The flora of the Nilgiri and Pulney hill-tops (1915; Government Press, Madras)” url=”” ]Ref:13[/tooltip] adds that the various species are marked in their variations in pollen grains due to which the Indian species should be divided into (four) different genera. Accordingly, Stobilanthes Bl. are those with four stamens,ellipsoid pollen grains with 12-20 longitudinal ribs. Mabberly[tooltip content=”Mabberly, D.J.,; The Plant book (1997, Cambridge University Press, UK)” url=”” ]Ref:14[/tooltip] sums up the situation for Strobilanthes with “sometimes split into many unsatisfactory segregate genera”.

Masting in Strobilanthes

Masting is uncommon among herbs and shrubs, perhaps because these plants are short-lived; this may suggest a relation between longevity and mast intervals for masting trees[tooltip content=”Silvertown” url=”” ]Ref:15[/tooltip]. The time required for the accumulation of starch, proteins, etc., to produce a mast crop of seeds – in sufficient quantity to satiate the predators and some to spare – may be an important factor in determining the length of the inter-mast period. The plants of the genus Strobilanthes are mainly shrubs or herbs and there is considerable variation in the life history of the species. A few points with respect to these discrepancies are that:

a. Most known plietesial Strobilanthes take between 10-15 years to flower gregariously, set seed and die, (though there are species that do this between 5-9 years);

b. The flowering periodicity in all individuals of a species is rarely 100%: some plants of the species do flower in non-mass flowering years;

c. In some species mass flowering occurs over a wide area on a species-specific cycle, in other species populations in different regions follow their own cycles[tooltip content=”Daniel, T.F., Synchronic flowering and monocarpy suggest plietesial life history for Neotropical Stenostephanus chiapensis (Acanthaceae); Proceedings of the California Academy of Sciences, Vol 57 No 38; p 1011-1018, 2008.” url=”” ]Ref:16[/tooltip];

d. Strobilanthes wallichii, a perennial specie flowers annually in the Eastern Himalayan portions of the range and flowers plietesially in the Western Himalayan portion[tooltip content=”ibid” url=”” ]Ref:17[/tooltip]. Such variation in the life history of a species and the occasional flowering of some plietesial species in non-mass flowering years may suggest that the behaviour of Strobilanthes is governed by some complex relationship between genetics and environment.

It has been noted that there are strong relations between plants and their pollinators, with such mutualisms dating back to the Cretaceous period[tooltip content=”Kearns, C.A., & Inouye, D.W., Pollinators, Flowering Plants and Conservation Biology; Bioscience Vol 47, No 5, American Institute of Biological Sciences, 1997.” url=”” ]Ref:18[/tooltip]. Insects acquired food from flowers and, in turn, flowers achieved higher reproductive success through insect pollination. It has been observed [tooltip content=”Anitha, K., & Prasad, S.N., Mass flowering of Strobilanthes consanguinea in the Western Ghats, India; Current Science Vol 92, No 12, June 2007.” url=”” ]Ref:19[/tooltip]that during the mass blooming of Strobilanthes kunthianus there was a migration of bees (both Apis dorsata and Apis cerana) in the Western Ghats of India, showing that plants attract pollinators by providing pollen and nectar as rewards. The extent of mass blooming is often so dense that seedlings of other forest species compete for survival with great difficulty.

CONCLUSION

The above note is essentially to show how little is known about such an important phenomenon as masting. Such characteristics in plants may lead to unexplained impacts on animal populations in any forest which may have larger ecological repercussions. It would be worthwhile to have field level botanists and biologists to document such ecological changes in their respective areas for a better understanding of our environment.