Friend or foe – The Lantana conundrumSun, 2013-09-01 16:05
By Geetha Ramaswami.
Lantana camara L. (lantana), a harmless looking thorny shrub with beautiful yellow and red flowers, is considered to be one of the world’s 100 most noxious weeds. Although a native of South and Central America, it is now found across dry tropics of India. It is often associated with reduced biodiversity and the chemicals it produces, called allelopathic substances, is speculated to be detrimental to growth of native species.
This shrub of central American origin made its way to India during British colonial era as an ornamental plant, almost two hundred years ago. British colonialists left us long ago but Lantana has continued its invasion and is thriving by widespead colonization across dry tropics. The threat of Lantana turning into a weed was recognized as early as 1895 in the Indian Forester (1895) where an anonymous writer asks “Lantana, a friend or an enemy?”. There is an overwhelming amount of support for the idea of lantana being an ‘enemy’. However, even more than a century later, the question is still pertinent and answers are elusive. How does Lantana affect growth of native plants? Can we take a step back and assess lantana simply as another cause of change in a landscape?
To understand ‘our guest’ lantana “who came for dinner and never left”, we tested a number of hypotheses on a permanent vegetation plot in the dry deciduous forest of Mudumalai. In this plot, set up by my PhD adviser Prof. Sukumar, the history of lantana invasion since 1989 was carefully recorded.
Our speculation was that lantana could affect the distribution of native woody species by acting as a filter for important ecological processes such as dispersal. Let us first look at dispersal of seeds, a process that is essential for a plant species to spread from one area to other. Seeds can disperse by physical means such as being blown away by wind or by biological means through gut of frugivorous animals. One could speculate that a lantana thicket could influence the kinds of seeds that arrive under it as it may form a barrier for dispersal by physical mechanisms, but it could also facilitate the occurrence of mammal- and bird-dispersed seeds, as animals have been observed to utilise lantana thickets. Lantana thickets could act as a barrier at the species’ niche level as well, by reducing the light reaching the ground, eventually affecting seedling growth and survival. The percent shade under lantana can be as high as 90%, whereas ambient shade in the forest is around 70%. One can then expect that moist forest shade-loving species would not be affected by lantana thickets, but dry forest light-loving species would, and species that have no habitat preference would be unaffected as well.
Surprisingly, our research found that lantana thickets were restricting only two types of species – those that were mechanically dispersed and had no habitat preference, and those that were mammal dispersed and had a preference for dry forests. Bird dispersed species with different habitat preferences were completely unaffected by the presence of lantana thickets. Is lantana, then, an enemy only to some species and a harmless bystander in the lives of others?
We decided to investigate whether lantana consistently affects light-loving species over a longer time span. To test whether lantana really affects the performance of already established seedlings (you could also call these ‘recruits’ or ‘resprouts’) by reducing light availability in this highly seasonal forest, we recorded light availability under dense lantana. We found it to be lower than ambient conditions in most months, but significantly so in three months out of the eleven months sampled. We tagged and monitored recruits every two months for a period of two years to test if lantana density, species habitat preference and rainfall affected their growth.
We found that the growth rates of recruits in this forest were very low – close to zero if not negative most of the time. Surprisingly, except for one species (Catunaregam spinosa), lantana did not seem to affect growth rates of recruits at all, and on the whole, light loving species grew faster than shade loving species. Recruit survival was very high overall, probably because these stems had the ability to sprout back after loss of above-ground biomass. Survival was higher for seedlings of dry forest preferring species than moist forest preferring species and increased with increasing rainfall. Seedling survival was also not affected by the presence of dense lantana.
Can we then conclude that lantana is in fact neither an enemy nor a friend to native woody plants, but simply a background fixture in this forest? Studies from other dry forests further north of Mudumalai seem to disagree, attributing the absence of the ‘sapling’ size class of woody plants in these forests to the indirect influence of lantana. Our studies could be contradictory simple because of non-standard methods of measuring the effects of lantana (springing largely from the lack of communication between researchers!). Or, the effects of lantana could be highly site specific – influenced by local environmental conditions (such as soils and fire regimes) and management history. But while woody plants may remain unaffected by the presence of lantana thickets, non woody plants such as grasses do get shaded out even in Mudumalai. How is the replacement of the grassy understory by lantana affecting fire regimes, nutrient cycles, and the herbivores that depend on this forage? Most answers to this question are anecdotal and inconclusive, offering a field of research ripe for exploration.
There are many reasons for why lantana spreads are still poorly understood. In other studies, we have found evidence that the combination of drought and fire occurrence favours the intensification of lantana invasion and the preference for stream habitats by this plant. The change from traditional management practices by forest dwellers – such as patchy, “cold” burning in winter - to the current practice of complete fire suppression by state forest departments, could also have triggered the spread of lantana. An ideal management plan would probably incorporate lessons from traditional forest dwellers and the results from scientific studies in small-scale exploratory experiments before prescribing practices to the landscape scale. This is a time-consuming endeavour that is required urgently.
Like the Lernaean Hydra of Greek mythology, lantana’s rearing head can literally sprout back manyfold when chopped – thanks to its coppicing abilities. While the debate about ‘Lantana – friend or enemy’ continues without a coherent answer in sight, effective management of this plant in dry forest landscapes of the world remains a frustratingly Herculean task indeed.
Bhagwat, S. A., E. Breman, T. Thekaekara, T. F. Thornton, and K. J. Willis. 2012. A battle lost? Report on two centuries of invasion and management of Lantana camara L. in Australia, India and South Africa. PlosOne 7.
Prasad, A. E. 2010. Effects of an exotic plant invasion on native understory plants in a tropical dry forest. Conservation Biology 24:747–757.
Ramaswami, G. & Sukumar, R. 2011. Woody plant seedling distribution under invasive Lantana camara thickets in a dry-forest plot in Mudumalai, southern India. Journal of Tropical Ecology. 27:365–373
Sundaram, B., and A. J. Hiremath. 2011. Lantana camara invasion in a heterogeneous landscape: patterns of spread and correlation with changes in native vegetation. Biological Invasions. 14:1127-1141.