Plastic Responses to Different Habitat Type Contribute to Hyptis suaveolens Poit. Invasiveness in the Dry Deciduous Forest of India

A new species, when introduced into a novel environment, must become established within the constraints of site characteristics and competition with resident vegetation. Local adaptation to environmental conditions is common among plants (1). Phenotypic plasticity is potentially an important mechanism for colonization success in environmentally diverse areas and contributes to the success of invaders in diverse habitats (2, 3). Richardson and Pyšek (4) suggested that for a species to become a widespread invader it should be plastic or evolve rapidly. Studies on several colonizing species have confirmed that their populations across broad geographic and environmental ranges consisted of genetically similar populations of highly plastic genotypes (5). Rapid adaptation to small-scale environmental conditions may increase both the number of invaded habitats within regions and the dominance within habitats (6). Here we consider phenotypic plasticity as the ability for the same genotype to produce different phenotypes in response to different environments. Phenotypic plasticity is believed to be the primary strategy for these invaders to adapt to new and heterogeneous environments (6, 7). Brock et al. (8) have shown that invasiveness involves adaptive plasticity for traits that increase fitness while maintaining constancy for traits such as high dispersal potential. Phenotypic plasticity buffers organisms from environmental stochasticity and confers the ability to function in different/new environment. The success of an alien species depends on the degree of invasiveness, i.e. the potential to establish and spread (9). A few biological features can be strong predictors of potential invasiveness of a species (9, 10). Plant height and seed mass may act as indicators of both the establishment and regenerative phases of the life cycle (11, 12) and are the most pertinent traits that address competitive ability (13). Many studies have demonstrated local adaptation in local populations (14–17). We suggest that Hyptis suaveolens Poit. (American Mint; Lamiaceae), an invasive alien weed, is mainly a ‘‘plastic generalist,’’ but local adaptation through plastic responses may also play a role in its successful occupation of contrasting habitats. The study intends to test that the colonizing ability of an invader is a function of plasticity for fitness-related traits, enabling it to cope with and perhaps to benefit from habitat conditions. We evaluated the response, in terms of plant height and seed mass, of H. suaveolens to different habitats in the dry deciduous forest of India. H. suaveolens (Lamiaceae) is an erect annual woody herb, commonly 1 m in height (maximum height 1⁄4 1.5 m), and reproduces by seed (18). It is commonly known as horehound weed or mint weed, is native to the tropical America, and is now a pantropic weed (19). The leaves are ovate, 4 to 9 centimeters long, pointed at the tip, pointed to somewhat heart-shaped at the base, and toothed on the margins. The flowers occur in the axils of the leaves in long stalks. The calyx in flower is hairy and about 4 millimeters long, but soon enlarges in fruit to about 1 centimeter long with erect, stiff teeth. The corolla is blue and about 8 millimeters long, with a limb 5 millimeters in diameter; nutlets are about 1.2–1.5 millimeters long (20). The seeds are flat and characteristically dimorphic. H. suaveolens is a prolific seed producer, and heavy infestations can yield up to ’3000 seeds per square meter, forming persistent propagule banks within a short period. It is found on a variety of habitats like rail tracks, roadsides, foothills of open forests, and forest clearings and can heavily infest wastelands, particularly on arid and rocky substrates (21, 22). In India H. suaveolens was not recorded in the Vindhyan highlands in 1903 (23). However, Agrawal (24) reported heavy infestations of H. suaveolens 2 kilometers north of the super thermal power plant at Singrauli, where as recently as 10 years ago the species was not prevalent. H. suaveolens has spread to considerable stretch in open areas of this forest and can be classified as a major common and widespread invader (25, 26). Richardson and Pyšek (4) suggested that invasive species establish and proliferate across a range of environmental conditions through their plastic responses. So, we sampled H. suaveolens in two habitats across the landscapes to observe the possible plastic response of this species. We identified two habitat types: tar roads verges and natural areas in the Vindhyan dry deciduous forest of India (24813 to 24819N; 83859 to 83813E and altitude of 315–485 masl). Climate is tropical monsoonal, with mean annual rainfall of 821 mm. H. suaveolens population densities were sampled during its peak growth period (September to early October 2006) using 30 1 1 m quadrats randomly located within each habitat. Each H. suaveolens population was sampled for height, number of spikes, number of flowers on each spike, seeds on each spike, total number of seeds, seed size, and weight per 50 seeds for an average individual in each quadrat. The Reproductive Index (Ri) was estimated as the ratio of total number of seeds to the height of the plant (27, 28). The effect of different habitats on plant growth and reproductive parameters was analyzed by one-way analysis of variance (ANOVA). Differences in H. suaveolens population between the habitats were tested by Tukey’s HSD test (at p , 0.05). Linear correlation regression was performed using the SPSS statistical package (29). H. suaveolens populations showed considerable differences in plant height in different habitat conditions ( f1,581⁄4925.18, p , 0.001). The average height ranged from 75 cm to 152 cm in natural areas and along tar roads, respectively (Table 1). Interestingly, ANOVA also indicated significant differences in the number of branchlets per plant ( f1,58 1⁄4 164.61, p , 0.001), number of flowers on each branchlet ( f1,58 1⁄4 88.43, p , 0.00), number of seed on each branchlet ( f1,581⁄4 123.13, p , 0.00), total number of seed per plant ( f1,58 1⁄4 131.38, p , 0.001), average seed size ( f1,58 1⁄4 216.33, p , 0.001), and average seed weight ( f1,58 1⁄4 148.25, p , 0.001). H. suaveolens density also differed significantly among the habitats ( f1,58 1⁄4 77.60, p , 0.001). Table 1 summarizes the vegetative and reproductive characteristics of H. suaveolens in different habitats. Reproductive index is indicative of the influence of plant size on seed production and proportion of energy allocated to seeds. Large plants seem to have reproductive advantage, as they produced a large number of seeds (30). Height is an indicator of relative size and may explain the greater ability of larger plants to