Trait plasticity: a key attribute in the invasion success of Ageratina adenophora in different forest types of Kumaun Himalaya, India

Successful invasion of biological entities does not merely depend on the rigorous habitat attributes of invaded regions, besides, the intrinsic traits of the invader species also play a significant role in the process of invasion. Evaluation of plants' functioning across habitats helps in understanding which traits are contributing to invasion success in response to changing environmental factors as well as the habitat preference and vulnerability of invasive plants. This study analyzed the effects of forest types on the growth and reproduction capacity of a dreadful invasive alien weed Ageratina adenophora (Spreng.) King and Rob. Different forest types selected for the present study were Sal, Sal-mixed, Chir-Pine, Banj-Oak, Cypress, and Kharsu-Oak forests in Kumaun Himalaya, India. At each forest site, twenty-four 1 x 1 m size quadrats were randomly laid during the peak growth period of A. adenophora (mid-March-April). The numbers of A. adenophora individuals/quadrats were also counted to compute the density of this weed in each forest type. Three plants were chosen at random from each quadrat and uprooted carefully followed by their morphological and reproductive traits assessment in each forest type following standard methods. To check the variability in soil physicochemical properties, the soil was collected from the surface layer (0-15 cm depth) of each forest type in replicates of three using soil corer. Other environmental variables like aspect, elevation, and solar irradiance were also recorded for selected forest types. ANOVA results showed a significant effect of forest types on all the measured plant traits except root length (p = 0.427), root/shoot ratio (p = 0.201), and root weight ratio (p = 0.225). Soil physicochemical properties also showed considerable variations across forest types except for soil pH. The maximum density (109 individuals/m(2)), plant height, (149.6 cm/plant), aboveground biomass (19.59 g/plant), and belowground biomass (2.82 g/plant) were recorded in the Cypress forest population, while the lowest growth was recorded in the Kharsu-Oak forest population with 10 individuals/m(2) density, 57.94 cm plant height, 3.36 g aboveground biomass, and 0.30 g belowground biomass/plant. Generally, A. adenophora allocated maximum biomass to shoots (57-81%), thus increasing carbon assimilation, while minimum biomass was allocated to reproductive parts (0-17%) indicating higher growth through vegetative propagation. Leaf area was highest in the Banj-Oak forest (2266.04 cm(2)), while reproductive performance in terms of seed numbers (13,646 seeds/plant), seed weight (0.54 g/plant), and reproductive index (119.26) was highest in the Sal-mixed forest. This study concluded that A. adenophora acclimates to heterogeneous environments in different forest ecosystems through higher plasticity in most of the observed traits. The maximum growth performance of A. adenophora recorded in open-canopied Cypress forest suggested some kind of shade treatment before the emergence of the invader as a control measure to check the invasion.