Stand biomass decreases towards the edge of a range expanding invasive plant, Mikania micrantha, but only on thick soil layers

During spread in the new range, invasive plants may evolve distinctive traits that make them rapidly occupy the suitable and empty sites at the invasion front, such as rapid growth rate and high seed dispersal ability. However, less is known about how competitive ability evolves during range expansion, and no study has examined whether stand biomass (i.e. biomass production of dense monocultural stands, which may indicate the invasion speed and impacts of invasive plants) changes during range expansion. In this study, using 27 populations of an invasive plant, Mikania micrantha, which is expanding its range on Hainan island of China, we examined how competitive ability and stand biomass change along the invasion route. We grew plants of M. micrantha either singly or with common intraspecific competitors originating from different populations in 3 liter pots to evaluate competitive ability, and grew six plants from the same population together in 3 or 11 liter pots to evaluate stand biomass. When grown alone or with competitors, plant biomass and competitive ability did not correlate with distance from invasion center. Stand biomass was negatively correlated with distance from invasion center in 11 liter pots with a thick soil layer, but not in 3 liter pots with a thin soil layer. Stand biomass was negatively correlated with root to shoot ratio (RSR) in 3 liter pots but not in 11 liter pots, possibly because excessive allocation to roots for belowground competition reduced the overall performance at the stand level when soil space was highly limited. Stand biomass was unrelated to competitive ability. Our results indicate that edge populations of invasive plants may have reduced stand level performance in habitats with thick soil layers. In habitats with shallow soil layers, populations of invasive plants may evolve greater allocation to roots and reduced stand level performance.