Growth responses and phenotypic plasticity of eight environmental weed species grown from seed in deep shade

Abstract

Shade tolerance is a key factor determining which environmental weeds can invade and persist in intact forest, where light availability can be as low as 1% of full sunlight. Understanding which species can survive under such low light is critical for predicting future invasions. We examined growth responses and phenotypic plasticity of eight environmental weeds: three palms (Archontophoenix cunninghamiana, Phoenix canariensis, Trachycarpus fortunei), four trees (Acer negundo, Maytenus boaria, Pinus patula, Prunus lusitanica), and one vine (Actinidia chinensis). We used a shade-house experiment with four shade levels (53%, 93%, 97%, 99%) to assess seedling establishment, biomass, root:shoot ratio (R:S), and specific leaf area (SLA). Palms showed high shade tolerance, with minimal variation in establishment and small biomass reductions at the deepest shade, but little plasticity. Maytenus boaria and Prunus lusitanica were moderately shade tolerant, with stable establishment but larger biomass declines between 97% and 99% shade. Actinidia chinensis and Acer negundo were less tolerant, showing greater biomass and establishment declines under deep shade. Pinus patula was least tolerant, with no seedlings surviving at 99% shade and the greatest biomass reduction between 53% and 93% shade. Maytenus boaria and Actinidia chinensis showed the greatest plasticity, with declining R:S and increasing SLA with shade. Prunus lusitanica was plastic in SLA but not R:S, while Acer negundo and Pinus patula showed minimal plasticity. Results indicate that all species except for Pinus patula have potential to establish in deep shade. Further research to determine their ability to invade intact native forests would be valuable.