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Keywords:

  • Coastal sage scrub;
  • Competition;
  • Exotic grasses;
  • Grass-shrub interactions;
  • Mediterranean;
  • Nitrogen;
  • Plant invasions;
  • Positive feedback;
  • Semi-arid;
  • Soil moisture

Abstract

Questions: Plant invasions are considered one of the top threats to the biodiversity of native taxa, but clearly documenting the causal links between invasions and the decline of native species remains a major challenge of invasion biology. Most studies have focused on impacts of invaders' living biomass, rather than on mechanisms mediated by litter. However, invasive plant litter, which is often of a very different type and quantity than a system's native plant litter, can have multiple important effects on ecosystem processes – such as nitrogen cycling and soil microclimate – that may influence native plants.

Location: We studied effects of litter of invasive grass species that are widespread throughout western North America on native shrubs in southern California's semi-arid habitat of coastal sage scrub.

Methods: We combined a 3-year field manipulation of non-native litter with structural equation modeling to understand interacting effects on non-native grasses, native shrubs, soil nitrogen (available and total), and soil moisture.

Results: Litter addition facilitated non-native grass growth, revealing a positive feedback likely to enhance invasion success. Contrary to a major paradigm of invasion biology – that competition with invasive plant species causes declines of native plants – we found that litter also facilitated growth of the native dominant shrub, a result supported by observational trends. Structural equation models indicated that enhanced soil moisture mediated the positive effects of litter on shrub growth.

Conclusions: We demonstrate that invasive plants, via their litter, can facilitate dominant native plants by altering soil moisture. Our results highlight that understanding the impacts and mechanisms of plant invasions may be enhanced by considering the role of invasive plant litter on native plants and ecosystem properties.