Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

Abstract

Questions Californian grasslands have a long history of invasion, starting with the introduction of exotic forage species a century ago, and followed by newer waves of invaders. Both exotic and invasive species produce large amounts of litter, but the importance of litter accumulation on the growth and expansion of these species has not been rigorously assessed. We addressed the following questions: (1) do litter type (exotic or invasive species) and litter quantity affect exotic and invasive plant performance; (2) at which plant life stages does litter accumulation effects occur; and (3) is litter accumulation impacting native species recruitment and diversity? Location Semi‐natural grasslands, northern California. Methods We created litter quantity gradients (0, 62.5, 125, 250 or 500 g·m−2) of three litter types (exotic or invasive species litter or a mixture of both). We evaluated litter effects on the germination, recruitment, production of flowers and biomass production of the exotic Avena fatua, the invasive Elymus caput‐medusae and a mixture of native species. We used litter depth as a proxy for litter accumulation and measured litter impacts on light availability at ground level, soil moisture, soil and microbial C and N. Results We found a significant litter depth impact on all species, but no litter type effects. Litter accumulation principally reduced light availability at ground level and positively affected exotic and invasive species at different life‐history stages. Above‐ground biomass production of the exotic species Avena increased with increased litter depth, suggesting a litter accumulation feedback. The invasive species Elymus responded to increased litter with increased seed production, rather than biomass production. In contrast, litter negatively affected the recruitment and diversity of native species. Conclusion Overall, our findings point to cascading effects in invasion with positive litter accumulation feedbacks of an exotic species simultaneously reducing native diversity and promoting seed production of a more recent invasive arrival. These results have strong consequences for invasion management and suggest that litter removal practices, such as burning and grazing, would better succeed at limiting the growth and spread of invasive species than herbicide application or seeding.

Invasive plant feedbacks promote alternative states in California vernal pools

“…The presence of an invasive litter layer can disrupt a variety of mechanistic pathways that in turn can inhibit germination and overall community structure (Henry et al ; Loydi et al ). These inhibitory pathways can include a physical barrier to germination and establishment (Vaccaro et al ; Kaproth et al ; Li et al ; Mariotte et al ). Little to no native species germination under the litter and high native germination less than 0.5 cm away from an areas containing litter (A.M. Faist, 2011, University of Colorado, personal observation ) suggests it could be the physical presence of the litter layer facilitating plant community composition (Amatangelo et al , Kaproth et al ).…”

“…Because these vernal pools are an annually dominated system, both through inundation and plant life cycles, after the spring peak growing season all plants senesce and deposit their biomass onto the soil surface. This deposited invasive plant biomass, or litter, has the potential to cause positive feedbacks facilitating the recruitment of species adapted to a deeper litter layer and simultaneously limit recruitment for those species unaccustomed to its presence (Amatangelo et al ; Kaproth et al ; Li et al ; Mariotte et al ). Furthermore, this positive feedback caused by increased plant litter depth, could represent a state change pathway that, depending on population and community responses, may transition the plant communities of vernal pools from native to invasive alternative states.…”

“…We empirically address these questions through abiotic field observations and biotic litter manipulations within the framework of restored and reference vernal pools. We hypothesize that the abiotic drivers allowing invasive species to proliferate will be directly linked to winter inundation depths (Gerhardt & Collinge ; Gosejohan et al ) and the biotic drivers will be a litter layer positive feedback created by the invasive species (Mariotte et al ) after integration into the pools. We further hypothesize that under different precipitation years, native species will have a lower or higher vulnerability to invasion, and especially under lower rainfall years native species will be vulnerable to invasive encroachment and a long lasting physical litter layer through positive feedbacks.…”

Novel fine-scale aerial mapping approach quantifies grassland weed cover dynamics and response to management

“…As illustrated (Fig 5), some of these patches were tens, sometimes almost hundreds, of meters across, and varied little across the two years, likely because of the persistence of accumulated thatch and thatch feedbacks (Fig 5A and 5B) [48]. In this ungrazed, weed-dominated landscape, distinct forage patches were also evident (Fig 5A) but during the drier year (2009) these patches lost some integrity and were invaded by the weeds (Fig 5B).…”

Plant-soil interactions of range-expanding plants

“…Research on plant-soil feedbacks under controlled greenhouse conditions has largely focused on legacy effects driven by interactions between 'living' plant roots and soil communities, whereas less is known about legacy effects driven by litter inputs to the soil (Ehrenfeld et al 2005, Kulmatiski et al 2008). However, it has been shown that plant-soil feedback effects mediated by plant litter can increase the performance of exotic plant species over the performance of resident native species (Eppinga et al 2011, Elgersma et al 2012, Mariotte et al 2017. In order to have positive effects of litter feedbacks to invasion, the litter of the exotic plant species has to alter soil nutrient status and, at the same time, the exotic plant species must benefit from this change in soil nutrients more than the native species.…”