Prescribed versus wildfire impacts on exotic plants and soil microbes in California grasslands

1. Prescribed fire is often used as a management tool to decrease exotic plant cover and increase native plant cover in grasslands. These changes may also be mediated by fire impacts on soil microbial communities, which drive plant productivity and function. Yet, the ecological effects of prescribed burns compared to wildfires on either plant or soil microbial composition remain unclear. 2. Here, we investigated the impacts of a spring prescribed fire versus a fall wildfire on plant cover and community composition and bacterial and fungal richness, abundance, and composition in a California grassland. We used qPCR of 16S and 18S to assess impacts on bacterial and fungal abundance and Illumina MiSeq of 16S and ITS2 to assess impacts on bacterial and fungal richness and composition. 3. Wildfire had stronger impacts than prescribed fire on microbial communities and both fires had similar impacts on plants with both prescribed and wildfire reducing exotic plant cover but neither reducing exotic plant richness. Fungal richness declined after the wildfire but not prescribed fire, but bacterial richness was unaffected by either. Yet increasing char levels in both fire types resulted in reduced bacterial and fungal biomass, and both fire types slightly altered bacterial and fungal composition. 4. Exotic and native plant diversity differentially affected soil microbial diversity, with native plant diversity leading to increased arbuscular mycorrhizal fungal richness while exotic plant diversity better predicted bacterial richness. However, the remainder of the soil microbial communities were more related to aspects of soil chemistry including cation exchange capacity, organic matter, pH and phosphorous. 5. Synthesis and applications. Understanding the different ecological effects of prescribed fires and wildfires on plant and soil communities are key to enhancing a prevalent management action and to guide potential management opportunities post wildfires. Our coupled plant and soil community sampling allowed us to capture the sensitivity of the fungal community to fire and highlights the importance of potentially incorporating management actions such as soil or fungal amendments to promote this critical community that mediates native plant performance.

The impact of multiple species invasion on soil and plant communities increases with invader diversity

Despite increasing evidence indicating that invasive species are harming ecological systems and processes, impacts of multiple invasions, and the linkages between these events and changes in vegetation and soil are inadequately documented and remain poorly understood. Addressing multiple invasions would help to highlight high priority invaders and would aid in designing more effective control strategies, contributing to environmental restoration and sustainability. In this work, we tested the impact of three concurring invasive plant species, Amorpha fruticosa, Fraxinus pennsylvanica and Acer negundo, on soil conditions and native plant diversity. The research was conducted in riparian ecosystem and included the following treatments: (1) co-occurrence of the three invasive plant species, (2) occurrence of a single invasive species, and (3) control, i.e., absence of invasive species. Our findings revealed that the impact of invasive plants on soil properties and native plant diversity is magnified by their co-occurrence. Soil in mixed plots (those populated with all three invaders) contained much higher levels of nitrifying bacteria (NB), organic matter (Om), nitrogen (N), and carbon (C) as well as lower carbon to nitrogen ratio (C:N) levels, compared to single species invaded plots and control plots. Mixed plots were also characterized by reduced native plant diversity compared to single species invaded and control plots. Differences in soil conditions and native plant diversity revealed the interactive potential of invasive plants in depleting biodiversity, and thus in affecting ecological and biogeochemical processes. Our results highlight the need to study the impact of multispecies invasion and suggest that sites in riparian areas affected by co-occurring invaders, should be prioritized for ecosystem restoration. Keywords: Acer negundo, Amorpha fruticosa, Fraxinus pennsylvanica, invasive plants, multiple invasions, soil properties

Agricultural land use curbs exotic invasion but sustains native plant diversity at intermediate levels

Unveiling the processes driving exotic plant invasion represent a central issue in taking decisions aimed at constraining the loss of biodiversity and related ecosystem services. The invasion success is often linked to anthropogenic land uses and warming due to climate change. We studied the responses of native versus casual and naturalised exotic species richness to land uses and climate at the landscape level, relying on a large floristic survey undertaken in North - Eastern Italy. Both climate and land use drove exotic species richness. Our results suggest that the success of plant invasion at this scale is mainly due to warm climatic conditions and the extent of urban and agricultural land, but with different effects on casual and naturalized exotic species. The occurrence of non-linear trends showed that a small percentage of extensive agricultural land in the landscape may concurrently reduce the number of exotic plant while sustaining native plant diversity. Plant invasion could be potentially limited by land management, mainly focusing on areas with extensive agricultural land use. A more consciousness land management is more and more commonly required by local administrations. According to our results, a shift of intensive to extensive agricultural land, by implementing green infrastructures, seems to be a win–win solution favouring native species while controlling the oversimplification of the flora due to plant invasion.

On linking mechanism to invasive species impact

Species invasion represents one of the major drivers of biodiversity change globally, yet there is widespread confusion about the nature of non-indigenous species (NIS) impact. This stems from differing notions of what constitutes invasive species ‘impact’ and the scales at which it should be assessed. At local scales, the mechanisms of impact on competitors can be classified into four scenarios: 1) minimal impact from NIS inhabiting unique niche space; 2) neutral impact spread across the community and proportional to NIS abundance; 3) targeted impact on a small number of competitors with overlapping niches; and 4) pervasive impact that is disproportionate to NIS abundance and caused by modifications that filter out other species. I developed a statistical test to distinguish these four mechanisms based on community rank-abundance curves and then created a scale-independent standardized impact score. Using an example long-term dataset, that has high native plant diversity and an abundance gradient of the invasive vine, Vincetoxicum rossicum, I show that impact resulted in either targeted extirpations or widespread biodiversity loss. Regardless of whether NIS impact is neutral, targeted or pervasive, the net outcome will be the homogenization of ecosystems and reduced biodiversity at larger scales, perhaps reducing ecosystem resilience.

Non-Native Eragrostis curvula Impacts Diversity of Pastures in South-Eastern Australia Even When Native Themeda triandra Remains Co-Dominant

Lowland grassy woodlands in Australia’s south-east face reductions in native plant diversity because of invasion by non-native plants. We compared the relative abundance and diversity of plant species among sites dominated by the native Kangaroo grass (KG) Themeda triandra with sites co-dominated by the non-native African lovegrass (ALG) Eragrostis curvula and KG. We found significant differences in plant species composition depending on the dominant species. Furthermore, our results revealed differences in several diversity parameters such as a lower species richness and forb diversity on sites co-dominated by ALG and KG. This was the case despite the functional similarity of both ALG and KG—both C4 perennial tussock grasses of a similar height. Therefore, our results highlight the critical function of the native KG in maintaining and enhancing the target plant species composition and diversity within these grassy woodlands. Herbivore grazing potentially impacts on the abundance of the dominant grass and forb species in various ways, but its impact likely differs depending on their evolutionary origin. Therefore, disentangling the role of individual herbivore groups (native-, non-native mammals, and invertebrates) on the plant community composition of the lowland grassy woodlands is essential to find appropriate grazing regimes for ALG management in these ecosystems.

On linking mechanism to invasive species impact

Species invasion represents one of the major drivers of biodiversity change globally, yet there is widespread scientific and popular confusion and controversy about the nature of non-indigenous species (NIS) impact. This confusion stems from differing notions and understanding of what constitutes invasive species ‘impact’ and the scales at which it should be assessed. I argue that the proximate mechanisms determining invasive species impact happen at smaller scales where species interact, and by understanding these mechanisms, we can scale up to a broader understanding of how invasive species impact biodiversity. The mechanisms of NIS impact on potential competitors can be classified into four scenarios: 1) minimal impact from NIS inhabiting unique niche space; 2) neutral impact spread across the community and proportional to NIS abundance; 3) targeted impact on a small number of competitors with overlapping niches; and 4) pervasive impact that is disproportionate to NIS abundance and ostensibly caused by ecosystem modification that filters out other species. I develop a statistical test to distinguish these four mechanisms based on community rank-abundance curves. Using an example dataset from plant communities invaded by the dominant invasive vine, Vincetoxicum rossicum, I show that in long-term plots that had high native plant diversity and where V. rossicum increased, impact resulted in either targeted extirpations (scenario 3) or widespread biodiversity loss (scenario 4). Regardless of whether NIS impact is neutral, targeted or pervasive, the net outcome will be the homogenization of ecosystems and reduced biodiversity at larger scales, perhaps reducing ecosystem resilience.

Invasive Johnsongrass, a threat to native grasslands and agriculture

Among the weedy plant species, Johnsongrass (Sorghum halepense) is one of the most destructive. Johnsongrass has invaded new habitats beyond its native Eurasian origin by outcompeting native flora and cultivated crops. The Johnsongrass habitat is expanding continuously due to clonal and self-pollinating reproduction strategy, accelerated growth and the progressing climate change. As a result, Johnsongrass has reduced native plant diversity in grasslands and inflicted economic damage to agriculture on every continent. Johnsongrass is a growing threat to crop production, as it serves as a refuge for a variety of agricultural pests and plant viral diseases. Over the past decades, herbicides extensively applied to control Johnsongrass have boosted selection pressure, resulting in the independent evolution of herbicide-resistant ecotypes across multiple locations. The apparent threat to native flora and agriculture caused by the invasive Johnsongrass is a subject to a long and ongoing research. This review provides a historical and research overview on Johnsongrass expansion, its current as well future impact particularly on North American and European grasslands and agriculture.