Expanding the invasion footprint:Ventenata dubiaand relationships to wildfire, environment, and plant communities in the Blue Mountains of the Inland Northwest, USA

Abstract

Questions: A recently introduced non-native annual grass, Ventenata dubia, is challenging previous conceptions of community resistance in forest mosaic communities in the Inland Northwest. However, little is known of the drivers and potential ecological impacts of this rapidly expanding species. Here we (1 identify abiotic and biotic habitat characteristics associated with the V. dubia invasion and examine how these differ between V. dubia and other problematic non-native annual grasses, Bromus tectorum and Taeniatherum caput-medusae; and (2) determine how burning influences relationships between V. dubia and plant community composition and structure to address potential impacts on Inland Northwest forest mosaic communities. Location: Blue Mountains of the Inland Northwest, USA. Methods: We measured environmental and plant community characteristics in 110 recently burned and nearby unburned plots. Plots were stratified to capture a range of V. dubia cover, elevations, biophysical classes, and fire severities. We investigated relationships between V. dubia, wildfire, environmental, and plant community characteristics using non-metric multidimensional scaling and linear regressions. Results: Ventenata dubia was most abundant in sparsely vegetated, basalt-derived rocky scablands interspersed throughout the forested landscape. Plant communities most heavily invaded by V. dubia were largely uninvaded by other non-native annual grasses. Ventenata dubia was abundant in both unburned and burned areas, but negative relationships between V. dubia cover and community diversity were stronger in burned plots, where keystone sagebrush species were largely absent after fire. Conclusions: Ventenata dubia is expanding the overall invasion footprint into previously uninvaded communities. Burning may exacerbate negative relationships between V. dubia and species richness, evenness, and functional diversity, including in communities that historically rarely burned. Understanding the drivers and impacts of the V. dubia invasion and recognizing how these differ from other annual grass invasions may provide insight into mechanisms of community invasibility, grass-fire feedbacks, and aid the development of species-specific management plans. | 563 Applied Vegetation Science TORTORELLI ET aL.

Community invasion resistance is influenced by interactions between plant traits and site productivity

“…In subplots where V. dubia was already established, community composition and biomass may have been influenced by the invader. In a previous observational field study, we found that V. dubia cover increased with decreasing Shannon Diversity (Tortorelli et al, 2020). However, whether diversity was reduced by invasion or diversity conferred invasion resistance was unknown.…”

“…Ventenata dubia had similar trait values to some resident species, including two introduced invasive annual grasses B. tectorum and B. arvensis, however these functionally similar graminoids were not abundant across the vegetation gradient, despite maintaining a scattered presence throughout the area for decades and heavily invading nearby Artemisia tridentata (big sagebrush) and burned forest communities (Johnson and Swanson 2005). This finding reflects the novelty of V. dubia's invasion into shallow soil communities in which few functionally similar native or introduced species have become abundant (Tortorelli et al, 2020). Furthermore, trait similarities to resident species suggest that standard below-and aboveground traits such as those we present here may not be driving V. dubia's relative success in these areas.…”

“…The relatively recent invasion of a Eurasian annual grass, Ventenata dubia , in North America's Inland Pacific Northwest provides an important and relevant opportunity to disentangle contrasting community assembly and invasion hypotheses. The rapid expansion of V. dubia into previously uninvaded natural areas (Tortorelli et al, 2020) and its potential to alter ecosystem processes through changing fuel characteristics and fire behavior have contributed to this species' high management concern (Kerns et al, 2020). Despite its high‐risk profile, relatively little is known regarding the factors influencing community susceptibility or resistance to V. dubia invasion, or how community–resistance relationships vary across vegetation types.…”

Forb community response to prescribed fire, livestock grazing, and an invasive annual grass in the Pacific Northwest Bunchgrass Prairie

“…One study in shrublands identified a negative correlation between Ventenata dubia cover and native plant community richness and diversity and a positive correlation with annual forbs (Jones et al, 2018). In dry conifer forests, richness of annual forbs decreased with increased Ventenata dubia cover only in burned areas while richness of perennial forbs decreased in both burned and unburned areas (Tortorelli et al, 2020). Overall native plant richness and diversity were lowest in plots with high cover of Ventenata dubia in the Palouse and canyon grasslands regions (Jones et al, 2020), but the specific response of forbs was not examined.…”

Fuel connectivity, burn severity, and seedbank survivorship drive ecosystem transformation in a semi-arid shrubland.

“…Understanding the mechanisms of hysteresis, and in particular how multiple redundant mechanisms act in concert, will provide important insights for ecosystem change on a global scale. At least 13 grass species initiate self-reinforcing feedbacks with fire in the U.S. (Fusco et al 2019;Tortorelli, Krawchuk, and Kerns 2020), and many more worldwide, including Australia (G. Miller et al 2010;Setterfield et al 2010), Brazil (Rossi et al 2014) and South Africa (Milton 2004). While the conversion of temperate forests and shrublands to grasslands may have a less per-hectare impact on carbon sequestration than tropical forests, the consequences are still relevant to the global carbon cycle, especially when forests (rather than the shrublands studied here) are replaced by herbaceous ecosystems (Kerns et al 2020).…”