Comparison of the non‐native herbAlliaria petiolatawith dominant native herbs in microhabitats of a Midwestern forest

Abstract

A mechanistic understanding of the causes of naturalization in terrestrial plant communities, and the consequences for associated ecosystems, relies on understanding whether and how native and non‐native plant traits differ. Traits of non‐native plants may either help them compete or avoid competition with natives, and depending on their interaction with the native community, the non‐native may act as more or less of an active driver vs. a passive passenger of change in the native ecosystem. Trait comparisons between native and non‐native plants are often laboratory‐based, providing insights that can be difficult to apply to natural communities. Studies of the impacts of non‐native plants, by contrast, are generally performed in natural communities, but usually do not compare observed impacts of the non‐natives with those exerted by dominant natives on each other and the community. Thus, the likely relative disruption of the non‐native to the community is unclear. In a four‐year observational study conducted at the microhabitat scale in the herbaceous layer of a Midwestern oak forest, we compared the non‐native herb Alliaria petiolata (garlic mustard) with dominant native herbaceous species in terms of correlational patterns related to three broad categories of traits: (1) environmental correlates (light, topography, soil nutrients) with presence and cover, (2) inter‐annual regeneration and phenology, and (3) associations with other species (competition and herbivory). We found that Alliaria differed strongly from dominant native herbs in all three categories. Compared with native herbs, Alliaria was more strongly associated with south‐facing slopes and high nutrient levels, displayed earlier phenology and enhanced dispersal ability, and was strongly avoided by the generalist native herbivore, Odocoileus virginianus (white‐tailed deer). Despite well‐described allelopathic effects of Alliaria on mycorrhizae and mycorrhizal plants, we found little evidence to suggest that Alliaria had negative interactions with native plants that were stronger than those of native community members. Overall, our data suggest that naturalization of Alliaria in this site is primarily due to differences from native plants in phenology, dispersal capability, and avoidance of herbivory, and that Alliaria is therefore more likely to be acting as a passenger than as a driver of change in our site.

Where Is Garlic Mustard? Understanding the Ecological Context for Invasions of Alliaria petiolata

“…Different mechanisms operate at different scales (Fridley et al 2007 ), and therefore one single mechanism is unlikely to be entirely responsible for the success of any individual plant species (Liebhold et al 2017 ). In addition, as with many other invasions, differentiating whether garlic mustard invades because of specific preexisting environmental site conditions or if garlic mustard is altering conditions for its own postinvasion success is difficult, begging the question as to whether garlic mustard is a passenger or a driver of change (MacDougall and Turkington 2005 , Phillips-Mao et al 2014 , Anderson et al 2019 ) in invaded sites. We begin in the present article by considering recent (2008–2021) progress in the literature regarding the mechanisms proposed by Rodgers and colleagues ( 2008a ).…”

“…Although invasive plants are often assumed to be better competitors than native species (Baker 1965 ), garlic mustard is not an overwhelming competitor with other understory plant species when grown head to head (Meekins and McCarthy 1999 , Wixted and McGraw 2010 , Davis et al, 2012 , 2015 , Leicht-Young et al 2012 , Anderson et al 2019 , Faison et al 2019 ). This holds true even when competing with a sympatric, co-occurring invasive mustard species with a similar life history strategy and growth form (dame's rocket, Hesperis matronalis; Leicht-Young et al 2012 ).…”

“…Garlic mustard lacks the mycorrhizal partners that facilitate soil phosphorus uptake for most other plants, and garlic mustard presence has been found to be positively associated with soil phosphorus availability (Castellano and Gorchov 2012 , Phillips-Mao et al 2014 , Anderson et al 2019 ). But work by Lankau ( 2012b ) found no correlation between soil phosphorus and garlic mustard cover.…”

“…But work by Lankau ( 2012b ) found no correlation between soil phosphorus and garlic mustard cover. It is unclear whether garlic mustard causes higher phosphorus levels because of increased mineral weathering (Rodgers et al 2008b ) or simply responds positively to higher phosphorus levels (Castellano and Gorchov 2012 , Anderson et al 2019 ). However, the removal of garlic mustard over 8 years at a site in Pennsylvania did not change soil phosphorus levels (Burke et al 2019 ), suggesting that garlic mustard was responding to, and not altering, soil conditions.…”

Impact of invasive and native dominants on species richness and diversity of plant communities

“…An analysis of data on global extinctions shows that alien species contributed to 25% of plant extinctions and 33% of terrestrial and freshwater animal extinctions; these figures are an order of magnitude higher than for native species, which are implicated in fewer than 5% and 3% of plant and animal extinctions, respectively (Blackburn et al 2019). Still, there is an ongoing debate on whether and how the impacts of invasive aliens differ from those of expanding native dominants (Richardson & Ricciardi 2013, Simberloff et al 2013, Simberloff & Vitule 2014, Thomas & Palmer 2015, Anderson et al 2019.…”