Herbaceous invaders in temperate forests: a systematic review of their ecology and proposed mechanisms of invasion

Abstract

We present a systematic literature review of exotic understory forest herbaceous invasions with a focus on the forests of East Asia (EAS) and Eastern North America (ENA), two dominant regions of the north temperate deciduous forest biome. We examined the biogeographic origins of herbaceous invaders in EAS and ENA forests, summarized their life histories and ecology, and compiled the relevant literature on the 10 leading mechanistic hypotheses proposed for these invasions. We asked whether invasions of EAS and ENA forests by herbs are shared between regions, and whether a common suite of ecological traits unite these invaders into a functionally distinct group. In a focused summary of the empirical literature, we investigated if leading hypothesized mechanisms for biological invasions at large are also invoked and supported for this ecologically important, but relatively understudied, group of species. In contrast to ENA, forest invaders in EAS are overwhelmingly herbaceous (78% of forest invasions vs. 34% for ENA) and originate from different regions. Plant families represented and species traits between regions differed. Within a single species, multiple invasion mechanisms were often supported, highlighting the need for future research that simultaneously investigates multiple mechanistic hypotheses. Further, because results for a single invader often differed across space and time, a shift in focus to incorporate the complex dynamics across temporal and spatial scales with the consideration of spatial heterogeneity and the interplays among natural and anthropogenic factors to study exotic invasions is needed.

Seeing beyond the trees: a comparison of tropical and temperate plant growth forms and their vertical distribution

“…Indeed, overbrowsing by large ungulates and fire suppression collapses temperate biodiversity and causes the formation of persistent depauperate understory communities across vast spatial scales (Côté et al , Nuttle et al ). These depauperate and open forest understories may subsequently become inundated with invasive species (Knight et al , Wavrek et al ). We suggest that these pressures on temperate forest understories, which have already caused plant diversity collapses, should become conservation priorities by forest managers.…”

Functional form and interactions of the drivers of understory non‐native plant invasions in northern US forests

“…Forests of the northern US have been particularly prone to non‐native plant invasions (Iannone et al, ; Schulz & Gray, ). These forests harbour a globally distinct suite of non‐native plant species (Fridley, ; Wavrek, Heberling, Fei, & Kalisz, ), many of which have had serious impacts on forest species composition, functioning and productivity (Fagan & Peart, ; Mascaro & Schnitzer, ; Peebles‐Spencer, Gorchov, & Crist, ; Rodgers, Wolfe, Werden, & Finzi, ). While several historical and contemporary factors are known to have facilitated the spread of non‐natives into northern US forests (e.g.…”

Carbon gain phenologies of spring‐flowering perennials in a deciduous forest indicate a novel niche for a widespread invader

“…However, similar patterns for invasive forest herbs are less commonly reported, despite the fact that there are several invasive herbaceous species now common to closed canopy forests in eastern North America (e.g. Microstegium vimineum, Alliaria petiolata, Ficaria verna; Wavrek et al, 2017). Given the diversity of phenological strategies among native forest herbs in North American forests compared with woody species (Uemura, 1994;Neufeld & Young, 2014), the role of leaf phenology in forest herb invasions in less clear.…”

“…Herbaceous species in temperate deciduous forests are increasingly of conservation concern due to anthropogenic stressors such as land-use change, overabundant herbivores, nitrogen deposition, and nonnative species introductions (Whigham, 2004;Gilliam, 2007). Nonnative plant invasions are now common in deciduous forests, and many mechanisms have been hypothesized to explain their understory success (Wavrek et al, 2017). Extended leaf phenology has been suggested as a dominant mechanism for forest understory nonnative species invasions (Wolkovich & Cleland, 2011;Smith, 2013), with some invasive species photosynthesizing earlier in the spring and/or later in autumn than co-occurring natives (Harrington et al, 1989;Engelhardt & Anderson, 2011;Fridley, 2012).…”