Impacts of an invasive plant are fundamentally altered by a co‐occurring forest disturbance

Abstract

Invasive species frequently co-occur with other disturbances, which can impact the same ecosystem functions as the invader. Yet invasion studies rarely control for the presence of these other disturbances, although their overlapping effects may influence the direction and magnitude of impacts attributed to the invader alone. Here, we ask whether controlling for the presence of a co-occurring disturbance, as well as the time since disturbance, yields different values of an invader's ecosystem effects than when these factors remain unaddressed. We used a chronosequence of six forest stands at a single site: five logged stands that each contained paired invaded-uninvaded plots of the forest understory invasive grass Microstegium vimineum, as well as one unlogged and uninvaded control stand. By controlling for the presence of both logging and invasion, we untangled the effects of each through time. We found that the co-occurring disturbance of logging can dramatically alter the measured effects of M. vimineum by amplifying, dampening, negating, or entirely reversing the direction of the invader's impacts. During its period of peak impact, logging amplified the invader's positive effect on the size of the soil microbial biomass pool by 24%, reduced the invader's positive effect on soil water holding capacity by 5%, negated the invader's positive effect on the particulate organic matter carbon pool (from a 9% increase to no significant effect), and reversed the direction of the invader's impact on net nitrogen mineralization rate from a 51% increase to a 52% decrease. Furthermore, the influence of logging on the invader's impacts was not static, but dynamic through time. The results from our site therefore demonstrate that failure to account for the impacts of a co-occurring disturbance, as well as the time since disturbance, can result in flawed inference about the nature of an invader's effects. Future research should determine how widespread such flawed inference might be among other invasive species and across different environmental contexts. To help guide such research, we describe a general framework for disentangling the overlapping effects of invasions and co-occurring disturbances through time.

To list or not to list: using time since invasion to refine impact assessment for an exotic plant proposed as noxious

“…Of particular concern is the possibility that pre-invasion decrements in native abundance and/or covarying factors such as disturbance favored the establishment and proliferation of invaders in a subset of plots to produce a negative correlation between native and invader abundance (Stohlgren et al 1998, Sokol et al 2017, Pearson et al 2018. Of particular concern is the possibility that pre-invasion decrements in native abundance and/or covarying factors such as disturbance favored the establishment and proliferation of invaders in a subset of plots to produce a negative correlation between native and invader abundance (Stohlgren et al 1998, Sokol et al 2017, Pearson et al 2018.…”

“…However, such factors would have to act at local scales to trigger the demise of native species coincident with invasion. While the analytical goal is to quantitatively isolate invader impacts from those attributable to environmental drivers, we note that these forces can certainly have interactive effects on native species (e.g., back-seat driver model; Bauer 2012), and mechanistic knowledge of such interactions will be key to effective management (Didham et al 2007, Johnson et al 2015, Sokol et al 2017. Native cover was higher in uninvaded plots relative to plots with increasing buckthorn cover, and the longer a particular plot had been invaded, the greater the decrement in native abundance.…”

“…In observational studies, negative correlations between invader and native abundance measured at local scales are assumed to represent invader impacts, that is, direct or indirect consequences of invasion. In the former case, invaders may simply be favored by conditions associated with reduced native abundance such as disturbance and/or increased resource availability (Stohlgren et al 1998, Davis et al 2000, Sokol et al 2017, Pearson et al 2018. In the former case, invaders may simply be favored by conditions associated with reduced native abundance such as disturbance and/or increased resource availability (Stohlgren et al 1998, Davis et al 2000, Sokol et al 2017, Pearson et al 2018.…”

“…In the former case, invaders may simply be favored by conditions associated with reduced native abundance such as disturbance and/or increased resource availability (Stohlgren et al 1998, Davis et al 2000, Sokol et al 2017, Pearson et al 2018. Yet relationships between invader and native abundance may be the product of either or both processes (MacDougall and Turkington 2005, Ortega and Pearson 2005, Bauer 2012, Sokol et al 2017). Yet relationships between invader and native abundance may be the product of either or both processes (MacDougall and Turkington 2005, Ortega and Pearson 2005, Bauer 2012, Sokol et al 2017).…”

“…Not surprisingly then, correlations between invader and native abundance are used to infer impact in some studies (reviews in Vil a et al 2011, Hulme et al 2013, Stricker et al 2015 and invasibility in others (Stohlgren et al 1999, Knight and Reich 2005, Mills et al 2012). More generally, variation in ecological factors affecting invader success (e.g., disturbance, resource availability) may confound, obscure, or alter relationships attributable to invader impact (Didham et al 2007, Kumschick et al 2015, Sokol et al 2017, Mueller et al 2018. More generally, variation in ecological factors affecting invader success (e.g., disturbance, resource availability) may confound, obscure, or alter relationships attributable to invader impact (Didham et al 2007, Kumschick et al 2015, Sokol et al 2017, Mueller et al 2018.…”