Cogongrass (Imperata cylindrica) invasions in the US: Mechanisms, impacts, and threats to biodiversity

Flory

2019

Ecology

Self Cite

Global change stressors such as drought and plant invasion can affect ecosystem structure and function via mediation of resource availability and plant competition outcomes. Yet, it remains uncertain how native plants respond to drought stress that co-occurs with potentially novel resource conditions created by a nonnative invader. Further, there is likely to be temporal variation in competition outcomes between native and nonnative plant species depending on which resources are most limiting at a given time. Interacting stressors coupled with temporal variation make it difficult to predict how global change will impact native plant communities. To address this knowledge gap, we conducted a 5-yr factorial field experiment to quantify how simulated drought, plant invasion (by cogongrass, Imperata cylindrica), and these stressors combined, affected resource availability (soil moisture and light) and competition dynamics between the invader and native longleaf pine (Pinus palustris), a foundation species in southeast U.S. forests. Drought and invasion mediated the survival and performance of pine seedlings in temporally dynamic and unexpected ways. Drought and invasion alone each significantly reduced pine seedling survival. However, when the stressors occurred together, the invader offset drought stress for pine seedlings by maintaining high levels of soil moisture, humidity, and shade compared to uninvaded vegetation. This facilitative effect was pronounced for 2 yr, yet shifted to strong competitive exclusion as the invasion progressed and the limiting resource switched from soil moisture to light. After 3 yr, pine tree survival was low except for pines growing with uninvaded vegetation under ambient precipitation conditions. After 5 yr, pines experiencing a single stressor were taller and had greater height to diameter ratios than pines under no stress or both stressors. This outcome revealed a filtering effect where poorly performing trees were culled under stressful conditions, especially when pines were growing with the invader. Together, these results demonstrate that although drought and invasion suppressed a foundation tree species, the invader temporarily moderated stressful drought conditions, and at least some trees were able to survive despite increasingly strong competition. Such unpredictable effects of interacting global change stressors on native plant species highlight the need for additional long-term studies.

Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Global change stressors alter resources and shift plant interactions from facilitation to competition over time

Alba

Flory

2019

Ecology

Self Cite

“…Therefore, it is difficult to predict how native plant communities will respond to C 4 grass invasion under predicted future changes in precipitation. To evaluate the individual and interactive effects of plant invasion and climate change on plant communities and how they change over time, we established a factorial field experiment with invasion by Imperata cylindrica (cogongrass), a rhizomatous C 4 grass native to Southeast Asia (Estrada and Flory 2015), and chronic drought (simulated with rainout shelters, Alba et al 2017). Imperata cylindrica is a globally problematic invader of warm temperate to tropical systems and a Federal Noxious Weed in the US with severe impacts on threatened longleaf pine ecosystems (Brewer 2008).…”

Section: Introductionmentioning

confidence: 99%

Grass invasion and drought interact to alter the diversity and structure of native plant communities

Angelini

Flory

2018

Ecology

Self Cite

Understanding the interactive effects of species invasions and climate change is essential for predicting future shifts in biodiversity. Because multiple stressors can interact in synergistic or antagonistic ways, it is notoriously difficult to anticipate their combined effects on species assemblages. However, some hypotheses predict that plant invasions will become increasingly problematic as climate change improves conditions for invaders or lowers the biotic resistance of native communities. In a 4‐yr field experiment, we quantified the individual and interactive effects of invasion by a globally problematic C4 grass, Imperata cylindrica, and chronic simulated drought imposed by rainout shelters on the whole plant communities of regenerating longleaf pine forest. Invasion both inhibited plant colonization and enhanced plot‐level extinctions, resulting in a severe (60%) loss of plant diversity across all functional groups, including perennial grasses and forbs, annual forbs, and woody species and dramatic shifts in community composition. Experimental drought reduced diversity by 20%, and caused a shift in the dominant functional groups, but had no significant effect on cover of the invader. The invader partially ameliorated water stress in the drought treatment such that invaded plots had higher soil moisture than uninvaded plots. Consequently, the combined effects of invasion and drought were lower than expected from an additive model of multiple stressors. These findings, which may have broader implications for how other C4 grass invaders will interact with drought to shift native community dynamics, challenge the perception that climate change will exacerbate invasions. In revealing that invasive species pose a major threat to the diversity and structure of native communities despite their moderating effects on abiotic stress, this work also highlights that management of aggressive invaders may be critical to preserving biodiversity regardless of future climate.

“…Cogongrass invades habitats ranging from intact forest understories to managed pastures and pine plantations (Dozier, Gaffney, Mcdonald, Johnson, & Shilling, 1998;King & Grace, 2000) and grows over a range of light, soil nutrient, and soil moisture conditions (Bryson, Krutz, Ervin, Reddy, & Byrd, 2010;Holzmueller & Jose, 2011). It covers more than 100,000 ha across the Southeast US (Estrada & Flory, 2015), where it often forms expansive monocultures that can drastically reduce light availability and native plant diversity in longleaf pine forests (Brewer, 2008). If cogongrass is better adapted to or can more efficiently alter trait expression in response to drought than resident plants, it may interact synergistically with drought stress to suppress pine trees and associated herbaceous species, potentially altering ecosystem functions.…”

Section: Study Systemmentioning

confidence: 99%

Methods to test the interactive effects of drought and plant invasion on ecosystem structure and function using complementary common garden and field experiments

Alba

NeSmith

et al. 2017

Ecology and Evolution

Abiotic global change drivers affect ecosystem structure and function, but how they interact with biotic factors such as invasive plants is understudied. Such interactions may be additive, synergistic, or offsetting, and difficult to predict. We present methods to test the individual and interactive effects of drought and plant invasion on native ecosystems. We coupled a factorial common garden experiment containing resident communities exposed to drought (imposed with rainout shelters) and invasion with a field experiment where the invader was removed from sites spanning a natural soil moisture gradient. We detail treatments and their effects on abiotic conditions, including soil moisture, light, temperature, and humidity, which shape community and ecosystem responses. Ambient precipitation during the garden experiment exceeded historic norms despite severe drought in prior years. Soil moisture was 48% lower in drought than ambient plots, but the invader largely offset drought effects. Additionally, temperature and light were lower and humidity higher in invaded plots. Field sites spanned up to a 10-fold range in soil moisture and up to a 2.5-fold range in light availability.Invaded and resident vegetation did not differentially mediate soil moisture, unlike in the garden experiment. Herbicide effectively removed invaded and resident vegetation, with removal having site-specific effects on soil moisture and light availability. However, light was generally higher in invader-removal than control plots, whereas resident removal had less effect on light, similar to the garden experiment. Invasion mitigated a constellation of abiotic conditions associated with drought stress in the garden experiment. In the field, where other factors co-varied, these patterns did not emerge. Still, neither experiment suggested that drought and invasion will have synergistic negative effects on ecosystems, although invasion can limit light availability. Coupling factorial garden experiments with field experiments across environmental gradients will be effective for predicting how multiple stressors interact in natural systems.