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

Abstract

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.

Nonlinear plant–plant interactions modulate impact of extreme drought and recovery on a Mediterranean ecosystem

“…For example, Alba et al . (2019) demonstrated that the interaction effect of drought and the invasive grass I . cylindrical shifted unexpectedly from facilitation to competition over time.…”

Contrasting latitudinal clines of nematode diversity in Spartina alterniflora salt marshes between native and introduced ranges

“…One of important global change factors is the introduction of species to areas outside their historic geographic ranges (Alba, Fahey, & Flory, 2019; van der Putten, 2012). Species that spread rapidly may not be well adapted to their new geographic ranges or to the species in their new communities (Allen et al, 2017; Zhang, Pennings, Li, & Wu, 2019).…”

Scaling up experimental stress responses of grass invasion to predictions of continental‐level range suitability

“…Due to its C 4 pathways, it can be drought tolerant, adapted to high temperatures, and also have greater water use efficiency, making it potentially more competitive in lower latitudes as compared to C 3 invaders. Since its initial introduction to the United States, cogongrass has expanded its range in the southeastern United States from Texas to Florida and as far north as Virginia (Jose et al 2002) and it is considered a primary threat to biodiversity and ecosystem functions (Estrada and Flory 2015, Fahey et al 2018, Alba et al 2019.…”

“…The watering frequency was modified according to the watering frequency used in Webster and Grey (2008) to have a close representation of the weather in Florida, which also resulted in similar soil water contents to what Burns (2004) used in her study. Indeed, a further lower water level could be used to mimic a stronger drought (Burns 2004, Alba et al 2019. Our watering frequency was designed based on a comprehensive consideration of watering period and soil water content to have a close representation of the weather in Florida, which also aimed to mimic meteorological drought to ultimately show vegetation physiological drought response.…”