Invasion resistance and persistence: established plants win, even with disturbance and high propagule pressure

Abstract

Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 9 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plantavailable soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.

Can Land Management Buffer Impacts of Climate Changes and Altered Fire Regimes on Ecosystems of the Southwestern United States?

“…Contemporary wet mixed conifer and spruce fir forests include ponderosa pine, which remains viable at these sites throughout the climate changes of the next century. In forests, types where none of the current overstory species remain viable, delayed uphill movement of lower elevation species will likely exacerbate and lengthen biomass declines and increase the potential for persistent type changes where shrub or cheatgrass communities establish and resist future invasion by tree species [118,123]. Because our models did not include insect disturbance, which has been a significant cause of recent tree mortality in the southwestern US and particularly in piñon pines in the southwest [124], the role of piñon pine in maintaining tree cover and initiating forest recovery could be unrealistic.…”

Wildland Fire, Forest Dynamics, and Their Interactions

“…As a result, fuel quantification systems that have a basis in fundamental fire properties will have a degree of universality by default. The adoption of a hierarchical system could provide for abstraction while allowing for base level fuel attributes to be reconstituted [25,123]. Such a hierarchy could be considered in terms of:…”

“…While fuels can be parameterised solely in terms of their potential contribution to fire behaviour, in order to understand their properties through time, it is important to also recognise that they are biological products that are a product of complex and dynamic processes [3,27,123]. To date, there has been a tendency to consider fuel separately from the vegetation it is derived from; however, to be truly understood, the biotic nature of fuel needs to be taken into consideration.…”

“…Such an attribute set would provide for the development of new, improved models, would allow integration with other ecological modelling systems and would allow broader uses of the data such as the analysis of ecological processes and spatial patterning in three dimensions [123]. An enduring challenge with the development of such a system is that there are multiple needs that require the quantification of fuels, in particular:…”

Importance of soil and plant community disturbance for establishment of Bromus tectorum in the Intermountain West, USA

“…Large gaps between perennial bunchgrasses (Rayburn et al 2014) and low perennial grass cover (Chambers et al 2007) can promote B. tectorum establishment. Presence of the native grass Elymus elymoides, even with aboveground biomass removal, can enhance resistance to invasion (McGlone et al 2011). Cover by B. tectorum is highest in areas with low cover and species richness of biological soil crusts (Ponzetti et al 2007), and lichen crusts can reduce B. tectorum abundance by 85% (Deines et al 2007).…”