Relative importance of suppression‐based and tolerance‐based competition in an invaded oak savanna

Abstract

Summary1 Invasive species dominate many ecosystems but the competitive strategies underlying this dominance are unclear. Are invasive species generalist competitors, or do they only thrive in certain environments? Do they occur mainly post-disturbance or can they persist throughout succession? 2 We tested the relative importance of resource acquisition (competitive suppression ability) and the ability to tolerate reduced resource levels (competitive tolerance ability) among four C3 perennial grass species in an invaded oak savanna. Two species ( Poa pratensis and Dactylis glomerata ) are exotic invaders and are thought to have replaced the two native species ( Bromus carinatus and Elymus glaucus ) as dominants. 3 Using glasshouse and field experiments we tested whether the two strategies were maintained with changing resource levels and successional conditions, and their relative roles in explaining exotic dominance. 4 The relative importance of suppression-and tolerance-based competition shifted with neighbour density, burning and planting order. Further, the relative importance of particular plant traits changed depending on the imposed conditions, and the exotic dominants were only competitively superior under certain circumstances. 5 Competitive suppression ability was maintained with changing resource levels but was confined to post-disturbance conditions. When planting of neighbours was delayed, the early establishing targets were dominant regardless of species, fertility and neighbour density. 6 Competitive tolerance ability determined long-term patterns of relative abundance and coexistence, but only under the current field conditions of low fertility and limited disturbance. Alteration of these conditions changed the relative abundance of the four grasses, and would probably reconfigure species patterns in the oak savanna community generally. 7 Exotic dominance is presently determined by tolerance-based competitive traits interacting with the long-term absence of disturbance. Dominance is therefore contingent on the interaction of competitive strategies, resource availability and disturbance history rather than any one factor alone. 8 Exotic flora dominate all stages of succession in this savanna because there exists both early (suppression) and late (tolerance) successional specialists. The identity of the dominant changes with succession based on the competitive strategy it employs. This result highlights the importance of examining the historical context of invaded communities and tracking their successional status over time.

Early emergence and resource availability can competitively favour natives over a functionally similar invader

“…Lovegrass demonstrated characteristics typical of many suppression-based competitors, including rapid post-germination growth rates (Goldberg and Landa 1991) and significant intraspecific suppression, to the degree that Lovegrass appears incapable of recruiting from seed in its own understorey (Goldberg and Landa 1991;Grime 2001;MacDougall and Turkington 2004). Lovegrass did not, however, suppress the growth of the native grasses under all water and nutrient treatments.…”

Disturbance governs dominance of an invasive forb in a temporary wetland

“…Lippia has a contrasting strategy in that it almost ceases growth when inundated but survives extended inundation (at least 75 days) and propagates vegetatively. Lippia establishes both by seed and vegetatively in the wet soils following drawdown of flood waters (Macdonald 2008;Price et al 2010Price et al , 2011 and then increases in cover in the dry inter-flood periods when the cover of native species falls due to decreased growth rate and grazing pressure (Earl 2003). A reduced frequency and duration of flood events would therefore reduce the length of time when conditions are favorable for water couch and increase the length of time when conditions favor lippia.…”

Oak Woodland Restoration: Understory Response to Removal of Encroaching Conifers

“…The moderate increase in non-native grass cover in the control treatment may have been associated with the commercial thinning that removed scattered Douglas-fir trees prior to the study. In southwestern British Columbia, disturbance of the understory in Oregon white oak savannas led to overall increases in invasive species and shifts in abundance of native and nonnative grasses as different competitive strategies were favored (MacDougall 2002, MacDougall andTurkington 2004). Following removal of Douglasfir from an Oregon white oak savanna in Oregon's Willamette Valley, there was a rapid increase in abundance of non-native forb and grass species, although the fraction of non-native species had begun to decline slightly by the fourth year after treatment (Vance et al 2006).…”