Biotic constraints on the establishment and performance of native, naturalized, and invasive plants in Pacific Northwest (USA) steppe and forest

Brian M Connolly,Richard N Mack,Jennifer Powers

doi:10.3897/neobiota.34.10820

Abstract

Factors that cause differential establishment among naturalized, invasive, and native species are inadequately documented, much less often quantified among different communities. We evaluated the effects of seed addition and disturbance (i.e., understory canopy removal) on the establishment and seedling biomass among two naturalized, two invasive, and two native species (1 forb, 1 grass in each group) within steppe and low elevation forest communities in eastern Washington, USA. Establishment within each plant immigrant class was enhanced by seed addition: naturalized species showed the greatest difference in establishment between seed addition and no seed addition plots, native and invasive species establishment also increased following seed addition but not to the same magnitude as naturalized species. Within seed addition plots, understory canopy disturbance resulted in significant increases in plant establishment (regardless of plant immigration class) relative to undisturbed plots and the magnitude of this effect was comparable between steppe and adjacent forest. However, regardless of disturbance treatment fewer invasive plants established in the forest than in the steppe, whereas native and naturalized plant establishment did not differ between the habitats. Individual biomass of naturalized species were consistently greater in disturbed (canopy removed) versus undisturbed control plots and naturalized species were also larger in the steppe than in the forest at the time of harvest. Similar trends in plant size were observed for the native and invasive species, but the differences in biomass for these two immigration classes between disturbance treatments and between habitats were not significant. We found that strong limitations of non-native species is correlated with intact canopy cover within the forest understory, likely driven by the direct or indirect consequences of low light transmittance through the arboreal and understory canopy. Considered collectively, our results demonstrate how seed limitation and intact plant ground cover can limit the abundance and performance of naturalized species in Pacific Northwest steppe and low elevation forest, suggesting that local disturbance in both habitats creates microsites for these species to establish and survive. Future studies evaluating interactions between multiple barriers to establishment using more representatives from each immigration class will further reveal how biotic interactions ultimately influence the demography and distribution of non-native plants within these communities.

Highlights

Naturalizations form the small fraction of those introduced species that have surmounted demographic and local environmental barriers to develop self-sustaining populations, but unlike invaders, naturalized species do not inevitably proliferate within the novel habitat (Blackburn et al 2011, Richardson and Pyšek 2012)
Not surprisingly seed addition plots had greater recruitment than plots without seed addition, but the magnitude of the positive effects of seed addition varied by habitat and disturbance treatment (Table 1, Fig. 1)
As of July 2012, the magnitude of the effect of seed addition was greatest for naturalized species, had an intermediate effect on native species, and contributed the least to invasive plant establishment (Table 1, Fig. 2B)

Summary

Introduction

Naturalizations form the small fraction of those introduced species that have surmounted demographic and local environmental barriers to develop self-sustaining populations, but unlike invaders, naturalized species do not inevitably proliferate within the novel habitat (Blackburn et al 2011, Richardson and Pyšek 2012). Despite the role of naturalizations as precursors to invasions, we know surprisingly little about how demographic, physical, and biotic factors interact within a novel range to curb, delay or prevent naturalized species from becoming invasive (Richardson et al 2000, van Kluenen et al 2010, Richardson and Pyšek 2012). The physical and biotic factors governing plant establishment are frequently quantified (e.g., Mack and Pyke 1984, Pyke 1986, Weiher and Keddy 1999, Myers and Harms 2009), but the effect of these factors on the fate of naturalized species compared to the fate of co-occurring invaders is unclear (van Kluenen et al 2010). Disturbance by the removal of competitors can differentially influence the establishment

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