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

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. Connolly et al. / NeoBiota 34: 21-40 (2017) 22 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.

Exploring the complex pre-adaptations of invasive plants to anthropogenic disturbance: a call for integration of archaeobotanical approaches

“…in undisturbed than in disturbed plots (Wallace and Prather, 2016). This result is similar to the seed-addition experiment of Connolly et al (2017), who showed that two European invasive species were able to establish in plots without disturbance and did not benefit more from disturbance than native species, even with a standardized amount of added seeds (predictions d) and e)).…”

“…Prediction c) has been tested with several large-scale multi-species experiments, which provided mixed support. Researchers studied the response of European invaders to different disturbance regimes such as biomass removal, nutrient addition and soil disruption, and they found no (Connolly et al, 2017;Broadbent et al, 2020;Bellini et al, 2022) or only partial (Maron et al, 2014) support for this prediction. With regard to prediction d), two multispecies experiments that focused on plant responses to soil disruption indeed found that some European species were unable to establish in undisturbed soil (Jesson et al, 2000;Pearson et al, 2022).…”

Improving Ecological Restoration to Curb Biotic Invasion—A Practical Guide

“…Particularly, new experiments using various biomass treatments have substantially added to our understanding of restoration success. For example, a recent study demonstrated how seed limitation and intact plant ground cover can limit the abundance and performance of naturalized species in Pacific Northwest steppe and low-elevation forests (Connolly et al 2017).…”