Impacts of native herbaceous and invasive woody vegetation on tree seedling performance in an urban riparian forest restoration

Background/Question/Methods

Community interactions are altered by invasive species, which complicates restoration efforts. Two woody invasive species that readily colonize forest edge habitat in northeastern North America are Lonicera maackii, bush honeysucke, and Pyrus calleryana, callery pear. Both of these invasive species dramatically alter the native herbaceous and woody understory vegetation through mechanisms including direct competition for light and nutrients. Important indirect effects, such as interference in mycorrhizal associations, competition for seed dispersal and pollination services, and modification of the intensity of herbivory pressure can also impact the native vegetation.Restoration efforts, consisting of the removal of these woody invasives and subsequent planting of native trees and shrubs, can be complicated by the altered community interactions in the invaded system. Planted native tree seedlings experience a novel environment with a radically different herbaceous community and intensity of herbivory pressure than their natural establishment niche. To investigate how these interactions affected the survivorship and growth of planted native tree species, we measured herbaceous cover, herbaceous community diversity, herbivory pressure, and prevalence of invasive species in an urban forest restoration project in Ohio.

Results/Conclusions

Three year (2019-2021) survivorship and growth rates of 648 seedlings of twenty species of native trees and shrubs were tracked as a function of total herbaceous cover, and percent cover of invasive species in surrounding 1-meter quadrats. To account for spatial heterogeneity in site conditions affecting seedling performance, we used a spatial autoregressive model, but did not find a significant effect of spatial autocorrelation on seedling growth rates. Greater herbaceous cover significantly (p< 0.05) improved the growth rates of several tree species, including Quercus macrocarpa, bur oak, and Cercis canadensis, redbud. These effects were substantially larger than the presence or absence of herbivory-excluding tree cages. The surrounding herbaceous community richness and diversity also positively correlated with seedling performance. Taken together, these results suggest that facilitation of transplanted tree seedlings by herbaceous cover, potentially through mechanisms of herbivory protection and reduced transplant stress, can improve native woody seedling performance.