Understanding the patterns of competitive and facilitative interactions within and among species in plant communities is a central goal of plant ecology, because these patterns determine species coexistence and community dynamics. Network theory provides tools that allow these patterns to be quantified, and can provide greater understanding of important community properties, including community stability, than can documenting pairwise species interactions.

I characterized the interactions of multiple, co-occurring invasive and native species in an old field woody plant community to build plant interaction networks at two different life stages. With the goal of identifying structural features that may operate to maintain species coexistence, I characterized the architecture of these networks at multiple scales: the entire network, the substructures that compose the network, and species' roles within substructures.

I found that species-level pairwise interactions alone did not provide an accurate or sufficiently detailed picture of community structure. Rather, using a network approach, I identified substructures that have the potential to promote and hinder species coexistence in interactions among seedlings. Characterizing the nuances of network substructures was illuminating, as the size of the substructures and the pattern of interaction intensities within substructures influence the expected effects on species coexistence. Including interactions at multiple life stages was also important; the seedling species that benefited most from the nested structure of facilitative interactions with adults occupied subordinate roles in substructures with other seedlings. This role reversal at different life stages is a potential factor promoting coexistence in the community. Last, the network framework was useful for comparing species' roles between native and invasive members of the community, and the three invasive species in this system had different, life-stage dependent strategies in interactions with co-occurring plants.

Synthesis. The interplay of network architecture and substructures within plant communities and among plants at different life stages is important for understanding species coexistence. In the plant community characterized in this study, there were several features that may promote coexistence, and these features were not observable in interactions within a single life stage or when considering pairwise interactions independently.

The raw aboveground biomass data are provided as a .csv file.  These data are aboveground biomasses (in g) of target seedlings from seedling-seedling garden experiments and adult-seedling field experiments.

Seeds were germinated in April 2016 after having implemented stratification and scarification treatments as needed. Seedlings were transplanted into the garden and field experiments in May 2016. In the seedling-seedling garden experiment, all intra- and interspecific pairwise interactions between the seven study species were measured using a target-neighbour design, in which a central target plant was surrounded by four evenly spaced neighbouring plants in pots. Study species included species that are invasive in northeast North America: Rosa multiflora, Elaeagnus umbellata, Berberis thunbergii; and species that are native in the region: Rubus occidentalis, Rhus typhina, Cornus racemosa, and Pinus strobus. Control treatments with a central target plant and no neighbours were also planted. The pots were placed outdoors at the field site (Yale-Myers Forest, Connecticut, USA). Pots were watered to field capacity as needed. In the adult-seedling field experiment, seedlings of all target species were planted under the canopy of adult individuals of R. multiflora, E. umbellata, B. thunbergii, R. occidentalis, and P. strobus. Control treatments with no adult individual were also planted. Plants were harvested in September 2016, and harvested aboveground biomass was dried to constant mass at 60°C and weighed.

The R code used to analyze the data is also included in this dataset as a .zip file. Please see the manuscript for more complete details regarding methods.