Presence of a dominant native shrub is associated with minor shifts in the function and composition of grassland communities in a northern savannah

Ecosystems are spatially heterogenous in plant community composition and function. Shrub occurrence in grasslands is a visually striking example of this, and much research has been conducted to understand the functional implications of this pattern. Within savannah ecosystems, the presence of tree and shrub overstories can have significant impacts on the understory herbaceous community. The exact outcomes, however, are likely a function of the spatial arrangement and traits of the overstory species. Here we test whether there are functional linkages between the spatial patterning of a native shrub and the standing biomass, community composition, and overall nutrient cycling of a neighbouring grassland understory communities within the Aspen Parkland of central Alberta, Canada. In a paired grassland-shrub stand study, we found the native shrub, Elaeagnus commutata, has relatively few stand-level impacts on the composition and standing biomass of the ecosystem. One factor contributing to these limited effects may be the overdispersion of shrub stems at fine spatial scales, preventing areas of deep shade. When we looked across a shrub density gradient and incorporated shrub architecture into our analyses, we found these shrub traits had significant associations with species abundance and root biomass in the understory community. These results suggest that stem dispersion patterns, as well as local stand architecture, are influential in determining how shrubs may affect their herbaceous plant understory. Thus, it is important to incorporate shrub spatial and architectural traits when assessing shrub-understory interactions.