Simultaneous, independent, and additive effects of shrub facilitation and understory competition on the survival of a native forb (Penstemon palmeri)

There is increasing recognition that both competition and facilitation are important drivers of plant community dynamics in arid and semi-arid environments. Decades of research have provided a litany of examples of the potential for shrubs as nurse plants for establishment of desirable species, especially in water-limited environments. However, interactions with the existing understory community may alter the outcome of interactions between shrubs and understory plants. A manipulative experiment was conducted to disentangle interactions between a native forb species (Penstemon palmeri A. Gray), a native shrub (Artemisia tridentata Nutt.), and a diverse understory of exotic and native forbs and grasses in a semi-arid shrubland of Northern Utah, USA. Seedlings of P. palmeri were transplanted in a factorial design: (1) beneath shrub canopies or into their interspaces and (2) with understory interactions retained or removed. Transplant survival was tracked for roughly 1 year. Shrubs appeared to facilitate P. palmeri survival while interactions with the existing understory community were equivalently negative, leading to overall neutral interactions. Further, positive shrub interactions and negative understory interactions appeared to operate independently and simultaneously. While the debate over the importance of facilitation and competition in driving plant community dynamics continues, our observations strongly suggest that both have considerable effects on plant establishment in A. tridentata communities. Furthermore, our results inform the conservation and restoration of P. palmeri populations, and suggest the utility of nurse shrubs and/or understory thinning as strategies for increasing the diversity of desirable species in the arid and semi-arid western United States shrublands.