Plant behavior and species coexistence: Stem elongation of the carnivorous subshrub Drosophyllum lusitanicum and coexistence with xerophytic shrubs

J. Stephen Brewer, Department of Biology, University of Mississippi, University, MS, Maria Paniw, Department of Conservation Biology, Estación Biológica de Doñana (EBD-CSIC), Seville, Spain and Fernando Ojeda, Biology Department, University of Cadiz, Puerto Real, Spain

Background/Question/Methods - Facilitation can result in transient species coexistence, but subsequent competitive suppression by the facilitated or the facilitator is generally believed to limit long-term species coexistence. Adaptive phenotypic variation in response to increased competition following facilitation, however, could be a means of prolonging species coexistence. This hypothesis remains largely unexplored, however. Resprouting shrubs following fire in Mediterranean heathlands can facilitate seedling establishment and growth of subshrub species such as the carnivorous plant Drosophyllum lusitanicum. Nevertheless, shrub canopies interfere with prey capture by established D. lusitanicum plants, potentially reducing survival, growth, or reproduction with increasing time since fire. In this study, we examined whether adult plants in this species exhibit altered stem morphology (through either vertical or horizontal stem elongation) with increasing shrub canopy cover, so as to avoid shade and/or interference with prey capture. In addition, we quantified performance responses (e.g., survival, flowering, flower production) of D. lusitanicum plants to shrub canopy cover, size, and stem elongation. Measurements of stem morphology and performance were conducted at three Mediterranean heathland locations in southwestern Spain.
Results/Conclusions - Results revealed morphological variation indicative of vertical (but not horizontal) escape from shrub canopy interference (i.e., plants showed greater vertical elongation in dense shrub canopies). The probability of survival and flowering and the number of flowers per plant all generally increased with increasing plant size. However, smaller plants had an increased probability of flowering as shrub cover increased, whereas larger plants did not, resulting in a significant size × shrub cover interaction. The number of flowers produced by flowering plants responded to the interaction between shrub cover and vertical elongation. When shrub cover was high, plants that produced elongated stems produced more flowers than did those with short stems. In contrast, when shrub cover was low, plants with shorter stems produced more flowers than those with vertically elongated stems. We summarize our results by illustrating how species coexistence of foundational shrubs and D. lusitanicum could be maintained by a combination of facilitation of D. lusitanicum seedlings and adaptive changes in stem morphology in established D. lusitanicum plants to avoid shrub interference and allow the replenishing of a fire-regulated, persistent seed bank.