title: Shifts from competition to facilitation between a foundation tree and a pioneer shrub across spatial and temporal scales in a semiarid woodland.
abstract: * Theoretical and empirical research has supported the hypothesis that plant-plant interactions change from competition to facilitation with increasing abiotic stress. However, the consistency of such changes has been questioned in arid and semiarid ecosystems. * During a drought in the semiarid south-western USA, we used observations and a field experiment to examine the interactions between juveniles of a foundation tree (Pinyon pine, Pinus edulis) and a common shrub (Apache plume, Fallugia paradoxa) in replicated areas of high and low stress. * The presence of F. paradoxa reduced P. edulis performance at low-stress sites, but had the opposite effect at high-stress sites. However, the intensity of the interactions depended on temporal variation in climate and age of P. edulis. Both above- and below-ground factors contributed to competition, while only above-ground factors contributed to facilitation. * These results support the hypothesis that interactions can change from competition to facilitation as abiotic stress increases in semiarid environments. A shift from competition to facilitation may be important for the recovery of P. edulis and other foundation species that have experienced large-scale mortality during recent droughts.
authors.affiliation: Northern Arizona University, Department of Biological Sciences, Flagstaff, AZ, USA. cms79@nau.edu
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Sthultz
authors.forename: Catherine A
authors.initials: CA
authors.lastname: Gehring
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Whitham
mesh_terms: D000222:Adaptation, Physiological; D002980:Climate; D017753:Ecosystem; D028223:Pinus / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D027824:Rosaceae / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*