title: Pinus contorta invasions increase wildfire fuel loads and may create a positive feedback with fire.
abstract: Invasive plant species that have the potential to alter fire regimes have significant impacts on native ecosystems. Concern that pine invasions in the Southern Hemisphere will increase fire activity and severity and subsequently promote further pine invasion prompted us to examine the potential for feedbacks between Pinus contorta invasions and fire in Patagonia and New Zealand. We determined how fuel loads and fire effects were altered by P. contorta invasion. We also examined post-fire plant communities across invasion gradients at a subset of sites to assess how invasion alters the post-fire vegetation trajectory. We found that fuel loads and soil heating during simulated fire increase with increasing P. contorta invasion age or density at all sites. However, P. contorta density did not always increase post-fire. In the largest fire, P. contorta density only increased significantly post-fire where the pre-fire P. contorta density was above an invasion threshold. Below this threshold, P. contorta did not dominate after fire and plant communities responded to fire in a similar manner as uninvaded communities. The positive feedback observed at high densities is caused by the accumulation of fuel that in turn results in greater soil heating during fires and high P. contorta density post-fire. Therefore, a positive feedback may form between P. contorta invasions and fire, but only above an invasion density threshold. These results suggest that management of pine invasions before they reach the invasion density threshold is important for reducing fire risk and preventing a transition to an alternate ecosystem state dominated by pines and novel understory plant communities.
authors.affiliation: Land Resources and Environmental Sciences Department, Montana State University, Bozeman, Montana, 59717, USA.
authors.forename: Kimberley T
authors.initials: KT
authors.lastname: Taylor
authors.affiliation: Land Resources and Environmental Sciences Department, Montana State University, Bozeman, Montana, 59717, USA.
authors.forename: Bruce D
authors.initials: BD
authors.lastname: Maxwell
authors.affiliation: Earth Sciences Department, Montana State University, Bozeman, Montana, 59717, USA.
authors.forename: David B
authors.initials: DB
authors.lastname: McWethy
authors.affiliation: Laboratorio de Invasiones Biológicas (LIB), Facultad de Ciencias Forestales, Universidad de Concepción, Victoria 631, Casilla, 160-C, Concepción, Chile.|Institute of Ecology and Biodiversity (IEB), Las Palmeras 3425, Casilla, 653, Santiago, Chile.
authors.forename: Aníbal
authors.initials: A
authors.lastname: Pauchard
authors.affiliation: Grupo de Ecologia de Invasiones, INIBIOMA, CONICET, Universidad Nacional del Comahue, Quintral 1250, San Carlos de Bariloche, CP 8400, Argentina.
authors.forename: Martín A
authors.initials: MA
authors.lastname: Nuñez
authors.affiliation: Earth Sciences Department, Montana State University, Bozeman, Montana, 59717, USA.
authors.forename: Cathy
authors.initials: C
authors.lastname: Whitlock
mesh_terms: D017753:Ecosystem; D005390:Fires; D058865:Introduced Species; D009520:New Zealand; D028223:Pinus / Q000502:physiology*; D000075923:Wildfires
keywords: 
  Pinus contorta
  ; biological invasion; fire; fuel load; invasion impact; invasion threshold; pine invasion; positive feedback