Drivers, impacts, and feedbacks of global Pinus contorta (Lodgepole pine) invasions

Abstract

Pine species (genus Pinus) have been introduced across the Southern Hemisphere for forestry and several species have invaded surrounding ecosystems. Pine introduction across biogeographic regions sets up an ideal natural experiment to test underlying theories and assumptions of invasion biology. We studied the factors determining patterns of Pinus contorta invasion across nine sites in both the native and introduced ranges to understand how the invasion drivers change across sites and invasion stages. We found that propagule pressure is the most important factor in explaining invasion density in young invasions, but that biotic factors play an important role at later invasion stages. Additionally, we found higher invasion densities in the introduced than native range which may be explained by faster growth and earlier and more prolific reproduction in the introduced range. We examined the impacts of P. contorta invasions on plant biodiversity across sites and found that species richness and native plant cover decline with increasing P. contorta cover across sites in both the native and introduced range. There were more significant changes in species composition and individual species cover at grassland and shrubland sites in the introduced range than in the native range or a native forest site in the introduced range. Finally, there is concern that invasive pines, which are fire adapted, will alter fire regimes in a way that promotes a new fire-prone state further increasing their success over native plants. We examined the potential for P. contorta to form a positive feedback with fire by quantifying fuel loads and fire effects across an invasive gradient. We also examined P. contorta and native plant recovery following fire across an invasion density gradient. We found that fuel loads and simulated soil heating increased with older P. contorta invasions. Following fire, P. contorta dominated communities only when the pre-fire density was high. Therefore, we expect that a positive feedback between P. contorta invasion and fire will form only above an invasion threshold. Our invasion-fire simulation model suggested that fire in older invasions will increase invasion rates, but that fires in young invasions will not affect the invasion rate.