Drivers and impacts of plant invasion vary globally: Evidence from pine invasions within six ecoregions
Pine (genus Pinus) invasions have become increasing prevalent across the Southern Hemisphere and have been shown to have significant impacts on native species and ecosystems, however their impacts on wildfire fuel loads is less certain. Various pine and other conifer species have also been shown to invade grasslands or shrublands in their native range in western North America. We aimed to determine biotic and abiotic controls on pine invasion globally within six ecoregions that included both introduced and native ranges. We also assessed the impacts that these invasions have on wildfire fuel loads. We collected abundance, size, and fuel data across invasion fronts of the widespread invasive tree species Pinus contorta at nine sites, representing a range of climates in the native and introduced range. We determined the relative importance of propagule pressure, abiotic characteristics, and biotic factors for invasion success. We also compared key plant population metrics such as individual tree growth rates and reproductive effort between native and introduced ranges.
Pinus contorta invasion density decreased with increasing distance from source population across sites, however the importance and shape of this relationship depended on local biotic communities and invasion age. Areas dominated by native southern beech forest consistently resisted invasion. This biotic resistance was not overcome by high propagule pressure. Abiotic factors had relatively small effects on invasion compared to biotic factors. P. contorta growth was faster, age to maturity was earlier, and reproductive effort was higher in the introduced ranges compared to the native range suggesting a demographic shift towards more rapid population growth in introduced regions. Wildfire fuel loads increased with P. contorta invasion and the differences found in growth rates contributed to different fuel load accumulation rates across sites. We demonstrate that although biological invasions are driven by propagule pressure across different ecoregions, these processes interact strongly with biotic factors (demographic shift and biotic resistance) that affect invasion success. Intriguingly, our results suggest that propagule pressure may become less important than biotic interactions as invasions proceed. Multi-region studies including both the native and introduced ranges are integral to understanding these changing interactions across regions and different invasion stages.