Complex planting histories of Pinus contorta subspecies and hybrids may help explain invasion success

Key message

Success of invasive non-native conifers in New Zealand may be in part due to introduced hybrids or unintentional hybridisation post-introduction, resulting in unexpected traits that impact control success.

Abstract

Exotic conifers are amongst the most successful weeds in the Southern Hemisphere, in part due to their frequent introductions for commercial forestry and sustainable land use. Multiple introductions of many species, subspecies, and races can help overcome genetic bottlenecks and shape unique invasive populations. Populations of Pinus contorta, the most vigorously spreading conifer in New Zealand, were believed to be dominated by a single non-serotinous subspecies, largely due to its perceived faster spread rate. However, recent work has identified serotinous cones in P. contorta stands leading to confusion around the identity of populations. Here, we investigated the historical planting literature to determine which P. contorta subspecies were introduced across New Zealand to better understand which traits may be expected in invasive populations. We also consider the effect that cone serotiny, expressed by two of four subspecies, may have on control operations by comparing the viability of serotinous seed banks in treated and untreated areas. We find that all four subspecies of P. contorta were established almost ubiquitously across New Zealand. Therefore, we postulate that invasive populations are unlikely to represent any one subspecies and observed trait shifts could be due to intentionally introduced hybrids or unintentional intermixing between subspecies. Additionally, we note that serotinous cone banks persist within herbicide-treated populations with no effect on seed viability. Despite lessening initial spread rates, cone serotiny may become more common across New Zealand by allowing populations to reinvade after herbicide treatments and confer fitness benefits as wildfires become more common under climate change.