Reproductive trait shift in Pinus contorta helps explain invasion success in Aotearoa New Zealand

Pinus contorta is one of Aotearoa’s worst weeds. Evidence suggests that its spread and growth rates in Aotearoa are greater than in its native range in North America, yet the underlying drivers remain poorly understood. In this study, we examine cone and seed traits of P. contorta across several major invasion sites in Aotearoa and compare their traits with values from the native range. Across six sites, the number of filled seeds per cone ranged from 1–146, with an average 74 seeds per cone, three times the number recorded in the native range and approximately 50% more than previous estimates for Aotearoa. Notably, one site (Craigieburn, Canterbury) averaged only 26 filled seeds per cone with a larger proportion of unfilled seeds. Cone length and seed holding capacity varied greatly within individuals with an average cone length of 43.6 mm, ranging from 15.9–62.0 mm. We predict that this increase in seed holding capacity has likely enhanced the invasion success of P. contorta by increasing propagule pressure in the environment. We found that the average number of seed scales per cone is similar between Aotearoa and the native range, and therefore we suggest that this trait shift towards a greater seed holding capacity may be due to an increase in the proportion of fertile scales.