Hiding in plain sight: Reinvasion risk from retained seed after dispersal events in introduced conifers

You have to have a grudging respect for Pinus contorta. It can pretty much grow anywhere and now scientists from Scion and Auckland Council have found that cones persist on trees even after mortality via common foliar applied herbicide-based control methods. They investigated whether open cones could present a reinvasion risk by retaining seeds after their primary dispersal event and disseminating them over time.

They found that 19 % of open cones retained viable seed after opening, with an average of 3.5 viable seeds (ranging from 1-39) per cone. The oldest cone retaining viable seed was 15 years old. The majority of seeds had separated from their wing 6 years after the initial cone opening, however intact wings were recovered up to 11 years post cone opening. These results suggest that cones can still contribute to propagule pressure even after their main dispersal event, potentially leading to the reinvasion of sites years after control operations. However, as the majority of seeds detach from their wings only a few years after cone opening their dispersal ability is hampered and reinvasion would likely occur close to the parent tree.

The results demonstrate that open cones of invasive Pinus contorta still present a viable seed source for populations to reinvade up to 15 years after their initial opening (Fig. 1B). However, the long-distance dispersal potential of seeds decreases as cones age due to the deterioration of wings, ultimately removing the primary dispersal capability of seeds older than 5 years (Fig. 2). Operationally, this has implications for the management of conifer infestations as delayed seed release could allow populations to reestablish after control operations, especially as residual herbicide effects wane after two years

The scientists identified a previously overlooked mechanism for reinvasion by highlighting that roughly one fifth of Pinus contorta cones can continue disseminating viable seeds into the environment years after their initial cone opening. The timescale of seed dissemination exceeds any recorded residual herbicide effects from control operations in New Zealand, suggesting that Pinus contorta populations have the potential to regenerate even without assistance from outside seed sources. However, dispersal ability is impacted by the deterioration of wing structures after ~5 years, meaning that treated invasion fronts are unlikely to expand until the next generation of invasive conifers establishes. These results are likely generalisable to other coniferous tree-weeds that retain their cones for any considerable time. In New Zealand, this may help explain how infestations of conifers such as Pinus banksiana, Pinus sylvestris, and Pinus muricata (among others) re-invade after control operations.

This is a paper well worth reading.