Do Pinus radiata hybrids represent a solution to forestry’s wilding conifer problem?

Abstract

Afforestation by non-native pines has led to substantial conifer invasions globally, damaging ecosystems and resulting in substantial ongoing control costs. Any further new non-native conifers being introduced to forestry programmes must therefore be assessed from the perspective of spread risk prior to their widespread establishment. Pinus radiata × attenuata is a hybrid taxon that has been trialled for forestry suitability in New Zealand. The taxon performs favourably compared to Pinus radiata at higher elevation sites due to its greater cold tolerance, and as such is now beginning to be planted by the forestry industry at higher elevation sites that are typically unsuitable for P. radiata afforestation. It has been assumed that the likelihood of invasive spread by P. radiata × attenuata from areas of afforestation is low, however this has not been formally assessed. Here, we undertook assessments of cone production, cone opening, seed viability, seed dispersal, seedling establishment, and sapling survival for P. radiata hybrids with P. attenuata. In addition, we undertook analogous assessments of P. radiata to provide a baseline for comparison, given its propensity to establish outside of cultivation. Our results indicate that while cone production and seedling survival are higher for the hybrids than P. radiata at high elevation sites, cone opening requires significantly higher temperatures. On average, only 3.6 % of ripe cones on mature (26-year-old) trees opened on the P. attenuata × radiata trees in field conditions suggesting that the strong cone serotiny may present a barrier to invasion for this taxon. Based on our current data, we suggest that the spread risk from these P. radiata hybrids is likely to be lower than that from P. radiata. Future work should examine the longevity of seed in aerial seed banks and include field surveys to assess for wilding spread from mature P. radiata with attenuata hybrid stands. We emphasise, however, that the high temperatures required to break cone serotiny appears to be the only factor limiting the spread of P. radiata hybrids with P. attenuata. Any back-crossing of these hybrids with P. radiata in future breeding programmes may lower the cone-opening temperatures and remove this barrier to spread. Further, the high temperatures generated by forest fires will lead to more extensive release of seed from the aerial seed banks and should thus be treated as incursion events. The likelihood of such fires is expected to increase in predicted future climate scenarios.