Seed release by a serotinous pine in the absence of fire: implications for invasion into temperate regions.

Sarah V Wyse,Jerusha E Brown,Philip E Hulme,Rafael D Zenni

doi:10.1093/aobpla/plz077

Abstract

In pines, the release of seeds from serotinous cones is primarily considered a response to the high temperatures of a fire. However, the naturalization of serotinous pines in regions where fires are rare highlights the need to quantify environmental conditions that determine seed release to allow accurate prediction of dispersal and spread risk. We investigated the conditions that break cone serotiny in Pinus radiata, a widely planted forestry species that has naturalized in temperate regions worldwide. We quantified the cone temperatures at which cones open in this species, while also assessing potential confounding effects of cone moisture and age on these temperature requirements. We compared our laboratory results with cone opening behaviour under typical field conditions during summer in Canterbury, New Zealand. Cones opened at a mean temperature of 45 °C, much higher than maximum ambient air temperatures recorded in New Zealand. We found no influence of cone age or moisture content on opening temperature. Under field conditions, cones opened upon reaching similar temperatures to those determined in the laboratory; however, passive solar heating caused cones to reach temperatures up to 15 °C higher than ambient conditions. This resulted in 50 % of cones opening in field conditions where maximum air temperatures never exceeded 30 °C. Our results highlight the need for complementary laboratory and field experiments for understanding seed release from serotinous cones. Our findings have important implications for weed risk assessments, showing that serotinous pines can release seed in temperate climates without fire.

Highlights

Serotinous cones retain their seeds for one or more years following maturity (Critchfield 1957; Lamont 1991), with seed release often occurring in response to an environmental stimulus such as high temperature
We investigated the conditions that break cone serotiny in Pinus radiata, a widely planted forestry species that has naturalized in temperate regions worldwide
The occurrence of serotinous cones has been viewed as an important pre-adaptation that has facilitated the widespread invasion of pine species into

Summary

Introduction

Serotinous cones retain their seeds for one or more years following maturity (Critchfield 1957; Lamont 1991), with seed release often occurring in response to an environmental stimulus such as high temperature. Pines have been introduced to regions outside their native climate niche, to regions with cooler, wetter and less seasonal climates (Essl et al 2011; Perret et al 2019) Such environments are often described as non-fire-prone (sensu Lamont and He 2017) and are typically characterized by plant species without unambiguous adaptations to fire Latifolia is often classed as serotinous, in the cooler, wetter climate of Sweden up to 70 % of trees in commercial plantations bear non-serotinous cones (Despain 2001) As a result, this species has naturalized widely in Sweden (Sykes 2001). In some species, seeds may be released from serotinous cones in the absence of fire, such as following hot or dry weather conditions (Keeley and Zedler 1998). Whether this is a function of cone age or cone moisture content remains unclear

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