Fire regime and climate determine spatial variation in level of serotiny and population structure in a fire-killed conifer

Fire-killed serotinous trees are often dominant species in fire-prone regions with contrasting wet and dry seasons. We studied a serotinous fire-killed conifer (Callitris preissii, Cupressaceae) to identify the influence of fire regime and climate on geographic variation in level of serotiny, recruitment and population structure. We measured population size distributions, seedling recruitment and level of serotiny for sample stands across a climatic gradient from mesic coastal (including two islands) to semi-arid inland sites in SW Australia. Trees on islands were mostly non-serotinous and were only weakly serotinous on the adjacent mainland in the higher rainfall West coast region. In the semi-arid Goldfields and South inland, and the higher rainfall South coast region, trees were more strongly serotinous with up to six cohorts of closed cones retained on trees. Level of serotiny was stronger at the drier end of the climatic gradient where severe to extreme fire danger weather occurs most frequently, and where vegetation type (e.g., shrublands) supports frequent crown-type, stand-replacing fires. Recruitment was most abundant post-fire in all regions, but seedlings also established inter-fire where there was regular seed fall due to low level of serotiny or spontaneous release of seeds from old cones on trees with a high cone load. Population structures for mainland stands shifted from approximately reverse-J (indicating continuous recruitment) in mesic regions to mainly unimodal (establishment mostly after fire) in drier, inland regions, although some long-unburned inland stands showed occasional inter-fire recruitment, likely linked to high rainfall years. Unimodal population structures characterized islands, where previous studies have identified herbivory on seedlings by refugial populations of native macropods as a limiting factor on inter-fire establishment. Understanding landscape context, response to disturbance, biotic interactions and reproductive traits of dominant species is essential in conservation planning in a rapidly changing global environment