Life history traits such as the timing of seed dispersal interact with fire regimes on both ecological and evolutionary time scales. A better understanding of these interactions will facilitate both reconstruction of historic fire regimes as well as prediction of species responses to shifts in fire regimes produced by climate change. The tendency of some plants to retain seed in their canopy for multiple years, a trait known as serotiny, is perhaps the most commonly cited example of plant adaptation in response to fire. However, our understanding of its expression and functional significance comes primarily from research on serotinous species in the families Pinaceae (in the northern hemisphere) and Proteaceae (in the southern hemisphere). I examined patterns of variation in the strength of serotiny (proportion of unopened cones that are older than one year) among populations of McNab cypress (Hesperocyparis macnabiana), a rare and increasingly threatened species in the family Cupressaceae. I also evaluated the extent to which variation in serotiny correlated with fire history and stand age structure.
Results/Conclusions
Although the strength of serotiny in McNab cypress was high overall, I also found significant variation in the degree of serotiny among populations. Consistent with studies of other serotinous species, strength of serotiny was generally higher in populations comprised entirely of even-aged stands that originated following stand-replacing fires. In contrast, serotiny was generally weaker in populations containing uneven-aged stands where some recruitment had occurred in the absence of stand-replacing fire. Interestingly, one exceptional population contained uneven-aged stands despite strong serotiny. Subsequent experimentation revealed that while cones from this population were pyriscent (opening in response to high temperatures) to the same extent as other populations, their cones were more highly necriscent (opening upon tissue death). This suggests that weak serotiny and strong necriscence may represent alternative adaptations to allow recruitment in the absence of stand-replacing fires. Overall, it appears that differences in historic fire regimes across the species’ range have influenced the evolution of serotiny and seed dispersal cuing which may, in turn, result in different population responses to future alterations in fire regime.