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CCevol2016 (talk | contribs) Added information and a few sources under "fire-mediated serotiny" and "evolution" |
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Since even non-serotinous [[conifer cone|cones]] and woody [[fruits]] can provide protection from the heat of fire,<ref name=Michaletz2013>{{cite journal|last=Michaletz|first=ST|author2=Johnson EA |author3=Mell WE |author4=Greene DF|title=Timing of fire relative to seed development may enable non-serotinous species to recolonize from the aerial seed banks of fire-killed trees|journal=Biogeosciences|year=2013|volume=10|pages=5061–5078|doi=10.5194/bg-10-5061-2013|url=http://www.biogeosciences.net/10/5061/2013/bg-10-5061-2013.pdf}}</ref><ref name=Pounden2014>{{cite journal|last=Pounden|first=E|author2=Greene DF |author3=Michaletz ST |title=Non-serotinous woody plants behave as aerial seed bank species when a late-summer wildfire coincides with a mast year|journal=Ecology and Evolution|year=2014|volume=4|pages=3830–3840|doi=10.1002/ece3.1247|url=http://onlinelibrary.wiley.com/doi/10.1002/ece3.1247/pdf}}</ref> the key adaptation of fire-induced serotiny is seed storage in a canopy seed bank, which can be released by fire.<ref name=Lamont2000>{{cite journal|last=Lamont|first=BB|author2=Enright NJ|title=Adaptive advantages of aerial seed banks|journal=Plant Species Biology|year=2000|volume=15|pages=157–166|doi= 10.1046/j.1442-1984.2000.00036.x|url=http://onlinelibrary.wiley.com/doi/10.1046/j.1442-1984.2000.00036.x/pdf}}</ref> The fire-release mechanism is commonly a [[resin]] that seals the fruit or cone scales shut, but which melts when heated.<ref name=Beaufait1960>{{cite journal|last=Beaufait|first=WR|title=Some Effects of High Temperatures on the Cones and Seeds of Jack Pine|journal=Forest Science|year=1960|volume=6|pages=194-199}}</ref><ref name=Johnson1993>{{cite journal|last=Johnson|first=EA|author2=Gutsell SL|title=Heat budget and fire behaviour associated with the opening of serotinous cones in two Pinus species|journal=Journal of Vegetation Science|year=1993|volume=4|pages=745–750|doi = 10.2307/3235610}}</ref> This mechanism is refined in some ''Banksia'' by the presence inside the [[follicle (fruit)|follicle]] of a winged [[seed separator]] which blocks the opening, preventing the seed from falling out. Thus the follicles open after fire, but seed release does not occur. As the cone dries, wetting by rain or humidity causes the cone scales to expand and reflex, promoting seed release.<ref name=Dawson1997>{{cite journal|last=Dawson|first=C|author2=Vincent JFV |author3=Rocca A-M |title=How pine cones open|journal=Nature|year=1997|volume=390|pages=668–668|doi=10.1038/37745|url=http://www.nature.com/nature/journal/v390/n6661/abs/390668a0.html}}</ref> The seed separator thus acts as a lever against the seeds, gradually prying them out of the follicle over the course of one or more wet-dry cycles. The effect of this adaptation is to ensure that seed release occurs not in response to fire, but in response to the onset of rains following fire. Flammability is also related to serotinous species such as Gamba Grass.
The relative importance of serotiny can vary among populations of the same plant species. For example, North American populations of lodgepole pine (''[[Pinus contorta]]'') can vary from being highly serotinous to having no serotiny at all, opening annually to release seed.<ref>Muir, P. S. and J. E. Lotan. 1985. Disturbance history and serotiny of Pinus contorta in western Montana. Ecology 66:1658-1668.</ref> Different levels of cone serotiny have been linked to variations in the local fire regime: areas that experience more frequent crown-fire tend to have high rates of serotiny, while areas with infrequent crown-fire have low levels of serotiny.<ref name=Hernandez2013 /><ref>Schoennagel, T., M. G. Turner, and W. H. Romme. 2003. The influence of fire interval and serotiny on postfire lodgepole pine density in Yellowstone National Park. Ecology 84:2967-2978.</ref> Additionally, herbivory of lodgepole pines can make fire-mediated serotiny less advantageous in a population. Red squirrels (''[[Red squirrel|Sciurus vulgaris]]'') and red crossbills (''[[Red crossbill|Loxia curvirostra]]'') will eat seeds, and so serotinous cones, which last in the canopy longer, are more likely to be chosen.<ref>Benkman, C.W., W.C. Holimon, and J.W. Smith. 2001. The influence of a competitor on the geographic mosaic of coevolution between crossbills and lodgepole pine. Evolution 55: 282-294.doi.org/10.1554/0014-3820(2001)055[0282:TIOACO]2.0.CO;2</ref> <ref>Talluto, M. V., and C. W. Benkman. 2014. Conflicting selection from fire and seed predation drives fine-scaled phenotypic variation in a widespread North American conifer. PNAS 111:9543-9548 doi:10.1073/pnas.1400944111</ref> Serotiny occurs less frequently in areas where this seed predation is common.
Pyriscence can be understood as an adaptation to an environment in which fires are regular, and in which post-fire environments offer the best germination and seedling survival rates. In Australia, for example, fire-mediated serotiny occurs in areas that are not only prone to regular fires, but also possess [[oligotrophic]] soils and a seasonally dry climate. This results in intense competition for nutrients and moisture, leading to very low seedling survival rates. The passage of fire, however, reduces competition by clearing out undergrowth, and results in an [[ash bed]] that temporarily increases soil nutrition; thus the survival rates of post-fire seedlings is greatly increased. Furthermore, releasing a large number of seeds at once, rather than gradually, increases the possibility that some of those seeds will escape predation.<ref>{{cite journal|last=Bradshaw|first=S|coauthors=Don Dixon, Kingsley W. Hopper, Stephen D. Lambers, Hans Turner, Shane R.|title=Little evidence for fire-adapted plant traits in Mediterranean climate regions|journal=Trends in Plant Science|year=2011|volume=16|issue=2|pages=69–76|doi=10.1016/j.tplants.2010.10.007|pmid=21095155}}</ref> Similar pressures apply in Northern Hemisphere conifer forests, but in this case there is the further issue of [[allelopathic]] leaf litter, which suppresses seed germination. Fire clears out this litter, eliminating this obstacle to germination.
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Serotinous adaptations have occurred in at least 530 species in 40 genera, which together constitute a paraphyletic group. Serotiny likely evolved separately in these species, but may in some cases have been lost by the related non-serotinous species.
In the ''Pinus'' genus, serotiny likely evolved because of the atmospheric conditions during the [[Cretaceous]] period.<ref name="He2012" /> The atmosphere during the Cretaceous had higher oxygen and carbon dioxide levels than our atmosphere. Fire occurred more frequently than it does currently, and plant growth was high enough to create an abundance of flammable material. Many ''Pinus'' species adapted to this fire-prone environment with serotinous pine cones.
A set of conditions must be met in order for long-term seed storage to be evolutionarily viable for a plant:▼
▲A set of conditions
* The plant must be phylogenetically able (pre-adapted) to develop the necessary reproductive structures
* The seeds must remain viable until cued to release
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