Serotiny in botany simply means 'following' or 'later'.
In the case of serotinous flowers, it means flowers which grow following the growth of leaves,[1] or even more simply, flowering later in the season than is customary with allied species. Having serotinous leaves is also possible, these follow the flowering.
Serotiny is contrasted with coetany. Coetaneous flowers or leaves appear together with each other.[1]
In the case of serotinous fruit, the term is used in the more general sense of plants that release their seed over a long period of time, irrespective of whether release is spontaneous; in this sense the term is synonymous withbradyspory.
In the case of certain Australian, North American, South African or Californian plants which grow in areas subjected to regularwildfires, serotinous fruit can also mean anecological adaptation exhibited by someseed plants, in which seed release occurs in response to an environmental trigger, rather than spontaneously at seed maturation. The most common and best studied trigger isfire, and the termserotiny is used to refer to this specific case.
Possible triggers include:[2]
Some plants may respond to more than one of these triggers. For example,Pinus halepensis exhibits primarily fire-mediated serotiny,[3] but responds weakly to drying atmospheric conditions.[4] Similarly, Sierras sequoias and someBanksia species are strongly serotinous with respect to fire, but also release some seed in response to plant or branch death.
Serotiny can occur in various degrees. Plants that retain all of their seed indefinitely in the absence of a trigger event arestrongly serotinous. Plants that eventually release some of their seed spontaneously in the absence of a trigger areweakly serotinous. Finally, some plants release all of their seed spontaneously after a period of seed storage, but the occurrence of a trigger event curtails the seed storage period, causing all seed to be released immediately; such plants are essentially non-serotinous, but may be termedfacultatively serotinous.
In theSouthern Hemisphere, fire-mediated serotiny, pyriscence, is found inangiosperms in fire-prone parts ofAustralia andSouth Africa. It is extremely common in theProteaceae of these areas, and also occurs in other taxa, such asEucalyptus (Myrtaceae) and even exceptionally inErica sessiliflora (Ericaceae). In the Northern Hemisphere, it is found in a range ofconifer taxa, including species ofPinus,[5]Cupressus,Sequoiadendron, and more rarelyPicea.
Since even non-serotinouscones and woodyfruits can provide protection from the heat of fire,[6][7] the key adaptation of fire-induced serotiny is seed storage in a canopy seed bank, which can be released by fire.[8] The fire-release mechanism is commonly aresin that seals the fruit or cone scales shut, but which melts when heated.[9][10] This mechanism is refined in someBanksia by the presence inside thefollicle of a wingedseed 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.[11] 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.
The relative importance of pyriscence 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.[12] 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 pyriscence, while areas with infrequent crown-fire have low levels.[3][13] Herbivory of lodgepole pines can make pyriscence less advantageous in a population. Red squirrels (Sciurus vulgaris) and red crossbills (Loxia curvirostra) eat seeds, and so serotinous cones, which last in the canopy longer, are more likely to be chosen.[14][15] 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, pyriscence occurs in areas that not only are prone to regular fires but also possessoligotrophic 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 anash bed that temporarily increases soil nutrition; thus the survival rates of post-fire seedlings is greatly increased. Releasing a large number of seeds at once, rather than gradually, increases the possibility that some of those seeds will escape predation.[16] Similar pressures apply in Northern Hemisphere conifer forests, but in this case there is the further issue ofallelopathic leaf litter, which suppresses seed germination. Fire clears out this litter, eliminating this obstacle to germination.
Pyriscence can function primarily through two active mechanisms of smoke- or heatshock-induced germination. Studies on invasive species success in fire-prone environments have indicated that invasive species of mediterranean environments tend to employ fire-mediated germination. Interestingly, in these environments there is a distinction between average invasive herbaceous and ligneous plant life strategies, with ligneous plants highly associated with heat shock based pyriscence and herbaceous plants significantly associated with smoke triggered germination.[17]
Serotinous adaptations occur in at least 530 species in 40 genera, in multiple (paraphyletic) lineages. Serotiny likely evolved separately in these species, but may in some cases have been lost by the related non-serotinous species.
In the genusPinus, serotiny likely evolved because of the atmospheric conditions during theCretaceous period.[5] 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. ManyPinus 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: