is an ecological adaptation exhibited by some seed 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 is fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
, and the term serotiny
is often used to refer to this specific case. The term has also been 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 with bradyspory
bradyspory is the gradual release of seed from a cone or fruit over a long period of time, as opposed to tachyspory, the more-or-less immediate release of seed as soon as they have matured....
Possible triggers include
- Death of the parent plant or branch (this form of serotiny has been technically termed necriscence)
- Wetting (hygriscence)
- Warming by the sun (soliscence)
- Drying atmospheric conditions (xeriscence)
- Fire (pyriscence)
- Fire followed by wetting (pyrohydriscence)
Fire is the most common and best studied case, and the term serotiny
is often used where pyriscence
is intended. Some plants may respond to more than one of these triggers. For example Pinus halepensis
exhibits primarily fire-mediated serotiny, but responds weakly to drying atmospheric conditions . Similarly, some Banksia
Banksia is a genus of around 170 species in the plant family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes and fruiting "cones" and heads. When it comes to size, banksias range from prostrate woody shrubs to trees up...
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 are termed strongly serotinous
. Plants that eventually release some of their seed spontaneously in the absence of a trigger are termed weakly 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 termed facultatively serotinous
In the southern hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
, fire-mediated serotiny is found in angiosperms in fire-prone parts of Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
. It is extremely common in the Proteaceae
Proteaceae is a family of flowering plants distributed in the Southern Hemisphere. The family comprises about 80 genera with about 1600 species. Together with the Platanaceae and Nelumbonaceae they make up the order Proteales. Well known genera include Protea, Banksia, Embothrium, Grevillea,...
of these areas, and also occurs in other taxa, such as Eucalyptus
Eucalyptus is a diverse genus of flowering trees in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia...
The Myrtaceae or Myrtle family are a family of dicotyledon plants, placed within the order Myrtales. Myrtle, clove, guava, feijoa, allspice, and eucalyptus belong here. All species are woody, with essential oils, and flower parts in multiples of four or five...
) and even Erica
Erica ,the heaths or heathers, is a genus of approximately 860 species of flowering plants in the family Ericaceae. The English common names "heath" and "heather" are shared by some closely related genera of similar appearance....
The Ericaceae, commonly known as the heath or heather family, is a group of mostly calcifuge flowering plants. The family is large, with roughly 4000 species spread across 126 genera, making it the 14th most speciose family of flowering plants...
). In the northern hemisphere, it is found in a range of conifer taxa, including species of Pinus
The genus Cupressus is one of several genera within the family Cupressaceae that have the common name cypress; for the others, see cypress. It is considered a polyphyletic group...
Sequoiadendron giganteum is the sole living species in the genus Sequoiadendron, and one of three species of coniferous trees known as redwoods, classified in the family Cupressaceae in the subfamily Sequoioideae, together with Sequoia sempervirens and...
The key adaptations of fire-induced serotiny are a cone
A cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity...
or woody fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
that provides protection from the heat of fire, together with a mechanism by which the passage of a fire can trigger seed release. Typically this mechanism is a resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
that seals the fruit or cone scales shut, but which melts when heated. This mechanism is refined in some Banksia
by the presence inside the follicle
In botany, a follicle is a dry unilocular many-seeded fruit formed from one carpel and dehiscing by the ventral suture in order to release seeds, such as in larkspur, magnolia, banksia, peony and milkweed....
of a winged seed separator
A seed separator is a structure found in the follicles of some Proteaceae. These follicles typically contain two seeds, with a seed separator between them...
which blocks the opening, preventing the seed from falling out. Thus the follicles open after fire, but seed release does not occur. Wetting of a follicle causes the wings to pull together, and then reflex back out again as the follicle dries. 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.
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 . 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.
Serotinous adaptations have occurred in at least 530 species in 40 genera, which together constitute a paraphyletic group. As such, it is likely that serotiny either evolved separately in these species, was lost by the related non-serotinous species, or a combination of the two.
A set of conditions must be met in order for long-term seed storage to be evolutionarily viable for a plant:
- The plant must be phylogenetically able to develop the necessary reproductive structures
- The seeds must remain viable until cued to release
- Seed release must be cued by a trigger that indicates environmental conditions that are favorable to germination,
- The cue must occur on an average timescale that is within the reproductive lifespan of the plant
- The plant must have the capacity and opportunity to produce enough seeds prior to release to ensure population replacement