Some tree species delay seed fall because their cones are dependent on a brief blast of heat to release seed. This dependency on heat during the seed production cycle is called "serotiny" and becomes a heat trigger for seed drop that may take decades to occur. Natural fire has to happen to complete the seed cycle. Although serotiny is primarily caused by fire, there are other seed release triggers that may work in tandem including periodic excess moisture, conditions of increased solar heat, atmospheric drying and parent plant death.
Trees that have a serotinous tenancy in North America include some species of conifers including pine, spruce, cypress and sequoia. Serotinous trees in the southern hemisphere include some angiosperms like eucalyptus in fire-prone parts of Australia and South Africa.
The Process of Serotiny
Most trees drop their seeds during and just after the ripening period. Serotinous trees store their seeds in the canopy via cones or pods and wait for an environmental trigger. This is the process of serotiny. Desert shrubs and succulent plants depend on periodic rainfall for seed drop but the most common trigger for serotinous trees is periodic fire. Natural periodic fires occur globally, and on average, between 50 to 150 years.
With naturally occurring periodic lightning fires over millions of years, trees evolved and developed the ability to resist high heat and eventually began using that heat in their reproduction cycle. The adaptation of thick and flame-resistant bark insulated the tree's internal cells to direct flame and used the rising indirect heat from fire on cones to drop seed.
In serotinous conifers, mature cone scales are naturally sealed shut with resin. Most (but not all) seeds stay in the canopy until the cones are heated to 122-140 degrees Fahrenheit (50 to 60 degrees Celsius). This heat melts the resin adhesive, the cone scales open to expose the seed that then drop or drift after several days to a burned but cool planting bed. These seeds actually do best on the burnt soil available to them. The site provides reduced competition, increased light, warmth and a short term increase of nutrients in the ash.
The Canopy Advantage
Seed storage in the canopy uses the advantage of height and breeze to distribute seed at the appropriate time onto a good, clear seedbed in satiating quantities enough for seed-eating critters. This "masting" effect increases the predator seed food supply to overabundance. With this abundance of newly added seed along with adequate germination rates, more seedlings than necessary will grow when moisture and temperature conditions are seasonally average or better.
It is interesting to note that there are seeds that drop annually and are not a part of the heat-induced crop. This seed "leakage" seems to be a natural insurance policy against rare seed failures when conditions are adverse just after a burn and result in a full crop failure.
What is Pyriscence?
Pyriscence is often a word misused for serotiny. Pyriscence is not as much a heat-induced method for plant seed release, as it is an organism's adaptation to a fire-prone environment. It is the ecology of an environment where natural fires are common and where post-fire conditions offer the best seed germination and seedling survival rates for the adaptive species.
A great example of pyriscence can be found in a southeastern United States longleaf pine forest ecosystem. This once large habitat is shrinking in size as fire is more and more excluded as land use patterns have changed.
Although Pinus palustris is not a serotinous conifer, it has evolved to survive by producing seedlings that go through a protective "grass stage". The initial shoot bursts in a brief bushy growth spurt and just as suddenly stops most top growth. Over the next few years, longleaf develops a significant tap root along with dense needle tufts. A compensating resumption of fast growth returns to the pine sapling around age seven.