I believe there is an additional influence that plays a part in the germination of the nut-like seeds: that of a colder pulse during the winter’s natural temperature fluctuations of day and night. I had not been able to germinate seeds of Mimetes stokoei and M. arboreus using naturally occurring winter temperature fluctuations in my area (about 8 deg C at night and 15 deg C during the day) and was attempting yet again to germinate seed during the winter temperature variations when our area received a colder than average winter cold front that delivered snow to the higher parts and intermittent hail for five days to some of my seed trays that were in the open. A few weeks after this they germinated well, while there was no germination from the batch that I had subjected to a hot pulse to imitate a fire. Apart from the hot pulse the two batches received the same pre-treatments. My impression is that the seeds were waiting for the temperatures of a “normal” winter to drop further, thus indicating a more than average wet winter might follow which would suit them better. The batch that had a hot pulse that did not germinate may indicate that the hot pulse of a fire plays a lesser part in that the fire may only be important to strip off the vegetation canopy that allows the ground temperatures to vary. More investigation required!
Or it could be that over thousands of germination cycles the normal (and maybe colder) cold fronts of the past are now unusual, occurring at greater intervals, leading to reluctant or delayed germination. This could have implications of global warming affecting germination cycles.
To germinate, seeds need water, warmth, cold and oxygen yet these factors are lessened underground in the ant’s nest where the temperature is more stable. If by chance they do germinate, they would be in deep shade at ground level provided by a living canopy while underground the new roots would have to compete with other mature roots. Evolution has worked to prevent the seeds germinating at the wrong time. The seeds need the sun’s warmth to penetrate though the soil above them and this will only happen after a fire when competition for scarce nutrients and root space is much less.
There is another unanswered question for me – is oxygen availability lessened underground in the ants’ nest and are the ants driven deeper underground by the fire for a time, allowing oxygen to penetrate to the higher stored seeds, thus triggering germination?
A fire heats the surface soil to an extent that pathogens and rodents are suppressed for a time and the fire ash nutrients have been watered into the soil by the first rains The prevailing conditions after a fire in the autumn is a sun-warmed soil and cooler and longer nights of the approaching winter with its promise of rain. There is a day to night fluctuation of temperatures that the seeds need to switch on as part of the dormancy break. If the seed germinates during this late autumn/early winter when the summer drought has broken it is assured of a lengthy period of regular rain, whereas an abnormally wet summer (fynbos gets most of its rain in winter) does not switch them on as the needed temperature fluctuations are not there yet. Germination in summer will only assure the seeds of heat and drought so the plants have evolved a mechanism that responds to the day/night temperature fluctuation of 10 deg C or more, and longer cool (autumn) nights.
I once tried to germinate some Leucospermum seeds at the correct time, but as they were less than a year old the germination rate was low, so I picked out the few seedlings that had germinated and forgot about the rest for a year, leaving them in the pot in full sun with no water except a little summer rain. When the temperature fluctuations started, and with no prompting from me, they started to germinate in the dry pot.
There are three protective coats surrounding the Leucospermum and Mimetes seeds, the outer, fleshy coat the ants use and the reason they take them underground safe from mice, then there is a much thinner coat that is not permeable to oxygen. This very thin inner coat when subjected to the heat of a fire flakes off in patches, allowing entry of oxygenated and smoke/ash saturated rainwater. This thin coat is difficult to mechanically abrade off and needs special attention. The last coat of the three is the strong nutlike coat that provides mechanical protection. It cannot be broken by squeezing it between thumb and forefinger, yet when exposed to a dry heat becomes brittle and crushable. Some of the stronger will germinate with simple prompting but with rare seeds, the weaker are as precious as the stronger.