How long do seeds of wind-dispersed Proteaceae remain mobile?
Steve Higgins and Frank Schurr (Department of Ecological Modelling UFZ - Centre for Environmental Research Leipzig) are looking at the migration potential of Fynbos Proteaceae. They are using a mechanistic model to simulate phase II/secondary dispersal of Proteaceae seeds by wind (i.e. the movement of seeds along the ground in post fire environments). An important parameter in this model is the time span for which seeds lying on the ground remain mobile. Unfortunately, this period is difficult to measure directly. Therefore they have asked various people to estimate this parameter. Getting an answer from several experts will allow them to estimate the confidence interval on the responses.
The question is:
How long do seeds of wind-dispersed Proteaceae remain mobile until they are permanently immobilized by germination, by being covered with sand/debris, or by damage to the dispersal structure?
(We are not considering the temporary trapping of seeds at obstacles (stones or plants) from which they can later be blown off again. Since we are only looking at successful dispersal, we are also not interested in seeds that become "immobile" due to predation.)
Because the period of time that seeds remain mobile varies, we would like you to estimate
A) the modal period of time a population of seeds remains mobile
B) the minimum period of time a population of seeds remains mobile
C) the maximum period of time a population of seeds remains mobile
Here is my response: if you have any improvements, please tell me.
A difficult question because there are so many parameters.
** there are three major losses of seeds: predation, germination and death
** the major loss of seeds from a mobility point of the seeds available is entrapment (obstacles and burial) and humidity.
Thus (I will confine myself to serotiny):
Losses of seeds to movement:
Seeds are released and lie/move around. Birds (and some rodents and some insects, but mainly birds) prey on them. There are two strategies: Sugarbush fruit - big and showy and Conebush fruit black and camouflaged to look like ash.
The Sugarbush fruit are thus open to heavy predation and get around the problem with 50-99% (mode 95%) of the fruit being duds and there only to fool predators. One problem with this is the sometimes fruit can clog up at obstacles and get caught in piles of fruit.
The Conebush fruit hide and are 90-100% viable.
Thus over time we have predation of fruit -this is high in Protea and low in Leucadendron. This is the explanation why spring fires yield low seedling numbers of Sugarbushes (less so, but still depressed in Conebushes), and autumn fires yield high numbers of seedlings. But three are two other explanations for this:
1. Spring fires are cooler and leave more litter and may thus allow higher rodent predation rates.
2. Spring fires have seeds blowing around for longer and these may become trapped in sand traps and gullies (mainly by wind movement of sand) and these may be buried too deep to reach the surface after germination.
But the current dogma is the predation story.
In almost all serotinous species there is a cold requirement (a minimum temperature - must be between 5-10*C - dunno for how long, perhaps just one night) and a wet requirement. Germination then follows en mass over about 1 month, but up to 3 months, thereafter very low. Thus after the first winter rains (the first heavy cold front) the seeds are gone: germinated. Some duds may lie around.
Sugarbush and Conebush seeds lose their viability after release: they are maintained on the plant, but do not store well. In soil trials Pat showed that most Sugarbushes were dead after 1 year, but that quite a few Conebush seeds survived into year 2. Thus for Sugarbushes there are no viable seeds after 1 year, for Conebushes none after year 2. However, in both, viability is about 6 months after that it declines rapidly - the longer lived ones are the leptokurtic tail. Seeds can be rescued with gibberellins (one of the secret added ingredients in Smoke Extract), but this is unlikely in the wild.
Factors stopping seeds moving
You have modelled this, but after the first rains and germination begins the environment becomes filled with obstacles as plants germinate and it is unlikely that after spring it will be possible for any seeds to move very far. Depending on rainfall, the vegetation may be up to 1m tall (unusual, usually 100mm) by spring.
After the first rains, there will be significant water movement of seeds and especially burial along gullies and runnels. In flatter areas flat "plateaus" occur with seeds and debris in lines along the edges of these flat areas.
I don't know about Conebushes - presumably any moisture in the soil will increase the adhesion of Conebush seeds to the soil surface and increase the forces required to get them moving.
But Sugarbushes have hygroscopic hairs and when dry these stand out like hedgehog spines, but when moist they collapse and the fruit becomes sleek and sexy. The hairs point the fruit base down allowing moisture absorption from the soil, and when wet these collapse onto the fruit, but the base will still be in contact with the wetter surface.]
Again, I don't know about Conebushes, but viable Sugarbush seeds absorb water when the soil gets wet and dry out if the minimum temperature is too high to allow germination and the soil dries out (a few seeds with at the warmer end of the response spectrum do germinate). This can happen many times. The weather data should give you the incidences of big fronts and median values for when the first cold-enough fronts hit the Cape.
How this operates under southeasters on mountain tops with significant cloud moisture I don't know. Lower down though southeasters are dry and this means seed movement. Use the weather records to get the incidence of southeasters from the summer fire to the first cold front.
In both cases the Berg Winds and Northwesters that precede the big cold fronts (usually around Easter) are very dry and can blow significant quantities of seeds (big bags full) the opposite way to the south-easterly winds. The weather data should give you relative durations and speeds of these respective winds.
I don't know how complex your models are, but southeasters and northwesters will follow different trajectories under different topographies so that the two are not necessarily just the opposite of one another.
The short answer to your question is:
A) after release a population of seeds typically remains mobile for xy minutes, days, months...
B) after release a population of seeds is mobile for at least xy minutes, days, months...
C) after release a population of seeds is mobile for at most xy minutes, days, months...
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