Friday John and George visited Seabranch State Park near Hobe Sound, a scrub area, or you might say a scrubby pine woods. Except for a mantis praying, not much out of the ordinary, so we enjoyed the ordinary dominant Sand Pine trees.
Two pines are native in our usual haunts, one being Sand Pine and the other Slash Pine. CLICK Sand Pine tends to be a small gnarly hurricane-whipped scrub tree, although, given a chance, it can reach 60-80 feet tall. They mature quickly, able under ideal conditions to form cones at under a year old, often reproducing by five years. Yesterday we saw mature cones on the trunks of youngsters. That’s one way of coping with fires and hurricanes: make seeds before trouble hits.
Todays the pines were maturing male cones resembling small yellow cigarettes, some already releasing pollen. The emerging female cones are about the size of a pea, but scaly. They take two years to mature into the familiar woody pine cones, and may remain on the tree for many more years until fire separates the scales and allows the seeds to fall onto the fire-cleared ground,
The Sand Pines in South Florida tend to have “serotinous” cones, that is, requiring fire to open up, although a walk in the woods shows some to open fireless. “Clausa” means closed-in. Oddly, there exists a second, northern, population in and near the Florida Panhandle separated from the southern distribution by a geographic gap. The northern population has non-serotinous cones. That is, they differ from the southern Sand Pines by having the cones open when mature and letting the seeds flutter without benefit of flames.
Now hold on there, that’s interesting. To reiterate, we have a species spread the length of Florida with a gap in the middle. North of the gap the cones open up, whereas south of the gap the cones wait for fire, although these tendencies are not 100%. The difference has led some botanists to classify the northern open-cone-pines as one variety and the southern closed-cone-pines as a separate variety. (Variety is a category below species in the classification hierarchy. Species can be subdivided into varieties.)
The formal designation of two distinct varieties isn’t very convincing and doesn’t interest me much, but how that north-south difference in cone-behavior came about is something to ponder. Three possibilities come to mind:
Possibility 1. Slow evolution. Perhaps the two populations have been apart long enough for each to experience its own evolutionary divergence, with the cone difference being the most prominent distinction. If that is so, was fire historically more a factor in the southern region than in the north? Or could it be that in the cold north it is better to drop the seeds into the protective earth than dangle them in the frosty air? (There are other serotinous pine species in northern regions.) Of the three possibilities this one strikes me as most likely.
Possibility 2. The southern population originating from a few closed-cone founders. Maybe the Sand Pines originated in the north while most of Florida was submerged, and then maybe just by chance a closed-cone great ancestor jumped whatever gap(s) existed in prehistoric times to populate the southern region with its closed-cone descendants. Scrub habitats at times of high water in millennia past were islands, and maybe a closed-cone “island” population developed and spread southward. This possibility is called the “founder effect.” There is a nice Wikipedia elaboration on this linked in the notes below.
Possibility 3. Environmental cues. Conceivably some environmental difference—soil, water, temperature—determines the cone type on any given specimen during its individual development. If you have 10 years and a truck, you could plant some closed cone-types in the northern zone and vice versa and see if the reciprocal transplants each conform to their new digs. Don’t hold your breath.
Possibility 4. None of the above, or a combination of factors.
Preliminary DNA study shows more variability within each of the two Sand Pine populations than between them. In other words, DNA so far does not support recognition of two different named varieties, although there seem to be minor differences in their reproductive cycles. The southern closed-cone types tend to be more uniform in age and in genetic variation (over small distances) than their northern counterparts. Such uniformity might result from uniform repopulation after fire or hurricanes like grass regrowing uniformly after mowing, or maybe it comes from possibility #2, the founder effect, given that a small founder population would be less diverse than the large, widespread population to the north.