Apical dominance in the Norfolk Island pine
July 15, 2020 9:23 PM   Subscribe

Walking around my parents' neighborhood in central Florida, I see a number of what I think are large (40 ft or more) Norfolk Island pine trees. About half of them are split some ways up the trunk into two or more tops about the same height. My question is about apical dominance. How could it happen that an additional shoot apical meristem would develop in the other fork(s) of the plant in a species with strong apical dominance like that one? What's the mechanism there? Scholarly articles and your own thoughts very welcome!
posted by pH Indicating Socks to Science & Nature (7 answers total) 2 users marked this as a favorite
Maybe there were utility lines up there before they took the telephone poles out? And the trees were pruned accordingly?
posted by sebastienbailard at 9:36 PM on July 15, 2020 [1 favorite]

Storm damage that broke the original leader?
posted by jon1270 at 3:25 AM on July 16, 2020 [1 favorite]

I don't have an answer for you, just an observation that here in NE Florida it's very common to see Norfolks bifurcated in the same way you describe.
posted by saladin at 5:22 AM on July 16, 2020 [1 favorite]

If these were white pines around here (New England), it would be because of pine weevils. It would be a sign that they attacked the top of the tree back when it had one trunk, after which the tree put out two trunks. I'm not sure whether pine weevils or something like them go after Norfolks, though.

I half-remember something about this that I learned from awesome eco-detective Tom Wessels. I think it was that the weevils only attack white pines of a certain height in a certain amount of sun, so pines with splits can be used as a clue to that land's history. E.g., if you see a bunch of white pines with two trunks starting at certain height, you can deduce that the land must have been pretty open at the time the trunk split, such as if the land had been used for pasture... or something like that. And maybe that there was likely a mast year for those pines a certain number of years before the split trunks, helping you assign a date to those woods. Anyway, sorry for the spotty account, but if that kind of thing sounds interesting, the real deal might be in one of Wessels' books.
posted by daisyace at 6:41 AM on July 16, 2020 [2 favorites]

This page about some historic plantings in Victoria says, without reference: A few individual trees are in poor condition, have a poor form or are bifurcated, a natural tendency of Norfolk Island Pines. , emphasis mine.
posted by SaltySalticid at 8:10 AM on July 16, 2020

On this Q&A page, someone says:
Norfolk Island Pine commonly will split trunks. They are susceptible to wind damage, which is often the cause of damage to these trees.

My research showed many reports of the tree splitting into two leaders.
This page on freeze damage says:
Even though Norfolk Island pines are remarkably salt-tolerant, they have a weak link: Very low temperatures (30 to 32 degrees Fahrenheit) can cause the growing tips to die and abort.
Something - wind, frost, insects, etc. - must have damaged the terminal leaders of those trees. This Wikipedia page on apical dominance explains that "the apical bud produces a hormone, auxin, (IAA) that inhibits growth of the lateral buds further down on the stem towards the axillary bud . . . When the apical bud is removed, the lowered IAA concentration allows the lateral buds to grow and produce new shoots, which compete to become the lead growth." So the terminal growing tip was damaged and two buds below it were released from growth inhibition, began putting out growth at the same time and became co-dominant.

For more detail about how this all works on the molecular level, you can look at the section on apical dominance and apical control starting on p. 3 of Molecular basis of angiosperm tree architecture.
posted by Redstart at 10:56 AM on July 16, 2020 [2 favorites]

I just got back from a run through the residential neighborhoods just south of downtown St Augustine, FL. I counted 14 NIPs (lol); all but one was multi-trunked.
posted by saladin at 5:22 AM on August 18, 2020

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