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Pines show alluring diversity, can thrive in unlikely sites

Posted: Friday, March 25, 2011

On a recent hike, I spotted a tall pine tree with just one living, vigorous branch, which was served by a narrow strip of bark winding up the otherwise naked trunk. Refusing to die, so to speak, although perhaps no longer able to reproduce. That tree prompted me to think about our local pine trees — but first, let’s put them in a broader context.

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Photo by Kathy Hocker
Photo by Kathy Hocker

About forty-two species of pine are native to North America, but there is only one in Southeast. That one species is called the shore pine, which is a coastal variant of the wide-ranging lodgepole pine. (The Interior variant of lodgepole is found near Haines and Skagway, as well as in the Yukon and down the Rocky Mountains.)

Among the North American species of pine, there is a fascinating diversity of cones and seeds. Cones of some species weigh over two pounds, more than five hundred times the smallest. Seeds of most species (including lodgepole pine) have well-developed wings, so they can disperse on the wind when the cone opens. Small seeds, weighing less than about ninety milligrams (less than a sunflower kernel), are generally wind-dispersed, but large seeds are dependent on animals for transportation. Some large-seeded species are dispersed by jays and nutcrackers that collect the seeds and cache them for winter food (but fail to retrieve them all); the eaten seeds are the price the tree pays to get the remaining seeds dispersed. Other large seeds may fall to the ground where rodents gather and cache them.

Some cones are heavily armored, with thick, tough scales, and sometimes with sharp spines on the scales as well; others have small, thin, rather flexible scales. Armored cones may defend the seeds against enemies, such as seed predators, insects, or drought. Spines on the scales are best developed in species whose cones open spontaneously; they deter some seed predators. For example, research has shown that spines hinder perching and probing by crossbills. Some types of red crossbills are specially adapted (by bill size and structure) to feed on pines, even specifically on lodgepole pines, but even these find the spines somewhat difficult to handle.

There is much variation in patterns of cone opening. Some species open their cones spontaneously, when the seeds are mature, although the opening may be spread over several months. In other species, including lodgepole pine, the scales of the cone may be glued shut with resin and a tree may simultaneously bear cones from many different years; all the seeds are stored in the cones, sometimes for decades. Cones with resin-sealed scales are termed “serotinous” (from the Latin for “coming late,” referring here to the delayed cone opening). Serotiny is generally most prevalent in areas where forest fires — especially ground fires — are relatively common (perhaps every one to two hundred years in a given area). The heat from a ground fire opens serotinous cones, and a little later the accumulated seeds fall onto the burn, where they can germinate and grow with little competition, creating dense stands of young lodgepoles.

Lodgepole pines typically produce a good cone crop annually, in contrast to spruce, hemlock and some other pines. The reliable availability of lodgepole pine seeds contributes to the stability of the populations of their seed predators, including red squirrels and crossbills.

Across most of the range of lodgepole pines, red squirrels are the main seed predator. The squirrels attack cones at the proximal or butt end, near the branch (in contrast to crossbills, which attack the cone at the distal end, away from the branch). Squirrels cut the cone from the branch and then start peeling back the scales. The proximal scales are bigger and tougher than the others and often bear no seeds, so they slow the harvest rate for the squirrels, and squirrels tend to favor cones with softer scales. For cones of similar size, a squirrel has to peel off the same number of sterile scales from few-seeded and many-seeded cones, so clearly the many-seeded cones are more worthwhile. The squirrels are able, by trial and error, to select cones from trees that have high numbers of seeds per cone.

Squirrels have caused lodgepoles to evolve various means of reducing the rate of seed predation. For example, lodgepole pine cones in areas with red squirrels tend to be wide at the base and sit very close to the branch. They often grow in whorls of two to five cones, all with their wide proximal ends close to the branch and each other. These traits all make it hard for squirrel to detach the cones from the branch. In turn, red squirrels may have evolved stronger bites, but the effect of lodgepoles on squirrels would be less than that of squirrels on lodgepole because squirrels exploit many different kinds of cones.

Now the fun begins, when we consider some more complex interactions!

• Red squirrels are absent from several small mountain ranges where lodgepole pines grow. In these places, almost all lodgepole cones are serotinous (fire-adapted). In contrast, where the squirrels are present, a smaller (but variable) proportion of the cones are serotinous. Retaining closed cones on a tree for many years means that those cones are exposed to squirrel predation for all those years. So serotiny has some disadvantages in such places; squirrels select against serotiny. As a result, in these locations, fewer seeds are released when exposed to fire, the density of seedlings is lower, and plant community dynamics there differ from the dynamics in areas where serotiny is the rule.

• In isolated mountain ranges where red squirrels are absent, resident crossbills are the main predispersal seed predators. Lodgepole pine cones there have larger and thicker distal scales than they do in places where squirrels are present. This makes it harder for crossbills to extract seeds. But the local crossbills have evolved deeper, more decurved bills, and concomitantly stronger bites, to counter these seed defenses. The stronger bills select for yet tougher scales, which in turn select for stronger bills. The absence of squirrels highlights the coevolutionary “arms race” between cones and crossbills.

Where does this leave our local shore pines? Their cones are not serotinous and forest fires are rare in Southeast. Serotiny is reported to be more common in pines growing closer to the Interior, however.

Shore pines vary greatly in size, from scrubby, slow-growing dwarfs in muskegs (and other relatively infertile sites) to full-size trees around the muskeg edges. They need a lot of light and cannot grow in deep forest, but apparently they don’t do very well in soggy conditions. Lodgepole pine is capable of producing cones at age four or five years, in good conditions. The dwarfed shore pines in our muskegs can produce small cone crops, but the trees are likely to be very old, because they grow so slowly in that habitat.

So cone crops are presumably better on the bigger trees around the edges of muskegs, but there are fewer pine trees where the spruce/hemlock forest begins, and therefore presumably rather few pine cones overall.

That is probably why we don’t have red crossbills that are adapted to using shore pine as a food source. Instead, we have mostly hemlock-adapted crossbills and a few spruce crossbills. It is possible that some lodgepole pine-adapted crossbills might pass through occasionally, but because lodgepoles tend to produce good cone crops annually, the lodgepole-adapted crossbills are apparently less nomadic than others.

I do not know to what extent our red squirrels forage on shore pine seeds. I often see middens of spruce cone scales and cores, and spruce is clearly the squirrels’ main seed source. We occasionally see small piles of squirrel-harvested hemlock cones, which offer fewer calories per cone than spruce. But none of my naturalist friends reports signs of squirrel foraging on shore pine. I wonder if squirrels use pine more often in years when other kinds of cones are few, or if the squirrel population just crashes — both outcomes have been observed in southern British Columbia. So here is another little question to be answered, perhaps, by further observation.

In early March, on a small muskeg exploration, we found shore pines with next summer’s male cones and new female cones, still very small and tight. Pines are rather strange, in that pollination takes place in one year, but the cones do not develop and the ovules aren’t even fertilized until the next year. In the second year, the seeds and cones develop to full size. So the small cones we saw originated last year, fertilization will happen this spring, and the cones will mature this summer. I have not found an hypothesis explaining why pine cones take two years to develop or why fertilization is so much later than pollination. One more little mystery!

• Mary F. Willson is a retired professor of ecology.



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