Last week, the Pine Collection beckoned—at least it seemed to. The pines at the Arnold Arboretum are among the most subtle of our collections, passing through the seasons without the showiness of a riotous spring floral display or the kaleidoscope of autumn leaf colors found among its deciduous tree brethren. Rather, each and every day, pines are measured in the subtle hues of green in their needles, the textures and shades of bark, and the slow nearly two-year maturation of their seed cones. Right now, the second-year seed cones are all still green, but brown and woody colors and textures will arrive with the end of summer. A favorite destination among the pines is a set of three 99-year old Table Mountain pines (Pinus pungens; 10706*A, 10706*C, 10706*E) that hail from the Appalachian Mountains. The seed cones have amazing umbos (jumbo umbos, morphologically speaking), the thorn-like structures protruding from each seed scale. Best of all, you can take in the life of a pine cone on these trees, starting with the cohort of first-year cones (upper left, small, and seemingly all umbo), second-year cones (upper right and bright green), cones that matured last year (lower left; beautiful light brown) and then from perhaps a decade or more ago, old seed cones (lower right) with a patina of lichens and other fungi.
What are such old pine cones filled with seeds still doing on these trees? Second-year cones in most pine species open to release the (winged) seeds in late summer and fall. But not so the Table Mountain pine (and a number of other pine species). These pines are serotinous; they hold their seed-bearing cones for years, tightly closed, until fire arrives. The woody cones insulate the seeds from the fire’s heat, but this heat also serves to then open the cones to release their extremely patient seeds. A beautiful evolved mechanism to ensure that stands of Pinus pungens are ready to repopulate their native ecosystems that are subject to natural episodic fires. So, when you visit these trees, look along older parts of each shoot and you will find a decade or more of cohorts or old cones quietly waiting for fire to arrive (strongly discouraged at the Arboretum).
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