The spruce plot and marcescent southern oaks

January 18, 2017

This morning was cold, damp, and murky. Biking in from the gate I could see about 30 feet by the headlight. The mist streamed steadily through the light, racing downward and toward me as I rode. I biked west to the eastmost shortcut road, which is gated off in the winter. The unused road past the gate pitches downward through the woods. Last week’s storm scattered branches over the pavement. I locked up at the walkway and headed east. I’ve walked or skied this trail many times, but picking it up at this point, in the middle, gives me a strange feeling. The woods directly to the east remind me of trails in the Finger Lakes region of New York where my wife grew up, dominated by red oaks and sugar maples, with American beech and ironwood and Carex platyphylla underfoot… the last of these species is missing here, but white bear sedge takes its place. Looking at the distribution map for the species in New York, I see the two counties flanking my wife’s birthplace have herbarium vouchers for the species, but her county does not. I ought to have collected it last time I saw it.

I hiked into the spruce plot, a foreboding place before dawn. This dark, monotonous monoculture of spruces planted in a rectangle with sharp margins doesn’t have a lot to recommend it in the day, at least during the growing season—mosquitoey from June onward, nothing to speak of growing in the understorey—but this foggy morning it was transporting. A little stream babbled on my right, nothing moved. The spruces towered above. I could have been on the Apostle Islands in November.

Coming out of the spruce plot was a bit jarring. It wasn’t yet six, but the glowing sky seemed awfully bright. I crossed the road and hiked up a slight rise on the other side where I found Carex woodii in my first year at the Arboretum. I looked around with the flashlight and found plenty of Pennsylvania sedge and something much greener, but no C. woodii that I could see. I’ll find it this week: it’s lurking along the edges of the road, distinctive with its bladeless, burgundy leaf sheaths and tall shoots. I’d like to see it in midwinter before the snow returns.

Hiking around the corner to the oak collection, I was struck by a few very small trees that were utterly marcescent. While many oaks, especially the black oaks and their relatives, have some marcescent shoots, basically all the trees in the collection have dropped the lion’s share of their leaves. But on these few trees, it appears that hardly a leaf fell in the autumn. Two are southern species—post oak and blackjack oak—and one is a widespread east Asian species, Quercus mongolica. Out of place, I could easily understand this degree of marcescence as a maladaptive consequence of evolving further south. But the fourth species is our common black oak, accession 185-92. When I returned to the building, I expected to find that the accession was a southern collection, something from downstate Illinois or the Ozarks. It turns out, however, to be from Merioneth Drive just east of Crete, Will County, Illinois. That is south of us, but less than a 1-hour drive. Can that be enough to explain the phenotypic difference? Victoria Sork wrote an arresting article in American Naturalist in 1993 showing local adaption to herbivores in red oak on the scale of 100s of meters. The article floored me at the time: knowing how far oak pollen moves, I had assumed that oaks would exhibit little if any local adaptation. It may be that a Will County population is just different enough to behave as we see in our oak collection. On the other hand, it may be that what I’m looking at is just young oak behavior (though I’ve listed all the young oaks I observed this morning to be marcescent). I need to keep my eyes peeled. I have another two months or so before the trees start going again. Once secondary growth starts up, the leaves are likely to drop.

I hiked the remaining distance to my bike. It was 6:20 by now, but still foggy enough that the sky wasn’t growing light very quickly.

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