Marcescence

Red oak in September:

Red oak in October:

Red oak today:

See all the dead leaves still clinging to its branches? This is called marcescence. A lot of oaks and beeches, especially younger ones (and especially on lower branches, as seen here), hang onto the past season’s old dead leaves throughout the winter. The dead leaves finally drop off in the spring when new ones grow in their place. No one is quite sure why some trees do this, but there are a few theories – well, a lot of theories.

  • Dropping dead leaves at the beginning of spring could provide a burst of fertilizer to start off the growing season as the leaves decompose.
  • Keeping the leaves in the winter could help trap more snow around the bases of the trees, giving them extra moisture when the snow melts in the spring.
  • The dead leaves could even provide some protection from cold and frost for the tree’s buds or even deter browsing by herbivores.

For more information, check out this great article on marcescence by a professional forester in Vermont. Are any trees marcescing (did I just make up a word?) in your area?

What Does an Oak Tree Sound Like in September?

If you blindfolded me and dropped me somewhere on the property at random right now, I would still be able to tell you whether or not there was an oak tree nearby, based on the sounds alone.

The chattering and scolding of the red squirrels. The screams of the Blue Jays. The knock-knock-knock of the jays’ bills against the acorns’ tough outer husks. The loud smacks of discarded caps and husks falling to the forest floor. It’s harvest season, and the animals are taking full advantage of this year’s acorn mast, which I first wrote about a month ago.

Like squirrels, jays are voracious acorn eaters (you’ll have to excuse the fact that I couldn’t get a good photo of one with my point-and-shoot camera). Researchers have found that a single bird can harvest and cache about 110 nuts a day. What they can’t eat right away, they cache (store for later) by burying it in the soil and leaf litter, and since they don’t find and eat every acorn they cache they’re also planting the next generation of oak trees. For more information about the relationship between jays and oaks, check out the article Jays Plant Acorns from the University of California’s Oak Woodland Conservation Workgroup.

Last winter I wrote about squirrel caches and posted a photo of the scraps of pine cone left behind after a squirrel’s meal – I suspect that this winter those middens will contain acorn husks as well!

Next week I will be leading a backpacking trip to the Porcupine Mountains for my students, so (obviously) I won’t be on the internet at all. I have three fabulous guest posts scheduled for you, but I won’t be replying to comments etc. until I get back, at which point I’ll hopefully have lots of new photos and stories to share.

What’s With the Little Holes in Acorns?

I’m sure you’ve all seen acorns with small holes in them, often right under the cap, like on this one. What causes that? To the best of my knowledge it’s actually an insect called acorn weevil, an insect in the genus Curculio. The female lays her egg in an immature acorn. The larva develops inside the acorn, and then when the nut ripens and falls it bores its way out to live in the soil until it’s ready to mature into an adult. If you want to see what the weevil actually looks like, it’s actually the subject of possibly one of the greatest insect photos of all time. For more information, the Michigan Entomological Society has a great page on acorn insects.

And now, because I’m talking about a species of Curculio, I’m sorry but I cannot resist posting the weevil joke from the naval movie Master and Commander. Again.

Capt. Jack Aubrey: Do you see those two weevils, doctor?
Dr. Stephen Maturin: I do.
Capt. Jack Aubrey: Which would you choose?
Dr. Stephen Maturin: Neither; there is not a scrap a difference between them. They are the same species of Curculio.
Capt. Jack Aubrey: If you had to choose. If you were forced to make a choice. If there was no other response…
Dr. Stephen Maturin: Well then, if you are going to push me… I would choose the right hand weevil. It has significant advantage in both length and breadth.
[the captain thumps his fist in the table]
Capt. Jack Aubrey: There, I have you! You’re completely dished! Do you not know that in the service, one must always choose the lesser of two weevils!

You’re welcome.

Oak Mast

We currently seem to be experiencing a bumper crop of acorns – enough so that it’s noticeable even though oaks aren’t all that common here. The technical term for this is mast. We are having a mast year.

Oaks don’t produce large amounts of acorns every year (same goes for other nut trees). Instead, the trees in area will synchronize themselves so that, at irregular intervals, they all go acorn-crazy at the same time. The most popular theory as to why is that when all of the trees go all out at the same time, there are just so many nuts around that the squirrels and other nut-eating critters can’t eat them all even if they gorge themselves, so at least a few will get to germinate and grow. If one oak tree were to be a rebel and produce a lot of acorns in a year when the other trees were holding back, all the squirrels in the forest would descend on that one tree and it wouldn’t get to reproduce. (Squirrels aren’t the only animals that eat acorns, of course – earlier this summer one of the people I work with spotted a black bear way up in the highest branches of an oak tree, feasting on them.)

Of course, the real question here is how the heck are a bunch of trees spread across a forest communicating with each other and reaching a consensus on when to produce mast? And how are they balancing this cooperation with competition, natural selection favoring the trees that manage to reproduce more than their neighbors?

The simple answer seems to be that no one is really sure, but that doesn’t mean there aren’t a couple ideas. Plants can communicate with each other through chemical signals, messages like “look out everybody, there are lots of herbivores around.” A new study that’s gotten some media coverage even suggests that some communicate by producing clicking sounds in their roots that other plants can sense – plants don’t have ears, but maybe they can sense vibrations? So maybe our oaks are doing something like this, but they’re not in dense stands here like the maples and hemlocks are, they’re scattered in among the other more common trees. It’s questionable whether chemicals and clicks would really be effective at transmitting information across an entire forest.

There are other ideas too, like maybe trees are responding to some environmental cue and we just don’t know what it is, or maybe they need a certain number of years to store up energy before they flower and, since they need each other’s pollen to produce acorns, eventually they all either get on the same cycle or fail to pass on their genes.

In any case, tree mast is a really cool phenomenon, and if you know more about it than this feel free to jump in via the comments. Is it a mast year where you are?