Tom's Blog

Sunday, March 20, 2011

Oak grubs

Oak grubs are small trees with multiple stems. They arise from oak seedlings whose tops are killed by fire. Oaks are uniquely adapted to fire and grubs may remain alive for many years without turning into saplings. According to Johnson et al. (Ecology and silviculture of oaks, 2009, CAB International) these small grubs may have large root systems that are many years older than their living stems.

As it turns out, this is a good time of year to spot oak grubs, especially in the savannas where there is a lot of open area. The photo above shows some black oak grubs (Quercus velutina) in Unit 12A. These are interesting because all of the saplings and large trees in this savanna are white oaks, yet all the grubs are black oaks. This species is a fairly weedy tree which thrives well in sandy areas, such as present in this particular unit. Without moving I counted at least a dozen grubs. In an open area like this, where prairie grasses also thrive, there is plenty of fuel for hot fires.

In a lower part of the savanna, near Pleasant Valley Road, I found lots of bur oak grubs (Q. macrocarpa), which are easy to distinguish because of their heavy corky bark. The specimen in the photo is a classic multiple-stemmed grub. Here, near a very large bur oak tree that is a prolific acorn producer, I counted at least a dozen grubs. The bur oak was historically an important savanna species in this part of the country, and was noted for its scattered presence among the prairies. Again, the multiple stems are due to frequent burns. (This area has been burned annually for more than 10 years.)

What would happen if the burns ceased? Since they have extensive root systems, all of these grubs would quickly turn into substantial trees. In a sense, they are just sitting there waiting for a chance to start growing.

Here is another possibility: due to weather conditions or other random events, one of these grubs might miss getting burned. If this happens, it will quickly turn into a small tree. Because of the big root system, it should send up a fire-resistant (because of the corky bark) trunk. In fact, near these bur oak grubs there is a small tree that has escaped fire. This tree is very close to Pleasant Valley Road, where prairie grass is not too well established, probably explaining why it is no longer a grub.

I noticed that there were lots more grubs in the open than in the closed savannas. Since oaks are not very shade tolerant, this makes sense.

Anyone interested in details of the ecology of oaks should check out the text by Johnson, which is an authoritative and very complete up-to-date reference book.


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