Tom's Blog

Saturday, October 9, 2010

Collecting acorns of red, black, and Hill's oak

Thursday we had a visit from Andrew Hipp of the Morton Arboretum. This was part of a research project he is doing on oak evolution. As part of this project, oaks of all New World oaks are going to be raised in the greenhouse and in a common garden, so large numbers of acorns are needed.

I knew Andrew when he was a Ph.D. student at U.W. In those days he was working on sedges (see his nice book on Wisconsin sedges published by the U.W. Arboretum). Since assuming his position as curator at the Morton Arboretum Herbarium, he has been working on the genus Quercus, especially Hill's oak (Q. ellipsoidalis).

I originally communicated with Andrew after Amanda had discovered that we had Hill's oak at Pleasant Valley Conservancy. See my earlier blog for details. It turned out that for his new project, he was looking for sites where he could collect lots of acorns. This year we have very good acorn production of oaks of the red oak group (red, black, and Hill's), and acorns are falling everywhere.

Since Andrew is the expert on Hill's oaks, I wanted first to have him confirm our identification of this species.

Although the acorns in the photo show distinct stripes, it turns out that this is not the definitive character. Other species can also have stripes at times. One of the best characteristics is pubescence on the inner surface of the acorn cap, which is very dense in black oak and essentially absent in Hill's oak. (See Andrew's 2010 paper published in the International Oak Journal.)

After lunch we went on an acorn collecting foray at Pleasant Valley Conservancy. Since not all trees were dropping acorns at convenient locations, we drove slowly along the North Fire Break, which passes between the savanna and woodland areas. We soon found a very fine red oak specimen with a large number of acorns on the ground.

Of course, not all the acorns had viable seeds. Some had tiny insect holes, and were empty of contents. Others had probably aborted during the acorn formation process. However, one could soon learn to distinguish good from bad acorns by feel. The bad ones, being very light in weight, were tossed aside.

Since we have a lot of acorn-loving squirrels at Pleasant Valley Conservancy, one might wonder why they had not carried all the acorns away. In a good mast year, as this one is, there are simply not enough squirrels to decimate the acorn crop. In fact, this is probably the major reason why oaks exhibit this masting process in the first place. If they produced large acorn crops every year, the population of squirrels (and other acorn eaters) would rise to consume the food supply. Instead, oaks keep the squirrel population low by producing low acorn populations most years. Then comes a big mast year, overwhelming the squirrels, but providing plenty of acorns for production of new oak seedlings. An interesting evolutionary process!


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