Tom's Blog

Friday, September 24, 2010

Hill's oaks at Pleasant Valley Conservancy

We have recently been able to confirm that we have a number of specimens of Hill's oak (Quercus ellipsoidalis), also known as northern pin oak or jack oak, at Pleasant Valley Conservancy. This species is an upland and northern version of pin oak (Q. palustris), and is an endemic in the upper Midwest (see Curtis, Vegetation of Wisconsin for background on this and the other Wisconsin oaks).

The recent paper by Hipp and Weber (Taxonomy of Hill's oak: Evidence from AFLP data; Systematic Botany, 2008, Volume 33 no. 1, pp. 148-158) provides a nice summary, including a map of its distribution.

Hill's oak is a member of the black oak section, and was originally identified as scarlet oak (Q. coccinea) (see Voss, Michigan Flora, for a lengthy discussion). The Hipp and Weber work provides firm data that Hill's oak is a distinct species.

According to Curtis, this is the most xeric species in the black oak group. It is predominantly a tree of dry, sandy places, and extensive pure stands are only found in Wisconsin in such sites. Curtis states that it is the most shade intolerant of all the Wisconsin oaks and cannot reproduce under the shade of any other tree species.

There are several characteristics that have been used to distinguish Hill’s oak: 1) Deeply lobed leaves with C-shaped sinuses (the sinus is the large opening in the leaf; see photo nearby). There are usually one or two deeply incised lobes (cut about one-half distance toward mid-rip of leaf. The leaves have sharp bristle tips like those of black and red oaks; 2) Acorns are ellipsoid often with longitudinal stripes, the cups enclosing one-third to one-half of the nut. The inner surface of the cup is much less hairy than that of black oak.

Another character that foresters use is that the lower branches are generally retained even though dead. It is from these stubs (pins) of dead branches that the name "pin oak” is derived. (However, this is not a defining characteristic.)

The first recognition of Hill's oak at Pleasant Valley Conservancy was by Amanda during her work on the tree database. Her identification was recently confirmed by Scott Taylor, an independent consulting forester. Identification is a little tricky, as Hill's, black (Q. velutina), and red (Q. rubra) oaks are fairly similar, and in fact are known to hybridize.

Although we have just begun to sort out which trees in our database are Hill's, we have identified several locations where they can be found. Our first "good" tree (shown in the photo with Scott) is in Unit 12A (the White Oak Savanna). It turns out that there is a small "grove" of Hill's oak at the eastern end of the unit, not too far from the steep ravine. Once this group was found we started looking uphill for an acorn source and found more Hill's at the east end of Toby's Prairie (on both the north and south sides) and in the area we call the Triangle (Unit 13D), which is also an area where we have red oaks as well.

I spent several hours yesterday cruising likely areas and found quite a few identifiable Hill's oaks. I found that the leaf structure was a good basis for identification. I used binoculars to check the structure. Because of the deeply incised sinuses (shown on the photo), the leaves look fairly "skinny" when viewed on the tree.

According to Internet sites, Hill's oak is popular as an ornamental tree because of its bright red fall color and tolerance of infertile sandy soils. As it turns out, several years ago I photographed a number of small trees in the Triangle that had interesting colors, one of which has now turned out to be Hill's oak. (See photo to left)

Obviously, we will now have to make some major corrections to our tree database, since all of the Hill's oaks were designated as black oaks.


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