Hill’s Oaks Resolved

September 10, 2010 at 6:02 am (botany)
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by Carl Strang

In a post more than a year ago I made a preliminary case for the possibility that a group of young oaks on a savanna ridge top in Mayslake Forest Preserve may be Hill’s oaks. There I discussed leaf and bud shapes, which were consistent with Hill’s, but I needed acorns to conclude the case. This year a couple of the trees have produced acorns.

Furthermore, a new paper came out by Andrew Hipp, one of the co-authors of the publication I discussed in the previous post. The new paper, available on line here, goes into detail to separate Hill’s, black and scarlet oaks (the complete reference is: Hipp, Andrew L. 2010. Hill’s oak: the taxonomy and dynamics of a western Great Lakes endemic. Arnoldia 67(4):2-14). The following account is drawn from that paper.

Hill’s oak, Quercus ellipsoidalis, also known as northern pin oak, was distinguished from scarlet oak (Q. coccinea) in 1899 by Ernest J. Hill, who studied populations in Cook County, Illinois. Hipp and Jaime Weber surveyed the genetics of these trees and concluded that Hill’s oaks and scarlet oaks are distinct, though they are one another’s closest relatives. Except for a few individuals introduced by people from farther south, scarlet oaks do not occur in northeast Illinois (though there may be some in northwest Indiana). Hill’s oaks and scarlet oaks apparently separate according to geology, with the former inhabiting savannas on dry or well drained soils that developed from glacial deposits. Hill’s oaks are distinct from other members of the black oak group, though they sometimes hybridize with black oaks (Q. velutina). Their habitat, buds and leaves distinguish them from all other species except black oaks.

The biggest difference between Hill’s and black oaks is in the acorns. Black oak acorns have caps in which the scales are fuzzy, and loose at the tips, as shown by this example from an enormous black oak growing in Culver, Indiana.

In Hill’s oak the caps are shiny and their scales are tight. This description fits acorns from the ridge top trees at Mayslake.

The inside of the black oak acorn cap is fuzzy, while that of Hill’s is smooth. Photos in Hipp’s paper illustrate this, and the Culver and Mayslake acorn caps separate accordingly. So it seems that this is one question I can shift from my mental file of mysteries unresolved to that of inquiries concluded.

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Hill’s Oaks?

April 3, 2009 at 2:20 am (botany)
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by Carl Strang

 

Last fall as I was making my initial acquaintance with Mayslake Forest Preserve, I was passing along the top of the savanna ridge east of Mays Lake when I noticed, in addition to the magnificent bur oaks all around, a cluster of smaller oaks with bristle-tipped leaf lobes.

 

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In that dry exposed spot, with leaves shaped like this, I ruled out red oak and pin oak, and assumed they were a cluster of planted Hill’s oaks. Then recently I was talking with Conrad Fialkowski, who has been steward at Mayslake for years, and he told me that no trees had been planted in the savanna. This aroused my interest, as my impression is that you don’t run into Hill’s oaks every day. District Plant Ecologist Scott Kobal said that his records mentioned black oak but not Hill’s oak as being present at Mayslake, but there has been some recent taxonomic work on the latter species, and he kindly sent me a copy of the paper [Hipp, Andrew L., and Jaime A. Weber. 2008. Taxonomy of Hill’s oak (Quercus ellipsoidalis: Fagaceae): evidence from AFLP data. Systematic Botany 33:1-11].

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Hill’s oak has been regarded as very close to the scarlet oak, Quercus coccinea, and possibly not separate from that species, though the oaks in general sometimes seem more like a hybrid swarm than a group of well behaved species. Hipp and Weber found, in a comparison of fragmented DNA’s of several species, that Hill’s and scarlet oaks are distinct both genetically and in their geographic ranges, to the point that only the former occurs in northeastern Illinois except for a few probably transplanted individuals. The kicker is that the distinction between Hill’s oak and the black oak, Quercus velutina, is not sharp and where the two co-occur they hybridize.

 

Which brings us back to Mayslake. The inventory of mapped trees near the mansion included a large black oak. Beneath that tree I found a fallen branch from a recent storm and got photos of its buds, which have the gray fuzzy coating and angular cross section typical of black oak.

 

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The buds of the savanna trees proved harder to photograph.

 

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They are smaller, but the gray coating is limited to the tips, a characteristic of Hill’s oak. However, the buds are angular like a black oak’s. What I need are acorns, which are distinctive between the species, but the trees all are still small, so I will have to watch and wait.

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