Quercus havardii and Quercus stellata - distinguishing intermediates

Work in progress
See Selected White Oaks of the Texas Panhandle Plains and adjacent Oklahoma

Distinguishing Quercus havardii from intermediates

Oftentimes intermediate individuals (grouped under Quercus havardii × stellata) and true Quercus havardii appear beside each other, even far from where Q. havardii is sympatric with Q. stellata. Because there can be a lot of variation when it comes to intermediates individuals, it can be more or less difficult to distinguish them from Q. havardii.

Abandoning Mottes for Trees

The easiest intermediates to distinguish are those which have more or less given up their rhizomatous growth form i.e. they are small trees:

Sometimes you may get one or a few trunks, as shown above. Other times you will get a grove of trees together, which are probably all the same individual. In any case it's pretty easy to sort these into Quercus havardii × stellata.

A multiple trunk example.

Out of Height Range

According to Flora of North America Quercus havardii does not get any higher than 1.5 meters (or about 5 feet) in height. Based on that assessment, anything far above that height is likely an intermediate individual.

Oak-obsessed taxonomist C. H. Muller (1952) notes that while other characters vary much among intermediates, the one thing in common was that all intermediate individuals had increased height:

Most of the characters except height vary quite independently of one
another. Examination of thousands of clones, especially in areas with a high percentage of variants, failed to reveal a single obvious leaf or fruit variant not associated with increased height.

Thus, this motte here would neatly fall under Quercus havardii × stellata, at about 8-10 feet tall:

With the information we have so far, it is safe to assume that anything around 1.5 meters or less can be considered Quercus havardii. Of course, there is likely to some extent some genetic admixture within individuals that fall around 1.5 meters or less, and which otherwise have all the morphological features of Quercus havardii, so one cannot make a strict, non-arbitrary cut-off point where there is probably a smooth continuum of intermediates. However, given our species-based system of taxonomy and a lack of molecular data, this is likely the best we can do for now.

For many intermediates, most if not all leaves will substantially differ from what you'd safely call Quercus stellata (although certain leaves may appear more or less cruciform - more on that later). The abaxial leaf blade texture will likely match up with Quercus havardii (tomentose - dense, overlapping branched hairs).

Other clues that may help:

Rounding/Squaring of Lobes

Muller (1952) notes that intermediate individuals tend to become "obscurely or markedly round-lobed." To put this into perspective, note these two Quercus havardii plants from Monahans Sandhills State Park, towards the far western portion of the range and very far from the current western edge of Quercus stellata's distribution:

Noticeably the leaves have shallow lobes, or perhaps better called teeth, ending in a rounded point. All the Quercus havardii from Monahans fall within that range, more or less.

Now, to look at some intermediate individuals:

The lobes here are either more round—like the arc of a circle—or more truncate (squared-off). The leaves on some individuals may start to take on a cross shape, but on other individuals the lobes can be more irregular in appearance. In many cases the depth of the lobes will increase as well.

These characters alongside the height of the plant should lend further evidence that these plants are intermediates rather than Quercus havardii.

Posted on January 16, 2024 11:25 PM by arnanthescout