Common hawthorn (Crataegus monogyna)
Old Presidential garden, north of Canada House
Depending on which botanist you ask, there are between 100 and 1100 species of hawthorns worldwide, and most of them are native to North America. Apparently a battle between "splitters" and "lumpers" has raged for some time. At the turn of the last century the splitters were ascendant; Jacobson relates that no less than 886 species of hawthorn were proposed and described between 1896 and 1910. Now, fortunately, the tendency is to recognize only a paltry 100 or so separate species, but allow them great variability. All of which will suggest that the species call on this tree (as well as its cousin, EH) is a wild guess.
Hawthorns are described as shade-intolerant. They are a pioneer species, being among the first plants to re-populate defoliated ground. One theory for their great variability is that they evolved in isolation in widely separated patches of ground newly freed from ice during the last retreat of the continental glacier. Another is that, for some mysterious botanical reason, they form hybrids with reckless abandon. Whatever: they are hard to identify.
Despite the profusion of native hawthorns, the two most common species of Crataegus encountered in North American gardens are English imports - this tree and EH, the English Midland hawthorn. These two are easy to distinguish - the haw (fruit) of monogyna contains only a single seed, whereas fruit of its cousin contain two or more. They appear to appeal to New World gardeners for reasons yet to be determined.
But enough serious stuff about hawthorns. This species of Crataegus is the quintessential hedge plant (in England, and perhaps elsewhere). They thrive under repeated pruning, and they are well armed with inch-long thorns. A hawthorn hedgerow is thus an inexpensive, and - in the spring, when they blossom - beautiful substitute for a barbed-wire fence. Hawthorn also is a plant with great mystical significance, much of it malign. (A surprising amount also is erotic.) For a G-rated sample, see the English midland hawthorn.
See Crataegus monogyna on Wikipedia.