Something peculiar on the dry limestone prairie of texas…
Last week I got a heads up from a concerned party that the rare Silphium albiflorum, a texas endemic member of a genus of prairie “sunflower” that I’ve been obsessed with ever since learning about the related Silphium terebinthinaceum that grows on the tall-grass prairie (or what’s left of it, at least, which is not very much) of Illinois. The entire genus Silphium is actually pretty cool, and if you live in the midwest or Eastern United States and enjoy plants, I suggest you quickly acquaint yourself with this fucking phenomenal genus.
Anyway, Silphium albiflorum first caught my attention because of how restricted it seems to be compared to the other dozen or so members of the genus. For one, Silphium albiflorum produces white flowers. And second, it occupies a very specific kind of habitat known as dry limestone prairie. We normally think of prairies having a very rich topsoil, due to the thousands of years of which plants have been growing and then dying on them, their old dead tissue forming a very nitrogen rich kind of natural compost. The prairie ecosystem is an ecosystem like no other, and I find it utterly fucking tragic if not outright offensive that many people - myself included - can grow up in regions where this ecosystem was once the dominant plant community and have no idea what a prairie is or what were some of the plants that once grew there. So much of it has been lost and bulldozed to erect the kind of garbage, automobile-slum suburban commercial cesspools that now constitute much of our intellectually and ecologically decrepit American landscape.
Most prairies have a rich topsoil, but dry limestone prairie barely has any topsoil. Dry limestone prairie acts as an “island” of dry habitat where much more arid-adapted plants will thrive compared to some of the more rich and soil-having areas further North or East. And as a result, this different and more stressful habitat provided an opportunity for the very cool Silphium albiflorum to eventually evolve and speciate out of its presumed sister-species (or their shared common ancestor), however many millions of years ago that was.
Silphium albiflorum resembles Silphium laciniatum in leaf, but that’s about all. Silphium laciniatum is a much more widespread species and a much more robust species, growing much taller than albiflorum and with yellow flowers. It also prefers deeper richer soils. Silphium albiflorum is a stouter and presumably much tougher plant, growing on very rocky, harsh limestone substrates (almost no soil present) and with a tap-root that has been measured at more than 15 feet in depth (you can watch a very cool youtube presentation on it here : https://www.youtube.com/watch?v=yznZ18uJABs). Not to mention that S. albiflorum produces white flowers and flowers much earlier in the season than most of the other Silphium species. Like all members of the genus Silphium, S. albiflorums ray florets (Asteraceae terminology knowledge required. If you don’t have any check out this video: https://www.youtube.com/watch?v=D44YgtQraXY&t=131s) are female (pistillate), and the disc florets are male (staminate), so only the ray florets (the ligulate florets) mature into seeds. Most Asteraceae flowers tend to be bisexual.
Overall, Silphium albiflorum seems to have evolved simply because its environment provided an opportunity to do so. Without the existence of these dry limestone prairies and their harsh growing conditions which acted as a selection pressure and a different habitat from the surrounding deeper-soiled blackland prairie, Silphium albiflorum would have never evolved. This species gives us a great example - in the heart of a very populated area - to understand how evolution works and how environment shapes the trajectory of plant evolution.
