I arrived back home from the SVP meeting a few days ago, and I’m still trying to catch up with all the things I ignored over the last 2 weeks. At some point I’ll add some additional posts about the places Tim and I visited on our trip. But while we were gone, a new Jeffersoniana issue was published on which I was a coauthor, so I want to spend some time talking about that.
This paper was a little different for me, in that it is largely a botany paper that resulted from my collaboration with DB Poli of Roanoke College. DB is also a coauthor on the paper, and the lead authors, Stephanie Vogel and Brian Piatkowski, are students in her lab.
Like so many projects, this one has a rather random genesis. DB and I had been working on the fossil plant material from the Boxley Beckley Quarry and the McLoughlin Collection, getting a plan together for how we were going to proceed with various projects stemming from those collections. DB was explaining aspects of lycopod anatomy to me (not my area of expertise) and mentioned, in an offhand way, that the spores of the modern lycopod Lycopodium (above) explode. Sure enough, there are lots of YouTube videos with exploding spore demonstrations. We’ve since uploaded our on version, showing about 1 ml of spores exploding in slow motion:
This got us thinking; what if fossil lycopod spores did the same thing? A big part of the Lycopodium spore flammability seems to be spore size (airborne clouds of various organic particles are prone to explosions if the particle size is in a certain range, hence the explosive properties of grain silos and non-dairy creamer), and the microspores of many fossil lycopods are similar in size to Lycopodium spores. The thing is, some of the fossil lycopods were huge tree-sized plants (below), and may have released millions of spores at a time. Even more intriguing, during the late Carboniferous atmospheric oxygen levels were much higher than today (perhaps as high as 35%); what would happen if a giant lycopod exploded in such an atmosphere?
We speculated that maybe fossil lycopods were actually fire-ecology plants, which “intentionally” caught fire during lightning storms to burn off the competition, leaving their spores to grow among the ashes. A forest of Lepidodendron could all go up in flames one after the other in a chain reaction. We hypothesized that if we forced modern Lycopodium to release a cloud of spores into a flame that we could make such a chain reaction on a small scale. Excitedly, we harvested some Lycopodium to conduct our experiment.
It was a dismal failure. Far from causing a chain reaction, we actually had a very hard time getting the plants to catch fire at all. Even when holding the strobilus (the “spore cone”) in direct, continuous flame, the tip of the strobilus would ignite, flare briefly, and then go out, leaving most of the strobilus unburned. Clearly, this was going to require more detailed experiments and more time to sort out.
It turned out that Lycopodium does react to fire, but in a different way than we expected. Lycopodium spores are stored in sacs called sporangia, which are located at the bases of leaf-like structures called sporophylls. Multiple sporophyllys form cone-like structures called strobili (the photo at the top of the page shows several strobili). When the spores are mature and ready for release, the sporophylls open and the sporangia crack, exposing the spores.
That’s how things work in a plant that hasn’t been burned. If we burned a Lycopodium strobilus, the open sporophylls would close up. The sporophylls don’t burn very well, so while the initially exposed sporophyllys would burn pretty much completely, the fire wouldn’t propagate very far (only about 2 cm), so a few sporophyllys would burn completely, a few would be lightly charred, and most were unaffected.
Then things got interesting. After the fire was out, the sporophylls that were lightly charred would open and begin releasing their spores. The sporophylls that weren’t burned would stay shut, as did the ones that were completely burned. There was some variation depending on how mature the strobilus was, but this was the general trend. Incidentally, if the strobilus had already released its spores, it didn’t react to the fire at all; it would completely burn up.
Our final surprise can when we planted the spores from the charred strobili to see if they had survived the fire. We used a protocol that should have resulted in the spores starting to germinate after about 9 months, assuming the spores had survived. The spores from the burned plants actually grew faster; almost a third of the spores germinated in 3 weeks instead of 9 months.
It’s surprising to find an apparently fairly sophisticated fire response in Lycopodium, because these plants are not known to be associated with fire-prone areas today. We speculated that the fire properties in modern Lycopodium may be a holdover from an earlier period in lycopod evolution. Lycopods first appeared in the Devonian, and flourished during the Carboniferous. The explosive properties of the spores may well not be an adaptive character, but an accidental one. The spores needed to be small for air dispersal (especially in the Devonian, when there weren’t many animal vectors), but this would make them prone to burning. That probably wasn’t a problem at first, but during the Carboniferous atmospheric oxygen was quite high, and another group of plants were becoming increasingly common; the gymnosperms. Many of the gymnosperms are full-fledged fire ecology plants, which use fire to suppress competition and whose seeds sometimes require burning before they will germinate. Lycopods may have found themselves in possession of potentially exploding spores, surrounded by gymnosperms that were more than willing to catch fire. A possible evolutionary response to such a scenario is to become more fire resistant, trying to put out any fire that gets started. Another possible response is to make sure that your spores are poised to take advantage of any fires that do start. It seems that lycopods could have adopted both of these strategies.
So what about our original idea, of giant lycopod firestorms? Even if that wasn’t a specific adaptation of lycopods, could it have ever happened, given a lightning storm, high oxygen levels, and a giant strobilus filled with millions of spores? Well, maybe. Based on extensive charcoal deposits, there were lots of wildfires in the Carboniferous, but lycopods are actually not all that common a component of Carboniferous charcoal. However, when you do get lycopod charcoal, it tends to form an unusual deposit with lycopod stumps that burned from the top down; in other words, an intense crown fire (Falcon-Lang, 1999). These deposits could represent lycopod crown fires started by spore explosions.