The Plants That Died with the Dinosaurs

When a meteor impacted with the Earth 65 million years ago, it did more than just wipe out the dinosaurs. The devastation of the impact winter also led to a mass extinction amongst plants, with up to 50% of North American species dying out. Scientists hypothesized that the cold and dark would have led to selection against evergreen species, but the suggestion has remained unconfirmed. Another possibility is that the impact was equally deadly to all plant species, without selection favouring any group of plants. In a paper appearing in PLoS Biology, a team of researchers put the selection hypothesis to the test and find evidence that deciduous species were better equipped to survive in the post-impact world.

The team took advantage of an exceptionally well-preserved assemblage of fossil plants in North Dakota. The fossils spanned a period of 2.2 million years around the impact, offering data on how the composition of plant communities changed after the event. The researchers measured two traits in the fossils, the mass per area, which indicates how much has been invested in the leaf, and the vein density, which reflects how much water and carbon can be moved in and out of the leaf. Together, these two metrics give an idea of the "economics" of the leaf. 'Fast return' leaves have a low mass per area and high vein density so they can quickly take up resources; they're cheap to produce, but are quickly lost when conditions get tough. By contrast, 'slow return' leaves, with a high mass per area and lower vein density, are a longer-term investment; they're more expensive to produce, but they're also longer-lived. Plants which live in variable environments tend to be deciduous, investing in fast-return leaves which they lose when conditions are poor, while plants in more stable environments tend to be evergreens producing slow-return leaves.

When they compared these metrics before and after the impact, the researchers found that post-impact plants generally had a lower mass per area and a higher vein density, suggesting that deciduous plants fared better and came to dominate after the impact. "Our study provides evidence of a dramatic shift from slow-growing plants to fast-growing species," said lead author Benjamin Blonder in a press release. "This tells us that the extinction was not random, and the way in which a plant acquires resources predicts how it can respond to a major disturbance. And potentially this also tells us why we find that modern forests are generally deciduous and not evergreen. The hypothesis is that the impact winter introduced a very variable climate. That would have favored plants that grew quickly and could take advantage of changing conditions, such as deciduous plants."

It's pretty amazing to think about how the world might have changed in the wake of such a dramatic event, particularly with someone who has a penchant for sci-fi, home of the post-apocalyptic tale. As deciduous plants replaced evergreen ones, what impact did that have on the ecosystems built around the plants — the animals and fungi that ate them or lived on them, which were also coping with the impact winter? It's fascinating to consider the knock-on effects of these events, which eventually created the world as we know it, and studies like this one offer glimpses to help paint in the picture.

Ref
Blonder B, et al. Plant Ecological Strategies Shift Across the Cretaceous–Paleogene Boundary. PLoS Biology 12(9): (2014) e1001949. doi:10.1371/journal.pbio.1001949