The Chicxulub bolide impacted Earth 66 million years ago with the force of 10 teratons of TNT, taking out all the non-avian dinosaurs and about half of all North American plant species. Now, researchers studying leaf fossils reveal that the dino-booming bolide decimated evergreen plants far more than their deciduous peers -- the latter were able to respond more rapidly to chaotic, post-apocalyptic conditions. Sometimes the “live fast, die young” strategy beats out the “slow but steady” one. Their findings are published in PLOS Biology this week.
"When you look at forests around the world today, you don't see many forests dominated by evergreen flowering plants," Benjamin Blonder from the University of Arizona says in a news release. "Instead, they are dominated by deciduous species, plants that lose their leaves at some point during the year."
To determine whether plant species went extinct randomly or non-randomly with respect to their traits, Blonder and colleagues examined about 1,000 fossil leaves of angiosperms (flowering plants, excluding conifers) from the Hell Creek Formation of southern North Dakota. These span across a 2.2-million-year period bracketing the Cretaceous-Paleogene (K-Pg) boundary mass extinction event.
The team measured two key traits: leaf mass per area (LMA), which indicates the amount of carbon invested per leaf area, and leaf minor vein density (VD), which signifies the ability to move carbon and water in and out of leaves. These traits help researchers understand plant survival strategies and suss out where particular species fall along the "leaf economics spectrum." In other words, were the leaves chunky and expensive or were they flimsy and cheap?
Leaves that are “fast return” (low LMA, high VD) are relatively cheap to make and allow plants to take up resources rapidly under favorably warm and wet conditions; but they’re lost when conditions are less favorable. Deciduous species acquiring resources in variable environments exemplify this strategy. On the other hand, leaves that are “slow return” (high LMA, low VD) are more costly to make but longer lived. Species on this end of the spectrum tend to live in less variable environments, and they’re usually evergreen, like hollies and ivies.
The researchers found that Cretaceous species with particularly high LMA mostly disappeared in the Paleogene, together with Cretaceous plant species with low VD. An impact winter with lower light levels and higher climatic variability would have selected against slow-growing evergreen species, favoring fast strategies -- which might explain why high-productivity, deciduous angiosperm forests dominate today.
"If you think about a mass extinction caused by a catastrophic event such as a meteorite impacting Earth, you might imagine all species are equally likely to die," Blonder says. "The impact is like a reset button. The alternative hypothesis, however, is that some species had properties that enabled them to survive."
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