The Impact That Killed the Dinosaurs Is Still Shaping Today’s Forests

Paleontologists have unlocked some of the most enduring mysteries about the meteor that wiped out the dinosaurs—along with at least half the world's species—66 million years ago. But it wasn't just animals that disappeared: the impact also deeply affected the ancient forests that sustained the dinosaurs, throwing the botanical balance completely out of whack.

Indeed, the fallout of the impact is still unfolding in modern forests, according to a study published in PLOS Biology today.

Based on analysis of about 1,000 fossilized leaves spanning 2.2 million years of the Cretaceous-Paleogene extinction event, the study's authors concluded that fast-growing deciduous plants fared much better in the catastrophic fallout of the impact than flowering evergreen plants.

"Deciduous and evergreen are two strategies that are both viable in different environments; neither is 'better' in an absolute sense," lead author and ecologist Benjamin Blonder told me. "Our paper shows that the particular environmental conditions after the [Cretaceous-Paleogene] boundary selected for deciduous species."

This marks one of the only instances in which "live fast, die young" can be considered solid evolutionary advice. Deciduous plants evolved to harvest a lot of sunlight in spring and summer, then shed their leaves in harsher conditions. The leaves essentially get evicted for not paying rent—once they start consuming more energy than they can collect, they are kicked off the branch. The slow-and-steady evergreens, however, are more conservative in both their intake and delegation of energy, because they never shed their leaves.

Dust from the impact created unpredictable climates and variable weather patterns, and the unstable conditions selected for fast-growing, seasonally adaptable deciduous trees. The slower flowering evergreens, meanwhile, died off in droves. Blonder's team was able to zero in on this transition using novel biomechanical formulas that helped reconstruct the ancient ecosystems.

"Think of a leaf as a flat object held up by a support point," he explained. "As the leaf gets bigger, the size of the support beam has to increase as well. Therefore we just have to measure the area of the leaf and the width of the petiole to estimate the leaf mass per area; this is easily done from a fossil."

"Once we have the leaf mass per area we have an index of the ecological strategy of the plant: higher values are 'slower' and lower values are 'faster,'" he said.

This post-impact dynamic between deciduous plants and flowering evergreens created a cascade effect that can still be observed in today's ecosystems. Most of the planet's forests are deciduous, and flowering evergreens are fairly rare. Some species survived and evolved into extant plants—holly and ivy, for example—but they never recovered their pre-impact distribution.

It's always tempting to use past catastrophes as predictive models for future events, but Blonder doesn't think this is one of those cases. When I asked if his study might help understand the effect of a meteor impact today, or perhaps the escalating effects of climate change, he said, "probably not, unfortunately."

"The [Cretaceous-Paleogene boundary] happened just once, and the Earth today is very different than the Earth of the past," he added. "That being said, our study does provide a very nice case study of how the Earth responds to rapidly shifting environments. Understanding the past is key to understanding the present and the future."