Fossil leaves reveal effect of "impact winter"

by Timothy Oleson
Friday, January 30, 2015

When the Chicxulub bolide struck the Yucatán Peninsula at the end of the Cretaceous about 66 million years ago, widespread extinctions of land and marine animals resulted. However, the blast’s lasting effects on plants, which tend to be more resilient against impact-related fallout, have been less clear. Now, a new analysis of fossil leaves dating to around the end-Cretaceous offers some of the first quantitative evidence of a substantial shift in plant communities — toward more deciduous plants — following the impact.

The new study, published in PLOS Biology, supports a hypothesis first put forth in the 1980s by Jack Wolfe of the U.S. Geological Survey and colleagues, who examined North American plant fossils and hypothesized that deciduous plants — which shed their leaves annually — were better suited to the years-long “post-impact winter” following impacts like the Chicxulub blast.

“After a meteorite impact you ought to see a shift toward more deciduous species of plants rather than evergreens” due to the cooler temperatures and decreased sunlight that would have prevailed after light-reflecting dust and debris were thrown into the atmosphere by the impact, says lead author Ben Blonder, a plant ecologist and science coordinator at the University of Arizona’s Sky School. Such conditions, unconducive to photosynthesis and growth, should favor plants that do not need to invest energy in maintaining their leaves year-round.

Deciduous leaves “need to have more rapid growth strategies than evergreen leaves in order to take up the same amount of carbon [through photosynthesis] over the course of the year,” says Blonder. “What that means is that [deciduous leaves] should have functional traits … associated with faster growth.”

In the study, Blonder and his colleagues analyzed hundreds of fossilized leaves ranging in age from 1.4 million years before to 0.8 million years after the Cretaceous-Paleogene (K-Pg) boundary. (The leaves were collected in southwestern North Dakota in the 1990s by co-author Kirk Johnson, now director of the Smithsonian’s National Museum of Natural History.) They focused on two functional traits in particular: leaf mass per area (LMA) and vein density (VD). Lower LMAs and higher VDs are thought to be traits of faster growth strategies in plants because they represent smaller carbon investments in leaves and a greater capacity for rapid photosynthesis, respectively.

By measuring LMA and VD, the team quantified the “overall strategy of [each plant] as fast or slow, and therefore if it was deciduous or evergreen,” Blonder says. And in crossing the K-Pg boundary, “we see a shift toward faster growth strategies.” Averaging pre- and post-boundary fossils into two groups, the researchers reported that LMA dropped by about 9 percent, while VD increased by about 31 percent. The range of LMA values observed also dropped in the post-impact group, largely due to the disappearance in the dataset of fossils with especially high LMA values, which likely belonged to evergreens.

The results support the hypothesis of an evergreen-to-deciduous transition just after the K-Pg boundary, he says, and suggest that in the “aftermath of this impact, there are some really big shifts in how terrestrial ecosystems are functioning.” The exact nature of those shifts and how they may have influenced the hydrological cycle or carbon storage, for example, is unknown, he says, but “this study [should] encourage other people to find clever ways to reconstruct those changes.”

Compared to the earlier work of Wolfe and his colleagues, who identified evergreen versus deciduous taxa based on overall plant characteristics, the approach of the new study was “in a sense, more rigorous because [the researchers used] quantifiable measures,” says Robert Spicer, a paleobotanist at Open University in England who was not involved in the work. He says he agrees the new observations back up Wolfe’s idea with regard to the region where the samples were found, but cautions that the effect may have only been local, as there is little evidence for similar transitions in flora elsewhere.

The changes observed are also “a lot more subtle than I would have thought,” Spicer says. This subtlety could indicate that LMA and VD do not correlate as strongly to evergreenness and deciduousness as suggested, he says, or that the transition in plant taxa was related more to longer-term climate change that began before the K-Pg transition — cooling and declining atmospheric carbon dioxide in particular — than to the Chicxulub impact.

Spicer favors the latter explanation, noting a discrepancy he says is overlooked in the new study: The reemergence of ferns — whose spores can only survive for a few years — in North America and beyond soon after the Chicxulub blast suggests the post-impact winter was brief. But if the impact winter only lasted a few years, he notes, “most of the woody plant seeds, including those of evergreen species, would have survived” because woody plants tend to produce large numbers of seeds that are known to survive for long periods in the soil. So whatever drove the observed transition among evergreen and deciduous leaves had to operate “over a much longer timescale” of at least hundreds to thousands of years in order “to overcome the resilience of the soil seed bank” and kill off so many species.

Blonder says that the evidence from this study, which is “based on one of the best fossil [leaf] datasets in the world for this period,” still points to a significant role for the Chicxulub impact in altering forests. But, he says, the “big limitation here” is that quality data — well-preserved, roughly 66-million-year-old leaves — from around the world are hard to come by. “We’re trying to make global conclusions” using the data on hand, Blonder says, but “it’s very possible that better data from other sources … will either provide a more nuanced view or potentially even challenge our conclusions.”