We often read about Earth’s mass extinction events and how they wiped out vast numbers of animal species, leaving survivors to evolve and repopulate the planet. But it’s rarer to hear about how plants managed these catastrophes.
A new study published last month by a team at University College Dublin, Ireland, in the journal Nature Plants shows how plants with thicker, heavier leaves were more likely to survive the Triassic-Jurassic mass extinction caused by an episode of global warming 200 million years ago – around the time when dinosaurs came to dominate the planet. The extinction event is known to have had a massive impact on life, both on land and in the oceans, with an estimated half of species on Earth going extinct at the time.
“Previously we knew a bit about plant survival in terms of their abundance – whether they are present or absent in fossil record,” says the study’s lead author Wuu Kuang Soh, of University College Dublin, “but we didn’t know how they survived or the intrinsic factors of plants that contributed to their survival.”
By looking at fossilized leaves of two plant groups uncovered in Greenland, the authors suggest that the reason for some plants’ survival is that those with heavier leaves were more resilient to environmental stressors such as high air temperature and rising carbon dioxide.
According to Barry Lomax, a lecturer in Environmental Science at the University of Nottingham, UK, who was not involved in the research, “being able to establish that different plant groups respond differently to stress, and that their capacity to adapt to this stress is reflected in their propensity for survival, is a great piece of detective work.”
New proxy development
The work presented in this study – a joint research effort by University College Dublin and Macquarie University, Australia – relied heavily on the development of a new proxy i.e. a physical characteristic which is used to infer details about some related, but immeasurable, trait. By showing how the thickness of a plant’s leaf cuticle (the protective layer covering a leaf) is related to the leaf mass per unit area (LMA) in modern leaves, the team were able to use their fossils to identify how heavy the paleo leaves were for their size.
For some scientists, this development itself is the study’s highlight. “I think the key finding of the work is that it gives us another set of tools to unpick how plants have responded to large scale perturbations in the carbon cycle which have influenced climate,” says Lomax.
By using the new proxy for LMA the authors were able to look at changes in two plant groups – Ginkoales (an gymnosperm order – see image) and Bennettitales (a now extinct order of seed plant)– across the Triassic-Jurassic mass extinction, and discover their opposing fates; the former flourishing and the latter experiencing sharp ecological decline.
“We found that plants with higher LMA had a higher chance of survival than plants with lower LMA during this global warming induced mass extinction event” says lead author Soh.
Commenting on the study, Charilaos Yiotis, a plant physiologist at University College Dublin who did not participate in the research, says that “under the greenhouse conditions of the Triassic-Jurassic Boundary – or, may I add, under near-future greenhouse conditions – plants with fast leaf turnover rates (low LMA) are outcompeted by those adopting more “conservative” strategies like robust, low turnover leaves (high LMA).” The turnover rate Yiotis refers to is the time taken for leaves to be produced and then fall.
“The spectrum describes how “fast” or “slow” the plant is turning over its nutrient resources,” adds Dana Royer, a paleobotanist at Weslayan University, Connecticut, who peer-reviewed the paper for Nature. “At the fast-return end (low LMA), plants have high photosynthetic rates, high nutrient contents, short-lived (deciduous, for example) and cheaply built leaves. This is the live-fast-die-young strategy. At the slow-return end (high LMA), plants show the reverse.”
The current mass extinction
The study’s findings are important in relation to the sixth mass extinction event which is currently underway. Scientists believe that a similar extinction process to those in the past is now taking place, as the variety of life on land gives way to the seemingly unstoppable human developments of agriculture, industry, and urbanisation. By looking at the paleo-evidence the authors tentatively suggest that today’s plant communities which host thicker, heavier leaves (high LMA) may be better adapted to deal with the current episode of anthropogenic warming, and therefore have a better chance of future ecological success than plants with lighter leaves (low LMA).
“More specifically, plants that can have leaves with both low and high LMA appear to do well after surviving a catastrophic mass extinction episode,” says Soh. “Our finding is important because it means that plants with flexibility in LMA will be the favourite to flourish during future global warming.”
“A shift to higher LMA is common for plants when they are exposed to high CO2” says Royer, “so the fact that the authors are finding the same response in a “natural” experiment – albeit 200 million years ago – lends support to the idea that we should expect a similar response in our own future.”
Further looking to the future, University College Dublin’s Charilaos Yiotis finds it alarming that most plants of economic importance today would probably never have made it through the Triassic-Jurassic Boundary.
“At a time where humanity’s biggest challenge is to feed an ever-growing human population,” he says, “this study should make us think again about how big a threat climate change is to future food security.”
By Conor Purcell, a Science & Nature Writer with a PhD in Earth Science.
Soh, W.K., Wright, K.L, Bacon, T.I., et al., Palaeo leaf economics reveal a shift in ecosystem function associated with the end-Triassic mass extinction event, Nature Plants, 3, doi:10.1038/nplants.2017.104, 2017.
Editor’s note: This blog post provides a summary to a research paper that is paywalled, unlike other scientific articles featured on GeoLog. The EGU supports and promotes open access, publishing 17 open access journals and having endorsed Open Access 2020, an initiative to promote the large-scale transition to open access publishing. Since research in the realm of palaeontology and evolutionary biology is rarely featured on GeoLog, an exception was made on this occasion to publish a story on a scientific paper not accessible to all. The lead author of the study is happy to be contacted with questions about the research; if you’d like to find out more please email Wuu Kuang Soh (email@example.com).