The K-T Boundary Asteroid May Have Sparked A Shift In The Composition of Earth's Forests

Extinctions have the power to radically alter the biological makeup of the Earth. The Permian Great Dying set the stage for dinosaurs to flourish whilst the decline and demise of the Ediacara biota heralded the arrival of most modern day animal phyla during the Cambrian Explosion. Yet it is the K-T boundary extinction, responsible for the death of the dinosaurs, which is the most famous of the events which have rocked life on Earth; known to adults and children alike. The reptile family was struck a serious blow, both in the loss of marine and terrestrial families, while mammals and birds only survived thanks to their small size, despite possessing higher metabolic rates than their cold- blooded cousins.

Yet people often overlook the effects of extinctions on plants. Palaeobotanists, of course, ponder these mysteries, but in most cases the way they are affected and the impact of their subsequent loss on larger ecosystems is not examined. Big flashy animals which leave solid bones in the fossil record seem preferable to study than a few, almost imperceptible, carbonaceous smudges on the surface of a rock. The balance, however, has been redressed by a recent fascinating paper published by researchers from the University of Arizona. This shows that the K-T extinction may have helped pave the way for forests with a modern floral composition.

Rhamnus cleburni from the Hell Creek Formation

The evocatively named Hell Creek formation in the north west USA preserves a record of the last days of the Cretaceous period and the transition into the Palaeogene, giving an unrivaled insight into the general effects of the extinction.

Dinosaurs are common finds in the sediments which were once deposited onto vast flood plains covering the ancient subcontinent of Laramidia. Plants, however, are numerous also. Using collections housed at the Denver Museum of Nature and Science, the researchers examined 1000 individual specimens spanning 2.2 million years, 1,400,000 years from the Cretaceous, 800,000 from the Palaeogene.

By applying biomechanical formulae to the specimens, they built up a picture of how the characteristics of plant species and the composition of the ecosystems they formed changed during the extinction phase. This yielded some intriguing results. 'When you look at forests around the world today, you don't see many forests dominated by evergreen flowering plants,' said the study's lead author, Benjamin Blonder. 'Instead, they are dominated by deciduous species, plants that lose their leaves at some point during the year.'

'If you think about a mass extinction caused by catastrophic event such as a meteorite impacting Earth, you might imagine all species are equally likely to die,' continued Blonder. 'Survival of the fittest doesn't apply - the impact is like a reset button. The alternative hypothesis, however, is that some species had properties that enabled them to survive. Our study provides evidence of a dramatic shift from slow-growing plants to fast-growing species. 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.'

Deciduous plants have the ability to quickly lose and regenerate leaves

The biomechanics study examined the mass of the leaf in relation to its surface area to determine whether a species had strong, chunky leaves or thin flimsy ones - effectively a measure of how much carbon and energy had been invested in their growth. The latter type were not long lasting, but would have enabled their deciduous owners to quickly build up a light-harvesting surface area. This would provide enough energy to grow and reproduce rapidly, in comparison to their evergreen cousins. In so doing, they were able to establish a foothold in the newly freed ecosystems and become the dominant plant group.

What is more, the ability to lose and regenerate leaves quickly would have been a distinct advantage in an extinction environment with a rapidly fluctuating climate and light availability, due to the injection of dust and ash into the atmosphere. The classic image of the world of the dinosaurs is one covered in evergreen pines. The world today is one of deciduous plants. In the final paragraph of the Origin of Species Charles Darwin asks the reader to a consider a tangled bank where not only the animals but also the plants are locked in an intense struggle for survival. This cannot be more true for the transition of evergreen to deciduous during the K-T extinction. On the surface plants may appear placid, benign, in reality they are just as vicious as animals in their quest to propagate and survive.