Wednesday, 17 September 2014

Why Modern Forests Evolved

Plant Ecological Strategies Shift Across the Cretaceous–Paleogene Boundary | PLOS Biology, September 16, 2014

The thing about science is that it all fits together to form a large picture, so it's always rewarding when one little piece of evidence is found which confirms another piece, or, if not exactly confirms it, certainly goes a long way towards supporting it. Take for example, the evidence from the fossil record that the dinosaurs all died out over a very short time amounting to an instant on a geological time scale - the so-called Cretaceous–Paleogene Boundary (also called the K-T Boundary after the German for Cretaceous-Triassic). Some authorities believe this may have occurred in just a few years, even one or two.

The speed of this extinction led to a handful of theories such as a virus, a super-volcano, or an impact from a large extra-terrestrial body such as a large meteorite, a stray asteroid or even a comet. The growing consensus is now that it was indeed a large object (bolide) slamming into Earth and throwing up millions of tons of dust and debris which precipitated Earth into several years of winter weather and dramatic changes in climate patterns. The impact is generally reckoned to have been at Chicxulub in modern-day Mexico.

Somehow, the long-term beneficiaries of this were the mammals and birds which were able to survive this climate change and were then able to diversify into the niches previously occupied by the dinosaurs.

Now a team of palaeontologists led by Benjamin Bonder of the Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA, has found that dinosaurs were not the only major taxonomic group to go extinct at the time. Evergreen trees also suffered and far more so than the broad-leaf deciduous trees as shown by a survey of the relative numbers of different leaf types either side of the Cretaceous–Paleogene Boundary. Generally, evergreen leaves are thicker and have fewer veins compared to deciduous leaves. The assumption is that an 'impact winter' would have favoured the faster-growing of deciduous plants over the slow-growing evergreen ones.

Abstract

The Chicxulub bolide impact caused the end-Cretaceous mass extinction of plants, but the associated selectivity and ecological effects are poorly known. Using a unique set of North Dakota leaf fossil assemblages spanning 2.2 Myr across the event, we show among angiosperms a reduction of ecological strategies and selection for fast-growth strategies consistent with a hypothesized recovery from an impact winter. Leaf mass per area (carbon investment) decreased in both mean and variance, while vein density (carbon assimilation rate) increased in mean, consistent with a shift towards “fast” growth strategies. Plant extinction from the bolide impact resulted in a shift in functional trait space that likely had broad consequences for ecosystem functioning.

Author Summary

Sixty-six million years ago the Chicxulub bolide impacted the Earth, marking the Cretaceous–Paleogene boundary (KPB). This event caused the planet's most recent mass extinction, but the selectivity and functional consequences of the extinction on terrestrial plants has been largely unknown. A key untested hypothesis has been that a subsequent impact winter would have selected against slow-growing evergreen species, a possible cause of the modern dominance of high-productivity deciduous angiosperm forests. We tested this hypothesis using fossil leaf assemblages across a 2-million-year interval spanning the KPB. We assess two key ecological strategy axes—carbon assimilation rate and carbon investment—using leaf minor vein density and leaf mass per area as proxies, respectively. We show that species that survive the KPB have fast-growth ecological strategies corresponding to high assimilation rates and low carbon investment. This finding is consistent with impact winter leading to the nonrandom loss of slow-growing evergreen species. Our study reveals a dramatic example of the effect of rapid catastrophic environmental change on biodiversity.

Blonder B, Royer DL, Johnson KR, Miller I, Enquist BJ (2014)
Plant Ecological Strategies Shift Across the Cretaceous–Paleogene Boundary.
PLoS Biol 12(9): e1001949. doi:10.1371/journal.pbio.1001949

So a hypothesis formed to explain the observed fact of a sudden mass-extinction of dinosaurs and the rapid rise of mammals is firstly supported by the geological evidence of a large bolide impact at the right time and now we have evidence of another mass-extinction - of evergreen trees, though not so total as that of the dinosaurs - occurring at the same time and meshing neatly with the hypothesis that it wasn't the actual impact with it's initial shock and heat waves, destructive though they must have been, but the ensuing impact winter which did for the dinosaurs. And of course this rules out a virus, which was only ever an also-ran as hypotheses go, but it doesn't exclude a super-volcano which could also precipitate a freak winter lasting several years.

It also explains why broad-leaf angiosperms (flowering trees) tend to predominate in the world's woods and forests and maybe why so many mammals and birds have co-evolved with these evolving forests.

Any creationist prepared to offer up an explanation for all this stuff which fits so neatly into their prefered notion of 'science', or to put forward a testifiable, falsifiable hypothesis to explain why the assumed creator created an Earth with so much evidence that it is very old? How about the evidence that any creator of it could not have been benevolent and seems to have indulged in regular periods of random destruction, or even explaining why the creator created a world with such a fragile ecosystem then failed to protect it properly?

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