Tales Written in Leaves, Part I ~ Blaming the Bolide

In which the blogger confirms, yet again, the dangers of assuming he knows something.

The process has nearly run its course, though oaks and beeches are holding on,  During November and very early December in this area of the northeastern U.S., deciduous trees let go of their leaves in numbers beyond imagination (well, my imagination, at least).  Given that such trees dominate many forests here, it’s not surprising that we were blanketed in leaves.  Metallic growls disturbed daylight hours as folks with leaf blowers tried to corral the fallen leaves, saving the users’ backs while sacrificing their hearing.  Being old school, I chose to sacrifice my back.

While creating my own piles of leaves, dragging a dog through the mounds of leaves that decorated curbsides, or traipsing through nearby woods, I puzzled over this question:  Why am I surrounded by deciduous trees?

A couple of weeks ago, in an instance of serendipity, as I sorted through a stack of recent Natural History magazines, I happened upon a brief review from 2014 of a research article apparently on just that question.  Science writer Ashley Braun posited that this research showed that the impact of the extinction event at the end of the Cretaceous period selected for deciduous plants with their faster growth and disposable leaves.  (Reading Tree Leaves, Natural History, November 2014.)

I wondered whether it was fair to blame the seemingly never-ending cascade of leaves here principally on the Chicxulub bolide that burned through the atmosphere about 65 million years ago and smashed into the shoreline of the Yucatan.  (For many of the writers I’m currently reading, bolide seems to be the term of art for this extraterrestrial object, be it asteroid, comet, what have you.)  The atmospheric consequences of this impact (including an impact winter) are widely believed to have brought the Cretaceous period to an end, causing the extinction of many, many groups of terrestrial and marine organisms, including non-avian dinosaurs.  In North America, over half of all plant species were extinguished.  I was suspicious of this assignment of responsibility, believing, as I do, that the workings of natural history are, more often than not, complex tales of myriad interplaying factors.  Blaming the bolide seemed too simple.

Cretaceous flora included gymnosperms (producers of “naked seeds,” including conifers) which, depending upon whom you read, either were dominant throughout this period or had already been shouldered aside before the bolide impact by the recently appearing angiosperms (flowering plants producing “seeds within a vessel,” including sycamores).  It's mostly (though not exclusively) angiosperms that are deciduous and so the sources today of these myriad throw-away leaves.  And, it should be noted, many angiosperms are also evergreen.

In the article Braun described, Plant Ecological Strategies Shift Across the Cretaceous-Paleogene Boundary (PLoS Biology, Volume 12, Issue 9, September 2014), ecologist Benjamin Blonder and his colleagues focused on fossil angiosperm leaves from two formations in North Dakota – Hell Creek (Upper Cretaceous period) and Fort Union (Lower Paleocene epoch).  By doing so, Blonder keyed in on a slightly more than 2-million-year period that straddles the End Cretaceous event.

To get a very general sense of the kinds of fossils he and his colleagues worked with, I have included a few pictures of material from both of those formations.  The first pictures below show leaves that are part of a slab of matrix from the Hell Creek Formation that is currently on display in the Last American Dinosaurs exhibit at the Smithsonian’s National Museum of Natural History.

The next image shows a single leaf which was found in an outcropping of the Fort Union Formation in Montana.  It comes from my fossil collection.

At the outset of his article, Blonder noted that he was generating quantitative evidence relevant to a qualitative analysis from 1987 by paleobotantist Jack A. Wolfe about the appearance of the deciduous trait in plants across the boundary at the end of the Cretaceous.  It’s helpful to see what Wolfe outlined nearly three decades earlier (Late Cretaceous-Cenozoic History of Deciduousness and the Terminal Cretaceous Event, Paleobiology, Volume 13, Number 3, 1987).  Broad-leaved deciduous plants, Wolfe posited, evolved the deciduous trait to cope with climates that included “periods unfavorable to growth.”  It made evolutionary sense in those situations to speed up the production of leaves, disposable ones at that, emerging quickly to capture sunlight and dropping away before needing to be protected from freezing temperatures.

The global climate from the Late Cretaceous through the Eocene was, Wolfe described it, “generally warm and equable,” a climate that should not have favored deciduous plants.  Not unexpectedly, in the Late Cretaceous, “broad-leaved deciduous plants were of low diversity,” but a change in the composition of the flora marked the epochs (Paleocene and Eocene) immediately following the Cretaceous.  By the Paleocene, the Northern Hemisphere featured a “high diversity of broad-leaved deciduous plants . . . .”  Something or some things, he concluded had selected for the deciduous habit in Northern Hemisphere vegetation between the Cretaceous and Paleocene.  Wolfe suggested the “change resulted from a brief low-temperature excursion, probably an ‘impact winter.’”

Enter Blonder.  He analyzed changes in two key leaf variables during this narrow time period – leaf mass per area and minor vein density.  The first gauges plants’ investment in leaf construction (mass per area); the second relates to the transportation of water and carbon (vein density).  Variations in these investments fall across a spectrum from evergreen, “slow-return” leaves that are long lived with significant carbon tissue (relatively high mass per area and low vein density) to deciduous, “high-return" leaves that are shorter lived with less carbon tissue (relatively low mass per area and high vein density).

With results supporting Wolfe's analysis, Blonder found that, across the end-of-the-Cretaceous boundary, leaf mass per area fell while minor vein density rose.  The decline in leaf mass per area apparently resulted from the extinction of species abundant during the Cretaceous with high leaf mass per area; the rise in vein density across the boundary was affected primarily by the loss of species with low vein density.  In other words, overall changes were in the direction of increasing deciduousness.  In the impact and immediate post-impact period, fast growing plants were favored as deciduous species were selected for while evergreen species were selected against.  Rapid growth with disposable leaves was apparently the ticket to deal with this unstable environment.  Blonder has observed,

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.  (As quoted by Daniel Stolte in Meteorite That Doomed Dinosaurs Remade Forests, University of Arizona, UA News, September 16, 2014.)

So, I guess that's where I end up in this post, where I started.  Though none of this suggests how challenging I found this whole exercise.  I became mired in the debate within the scientific community over whether angiosperms dominated Cretaceous flora or, instead, were widespread but marginalized.  Compounding my frustration was that it took me  much too long to realize that I was essentially equating angiosperm with deciduousness which made the conflicting positions on angiosperm dominance all the more confusing.

In closing, I hesitate to note that there’s some dissent (of course there is) from the position that angiosperms hold sway today.  See, for example, Timothy J. Brodribb et al., Elegance Versus Speed:  Examining the Competition Between Conifer and Angiosperm Trees (International Journal of Plant Sciences, Volume 173, Number 6, July/August 2012).  But I won't get into it.


[In a second post, tentatively titled Tales Written in Leaves, Part II ~ The Damage Done, I will turn to the stories that can be read on leaves about the travails of the summer just past, or of summers gone by millions of years ago.]