Sunday, March 31, 2019

When Glossopteris Missed the Spring

Late in the Permian Period, springtime had clearly changed in what is now the Sydney Basin (New South Wales, Australia).  No longer did forests of Glossopteris trees greet the changing of the seasons with new leaves.  The collapse of this Glossopteris flora in the Basin’s ecology occurred some 370,000 years before the mass extinction of marine life on the planet which happened roughly 251.9 million years ago as part of the End-Permian-Extinction (EPE).  This is the picture that geologist Christopher R. Fielding and his colleagues paint in a new study drawing on careful study of geologic formations in the Sydney Basin that allow for fine tracking of changes in flora from the late Permian through the EPE, and into the first half of the Triassic.  (Fielding, et al., Age and Pattern of the Southern High-Latitude Continental End-Permian Extinction Constrained by Multiproxy Analysis, Nature Communications, January 23, 2019.)

The glossopterids were large, very likely deciduous gymnosperms with trunk diameters that could reach 80 centimeters (over 2.6 feet) and heights of up to 30 meters (over 98 feet).  They lived in water-soaked environments, creating “moderately dense mire forests.”  (For an excellent article on the Glossopteris, see Stephen McLoughlin’s Glossopteris – Insights Into the Architecture and Relationships of an Iconic Permian Gondwanan Plant, Journal of the Botanical Society of Bengal, 2011.)

Pictured below are my sole Glossopteris fossils - multiple impressions of Glossopteris sp. leaves stained red-orange with hematite (iron oxide) during the fossilizing process.  They stand out dramatically from the cream colored matrix of claystone, a sedimentary clastic (composed of rock fragments).  These were found near the town of Dunedoo in New South Wales, on the northwestern edge of the Sydney Basin.

The demise of glossopterids in the Sydney Basin was a change with probably profound implications for that location at that time,and possibly for us today.  In the Late Permian, the Glossopteris was under threat across the mid to high latitudes of the Southern Hemisphere.  Until then, these trees had been so abundant and widespread in the southern expanses of the world that paleontologists refer to the vegetation found fossilized in Permian alluvial settings of the Southern Hemisphere as “Glossopteris flora.”  Indeed, the Glossopteris is an index fossil used to identify Permian strata.

Fielding et al. attribute the demise of the glossopterids in the Basin to a relatively short period of climate change.  Temperatures rose.  As they put it, there was “a brief perturbation to warmer, more humid climate conditions.”  The largely complete disappearance of the Glossopteris in the Sydney Basin was a piece of a much more widespread vanishing of this plant across the southern latitudes prompted by subsequent changes in climate.  As paleontologist Stephen McLoughlin has written (see article cited above), “it is clear that the demise of the Glossopteris flora in most regions is intimately associated with the global environmental changes and biotic crisis at the close of the Permian.”  (p. 10)  The plant did not go extinct in the EPE, but, instead, hung on into the Triassic for awhile in the colder climates found in higher latitudes further south.

Significantly, Field et al. find no evidence in the Basin of an “exceptional erosional event or catastrophic physical degradation of the landscape” coincident with the EPE.  The damage to the glossopterids had been done hundreds of thousands of years earlier.

Geologist Robert A. Gastaldo, in an article that considers this work on the Sydney Basin, observes that Fielding et al. delineate a “scenario for how vegetation might respond to current global warming.”  (Ancient Plants Escaped the End-Permian Mass Extinction, Nature News and Views, March 1, 2019.  More on that title later.)  The experience of plants, such as the Glossopteris, so clearly sensitive to relatively small changes in the average temperature in the climate, “might be a harbinger of the plant group’s ultimate extinction.”  And it’s that related aspect of the research on this once amazingly abundant tree that intrigues and worries me.

It’s now very evident that spring is here in the Northeast United States, though for several weeks many subtle signs had been there to see.  In the 1930s, botanist and nature writer Donald Culross Peattie (1898 – 1964) noted in An Almanac for Moderns (1935) (written, I believe, when Peattie lived in the D.C. area and had a nature column in the Washington Star newspaper) that it was in the last week or so of March that one could say that spring had, in fact, arrived.  That was not too early, he thought, though “poets and musicians” tended to announce its coming in late April and early May when, he noted sarcastically, no fool could miss the signs.

Peattie wrote in his Almanac entry for March 24th:
So does our spring begin, in a slow flowering on the leafless wood of the bough of hazel and alder and poplar and willow, a hardy business, a spawning upon the air, like the spawning in the ponds, a flowering so primitive that it carries us back to ancient geologic times, when trees that are now fossils sowed the wind like these, their descendants – an epoch when the world, too, was in its naked springtime. (The Norton Book of Nature Writing, edited by Robert Finch and John Elder, 1990 p. 452.)
“Naked springtime” is a wonderful phrase for that very brief period when the woods are still airy and filled with myriad shafts of light, even as buds open and leaves unfurl.  Without the Glossopteris in the Sydney Basin, the nakedness was probably much more pronounced and long lasting.

Peattie’s observation for March 24th about the “spawning upon the air” describes a phenomenon with which we are very familiar, to wit, car windshields thickly coated in sticky yellow-green pollen.  But that pollen-based covering of cars is only just in its early stages here in the waning days of March.  Is this late for us?  Were we vigorously cleaning our windshields last year at this time or the year before?  I don’t remember, but I seriously doubt that spring typically arrived for Peattie earlier in the 1930s than it does for us today.

In fact, with the relentless climate change we’ve set in motion for the planet, spring is arriving earlier and earlier in many places.  Data from the citizen-science project, USA National Phenology Network, show that around the country, spring, or, at least, certain harbingers of spring have been arriving on average several days earlier in recent years compared to prior decades.  A recent study, based on USA-NPN data for the four major migratory bird flyways and for the National Wildlife Refuge System, finds that in those areas:
the onset of spring is now earlier in 76% of all wildlife refuges and extremely early (i.e., exceeding 95% of historical conditions) in 49% of refuges. . . .  This differential rate of advance in spring onset is most pronounced in the Atlantic flyway, presumably because of a “warming hole” in the southeastern US.  (Erik K. Waller, et al., Differential Changes in the Onset of Spring Across US National Wildlife Refuges and North American Migratory Bird Flyways, PLOS One, September 12, 2018, abstract.)
Some do argue that earlier plant growth in the early spring will lead to greater absorption by plants of carbon dioxide with an ameliorating impact on the warming climate, but recent research seriously challenges that contention, showing that early spring growth comes at a cost.  Water is absorbed from the soil earlier in the year and the plants are unable to sustain their growth spurt throughout the summer leading to increased drought conditions and early death of the plants.  Indeed, climate change modeling apparently assumes some degree of increased absorption of carbon dioxide which this research asserts is overestimated.  (See, for example, Marlene Cimons’ blog post titled With Shorter Winters, Plants Bloom Early and Die Young, National Geographic Blog, October 19, 2018.)

Earlier springs can have dire consequences for some species as dependent relationships that have built up over millennia are broken.  Mismatches between feeders and food sources are increasing.  For instance, research has shown that some migratory songbird species are arriving at their springtime destinations too late for their offspring to be sustained on caterpillars and other insects because those insects have long since emerged in order to feed on the plant leaves that have come out early.  Ecologist Stephen J. Mayor, as quoted in a Washington Post article on this research, observed, “The rate at which birds are falling out of sync with their environment is almost certainly unsustainable.”  He added, “We can end up with these increasingly quiet springs.”  (Ben Guarino, Experts Fear ‘Quiet Springs’ as Songbirds Can’t Keep Up With Climate Change, Washington Post, May 16, 2017.)

Peattie’s naked springtimes may become also very quiet springtimes.

I find it worrying that a dominant tree like the Glossopteris could be undone by relatively small increases in temperature in the Sydney Basin in the Late Permian, well before the severe changes in climate that marked the EPE elsewhere on the planet.  On point (and fueling my anxiety) is a recent study of the distribution today of 86 tree species in the Eastern U.S. that found that among important factors influencing the distribution of tree species, climate change had a significant (though not sole) impact, possibly driving some species northward to cooler climates and some species westward to areas of greater precipitation.  (Songlin Fei, et al., Divergence of Species Responses to Climate Change, Science Advances, May 17, 2017.)  Ecologist Fei and his colleagues, focusing on data covering a 30-year period that describe the geographical abundance of different tree species, found that 73 percent of the 86 species experienced a westward shift (mean shift per decade was 9.6 miles) and 62 percent of the 86 species moved poleward (per decade mean shift of 6.8 miles).  Clearly, some species moved both westward and poleward.  The angiosperms in the study group were more likely to more westward, most of the gymnosperms moved north (ah, there go the Glossopteris trees in the Late Permian).  The authors raised the possibility that this divergence might be related to the fact that most gymnosperms are pollinated by the wind, while angiosperms are pollinated by animals.  Nevertheless, trees are on the move today, a phenomenon with potentially profound affects.  Fei et al. write:
The reduction or replacement of certain species in a community can be consequential, because species can have substantially different effects on ecosystem structure, function, and services, and the impacts can cascade through a broad range of ecosystem processes.
Almost a no brainer to suggest that the interdependence of organisms in ecosystems means that the loss of any taxon (through extinction or movement to more hospitable environments) may have adverse consequences for other organisms in those systems.

As noted earlier, Robert Gastaldo's article has what would appear to be a rather contradictory title – Ancient Plants Escaped the End-Permian Mass Extinction – given its coverage of the research findings by Fielding et al. just described.  But Gastaldo reviewed not just Fielding’s work but also a study by Hendrik Nowak et al. of a database on the plant fossil record across the Permian-Triassic boundary.  Nowak and his colleagues found that the evidence of substantial plant extinction cross this boundary was largely a function of biases in the fossil record.  In fact, they found that diversity of plant genera was largely unchanged, even though species diversity declined in this period.  Gastaldo observes, “In contrast to prevailing wisdom, Nowak and colleagues demonstrate that land plants did not experience widespread extinction during Earth’s most severe biological crises.”  This finding, Gastaldo asserts, is supported by Fielding and his coauthors because they posit that, at the EPE, the environment in the Sydney Basin was not marked by any dramatic change or decimation of flora.

But I would caution that, though the analysis by Nowak and his colleagues does cast doubt on a significant extirpation of plants at the EPE, their work should not be read as evidence that all was well for plants at that point and that, more generally, plants have been able to skate largely unscathed through those events across deep time that decimate the ranks of marine organisms and land animals.  Changes in flora occur at these dramatic junctures in deep time, something that even Nowak et al. do acknowledge:
[T]he compositions of floras changed repeatedly throughout the history of land plants.  By all accounts, their dominance structures were also drastically altered during the Permian-Triassic transition both on the short term and long term.  (No Mass Extinction for Land Plants at the Permian-Triassic Transition, Nature Communications, 2019, p. 6.)
Further, Nowak and his colleagues observe that plants that rely in some fashion on the activities of animals are particularly susceptible during periods of mass extinction because animals are generally more at risk during periods of rapid environmental change.

Most troubling to me is the note of optimism (seemingly directed to those of us who worry about the impact of climate change) that Nowak et al. strike at the very end of their analysis for those plants that do not partner up with animals:
[O]ther major plant groups that rely on abiotic vectors have a comfortable chance of survival.  (p. 6)
Frankly, I found their conclusion puzzling and offering only cold comfort.  “Abiotic vectors” are those nonliving elements in the environment – light, temperature, atmospheric gases, pollution, geography, etc.  If the Glossopteris would be included in the abiotic plant groups then clearly there is something to worry about with changes in climate.  And, now revealing my ignorance and bias toward a belief that all living creatures are interdependent, I must ask, Are there really significant plant taxa reliant solely on abiotic vectors, that is, independent of any animal partners?

Even if there are, all other plants that clearly survive and flourish in partnership with animals are at risk and the consequences of the loss of such plants – think, for example, food crops – would be profound.  I conclude that, whether or not plants generally managed to make a transition across the Permian-Triassic boundary without the same level of extinction that marine organisms or land animals experienced, the earlier blinking out of Glossopteris in the Sydney Basin means something troubling for us today in this 2019 springtime.
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