Araucarias gauge ancient levels of carbondioxide29 April
2011
Branches of young Wollemia nobilis.
One way of telling how much carbon dioxide was inthe atmosphere
in the past is by counting pores (orstomata) in leaves - the tiny
openings plants use toabsorb CO2 and lose water. It may seem
far-fetched, but plants tend to decrease the number ofpores when
the atmospheric CO2 is increased.
'With this observation it is possible to use thenumber of
stomata on the leaves of fossil plants tocalculate the
concentration of atmospheric carbondioxide in which those leaves
developed,' explainsMatthew Haworth, now based at the Istituto
diBiometeorologia in Florence, Italy.
The relationship between number of stomata andCO2 varies from
species to species and can bedetermined by counting the pores of
plants grownin the lab, under different amounts of CO2.
Anotherpossibility is to count the stomata of leavespreserved in
herbariums over the past 200 yearsand then compare them to recorded
atmosphericCO2 concentrations.
This so-called stomatal method 'works beautifullyover the last
1.5 million years where you have thesame species living as today,'
says Haworth. Themethod is relatively cheap and offers a good
time-resolution, which is important to understand
'rapidperturbations to the climate system, such as mass
extinction events or disruptions to the global carboncycle,' he
adds.
The catch is that most plant species stopdecreasing the amount
of stomata if the CO2concentration is above 400 parts per million.
This isknown as the ceiling response and 'really limits
theeffectiveness of the technique during periods ofhigh
concentrations of carbon dioxide, which areobviously the periods we
are most interested in tocompare with current rising CO2,' says
Haworth.
And this is where araucarias come in handy.
Araucarias nowadays are not very common, but200 million years
ago they represented a largechunk of the world's forests. These
trees evolved ina period when the concentration of CO2 in
theatmosphere was a lot higher than it is today and forthat reason
araucarias are good candidates togauge the levels of CO2 in the
atmosphere duringthe age of dinosaurs.
However, not much work has been done tounderstand how the number
of stomata in araucarialeaves relates to CO2 concentration. To do
this,Haworth and his team counted the stomata inleaves preserved in
herbariums 'to give us an ideaof how these species respond to
CO2concentrations below current ambient levels andalso over longer
timescales,' he says.
Additionally, the team grew three species ofaraucaria (Araucaria
bidwillii, Agathis australis andWollemia nobilis in a controlled
environment 'to seehow these ancient conifers respond to CO2
levelsway above current ambient,' Haworth says.
Working with the Wollemia araucaria wasespecially significant.
This species was discoverednear Sydney, Australia in 1994 when
scientiststhought it was extinct for 15 million years. 'It
wasreally exciting that I got to see the herbariumspecimens of this
conifer and that we were able to
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grow them,' says Haworth.
The results, published in Oecologia, suggest thatunlike most
plants araucarias don't show arelationship between number of
stomata and CO2below the level of 400ppm. However, when theCO2
concentration is above this threshold, 'thearaucarias reduced their
stomatal density by 11 to25 per cent, which is a very significant
decrease,'Haworth says.
This means that araucarias do not show the sameceiling effect as
other plants. In practice the resultsshow that araucarias can step
in and be usefulgauges of past CO2 levels, especially during
timeswhen CO2 concentration was higher than today.
This story is republished courtesy of Planet Earthonline, a
free, companion website to the award-winning magazine Planet Earth
published andfunded by the Natural Environment ResearchCouncil
(NERC).
More information: Matthew Haworth, et al. Thestomatal CO2 proxy
does not saturate at highatmospheric CO2 concentrations: evidence
fromstomatal index responses of Araucariaceaeconifers. Oecologia,
doi:10.1007/s00442-011-1969-1
Provided by PlanetEarth OnlineAPA citation: Araucarias gauge
ancient levels of carbon dioxide (2011, April 29) retrieved 3
February2019 from
https://phys.org/news/2011-04-araucarias-gauge-ancient-carbon-dioxide.html
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