Gas exchange analysis in fossil leaves and climate change

Next month, experts in climate reconstruction will meet to create a consensus record of atmospheric CO2 over the past 66 million years.  Stomata will play an important role in the coming meeting when paleoclimatologists reconcile their calculation using leaf gas exchange in fossil leaves with other sources1.

It was Woodward in 1987 who first observed that a significant inverse relationship existed between plant stomatal density (number of stomata per mm2) and atmospheric CO2 concentration. Using herbarium specimens, he demonstrated that the stomatal densities and ratio in 8 temperate woody specimens collected 200 years ago were significantly higher than those of the same plant species today2.

In 2014, Franks et al.3 applied the same idea of counting stomata to fossil leaves and integrated their counts with the available models for leaf gas exchange. Another important parameter used in common leaf gas exchange analysis is the internal CO2 content. In fossil leaves, this concentration can be obtained using the isotopic signal of δ13C to discriminate between leaf and atmospheric carbon. A worrisome finding from this method is that the Earth’s climate could be more sensitive to CO2 concentrations than previously thought.

 

conceptual-model-for-gas-exchange-analysis-in-fossil-leaves

Conceptual idea for the analysis of leaf gas exchange in fossil leaves (taken from Franks et al. 2014)

 

Understanding the sensitivity of climate to carbon dioxide concentrations is a key point in the discussions of climate change. Scientists are trying to come up with better methods, with less uncertainty, which can help arrive at more accurate conclusions about our past climate. Hopefully, the stomata in fossil leaves can help in this effort. Another promised result from the meeting is an open source paleo-pCO2 database.

Thanks to the people in the plant bio journal club for pointing out this news.

  1. Hand, E. Fossil leaves bear witness to ancient carbon dioxide levels. Science 355, 14–15 (2017).
  2. Woodward, F. I. Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels. Nature 327, 617–618 (1987).
  3. Franks, P. J. et al. New constraints on atmospheric CO2 concentration for the Phanerozoic. Geophys. Res. Lett. 41, 4685–4694 (2014).

Desperately lacking tropical data

A version of this post has also been published as an online comment in PLoS Biology

In his recent meta-analysis, “Climate-Related Local Extinctions Are Already Widespread among Plant and Animal Species“, Wiens looked at changes in species ranges and local extinctions driven by climate change.

Wiens claims to have broad taxonomic and geographic coverage of studies.  Unfortunately, this is not the case. In fact, from the true tropics (i.e., excluding studies from the Santa Catalina Mountains of Arizona, USA, the Appalachian Mountains of north Georgia, USA, and the high eastern Himalayas that Wiens categorizes as “tropical” for the purposes of his analyses), only 5 studies representing a total of just 341 species (35% of species, 18% of studies) are included.  All but one of these tropical studies (of 55 Andean bird species) are from oceanic islands (Borneo [insects], New Guinea [birds], Madagascar [amphibians], and Hawaii [plants]).  All tropical plants are represented by just 4 grasses in Hawaii.

The lack of data from the tropics is not Wiens’ fault but rather reflects a true underlying disparity in the state of knowledge about different systems of the world. Simply put, we know much more about the effects of climate change in North America and Europe than we do the effects of climate change in the tropics.  That said, Wiens needs to be more forthright in acknowledging this disparity.  Furthermore, given this extreme lack of data, it is clearly premature to conclude that “there were significant effects of climatic region overall, with extinction more common in tropical regions” and that “this pattern of more frequent tropical extinction arose from a much lower frequency of extinctions for temperate plants”. Four grasses from Hawaii tell us next to nothing about how the thousands of tropical plants are responding to climate change.  Or even if we lump the tropics and subtropics together as does Wiens, 4 grasses from Hawaii, 27 mountain desert plants from Arizona and 124 high-elevation Himalayan plant species (all with ranges restricted to elevations >3500 m asl) provide little information about how the thousands of other tropical and subtropical plants are responding to climate change.  The tropical data void is real and it is troublesome (Feeley et al. 2016a,b).  But before we can begin to address this lack of data it needs to be acknowledged and recognized for the problem that it is.

 

Wiens JJ. 2016. Climate-Related Local Extinctions Are Already Widespread among Plant and Animal Species. PLOS Biology 14(12): e2001104. doi: 10.1371/journal.pbio.2001104

Feeley KJ, Stroud JT, and Perez TM. 2016. Most “global” reviews of species’ responses to climate change aren’t truly global. Diversity and Distributions. In Press.

Feeley KJ, Silman M, and Duque A. 2016. Where are the tropical plants? A call for better inclusion of tropical plants in studies investigating and predicting the impacts of climate change. Frontiers of Biogeography. 7(4). fb_27602.