The ratio of leaf dry mass to leaf area (LMA) is a key trait for understanding the leaf economic strategies of plant species, but how can we estimate LMA for species long since gone extinct? Given very nicely preserved fossil leaves, one method is to directly measure the leaf area, and then divide it by an estimated value for leaf dry mass. Back in 2007 we (led by Dana Royer) showed that this estimated value can be calculated, using biomechanical principles, from the width of the petiole – the structure that physically supports the leaf blade.
In a new paper (led by postdoc Wuu Kuang Soh) we have revisited this question of estimating LMA from fossils, this time showing that LMA can be directly estimated from measurements of leaf cuticle thickness. Cuticles fossilize beautifully, for example leaf stomata may be exquisitely preserved. In this work we first showed that cuticle thickness (CT) is a good predictor of LMA across a variety of broad-leaved gymnosperm species (many growing in the Royal Botanic Gardens here in Sydney).
Next, Jenny McElwain (Dublin) sent out a set of fossil leaves from a well-studied sequence in Greenland that includes fossil beds either side of the Triassic-Jurassic boundary (a mass extinction event that occurred approx. 200 million years ago). We developed a method for carefully cutting cross-sections of these fossils, then predicted the LMA of each leaf using the predictive equation derived for living species.
Because Jenny and collaborators had already quantified the relative abundance of the various fossil species, we were able to estimate community-weighted mean LMA values, and so ask whether there were clear shifts in plant ecological strategies across the T-J boundary. In short, it looks like the extreme climatic conditions of the T-J boundary selectively knocked out low-LMA species, species that presumably had faster metabolic rates. The Early Jurassic plant communities were not only made up of completely different species to those from the Late Triassic, they also had more stress-tolerant, “conservative” leaf economic strategies.
Royer DL, Sack L, Wilf P, Lusk CH, Jordan GJ, Niinemets U, Wright IJ, Westoby M, Cariglino B, Coley PD, Cutter AD, Johnson KR, Labandeira CC, Moles AT, Palmer MB & Valladares F (2007). Fossil leaf economics quantified: calibration, Eocene case study, and implications. Paleobiology 33:574-589.
Soh WK, Wright IJ, Bacon KL, Lenz TI, Steinthorsdottir M, Parnell AC, McElwain JC (2017). Palaeo leaf economics reveal a shift in ecosystem function associated with the end-Triassic mass extinction event. Nature Plants 3:17104.