Text by Linda-Liisa Veromann-Jürgenson and Tiina Tosens
We just published a paper about the plausibility of using cuticle thickness in gymnosperms as a proxy for leaf mass per dry area (LMA). It was as the result of a wonderful collaboration between six academic institutes from four countries. The paper titled “Predictability of leaf morphological traits for paleoecological reconstruction: the case of leaf cuticle and leaf dry mass per area” is one of the two papers representing our team in the International Journal of Plant Sciences special issue – Functional Trait Evolution.
The reasoning behind this paper was to test a paleoproxy for estimating LMA from cuticle thickness (CT) in broad-leaved gymnosperms, and expand it across different foliage types and through the light gradient. This LMA-CT paleoproxy is a very attractive concept for assessing past ecosystem properties as cuticles are much more likely to be preserved in fossils than mesophyll. At the same time LMA is connected to many traits underlying the leaf economics spectrum as well as to some growth conditions like CO2 concentration and light availability. Paleoproxies are indeed a great tool to reconstruct the past environmental and ecological conditions for the plant, whose minute piece paleobotanists are studying millions of years later. However, as large generalizations are made based on tiny tiny preserved plant bits, we must make sure the correlations hold across many species and in different conditions. Thus, we tested the LMA-CT relationship on 86 gymnosperm species with broad leaves, needles and scales and used a sub-set to study the effect of growth light conditions on CT as its effect on LMA has been previously well established.
Our results were promising! The proxy could be used for broad- and scale-leaved species, while the correlation does not hold for needles. Importantly, the reliability of the proxy increases for species at the lower end of the leaf economic spectrum (LES) – for species with tough robust leaves with high LMA – which is good considering that many of the so-called “living fossils” belong to that end of LES. However, we advise caution as taxonomy and light conditions affected the LMA-CT relationship, so just measuring CT from a diverse set of fossils may give you wrong results. Further tests distinguishing the morphotype of the fossilized leaf and the LMA-CT relationship in the nearest living relatives should be carried out. Nevertheless, CT on itself can give valuable information about the environmental conditions and stresses for the plant!
Full citation: Veromann-Jürgenson, L. L., Brodribb, T., Laanisto, L., Bruun-Lund, S., Niinemets, Ü., Nuño, S. L., Rinnan, R., Puglielli, G. & Tosens, T. (2019). Predictability of Leaf Morphological Traits for Paleoecological Reconstruction: The Case of Leaf Cuticle and Leaf Dry Mass per Area. International Journal of Plant Sciences, 181(1), https://doi.org/10.1086/706342 (link to full text)
Our power to predict the future relies on our knowledge of the past. Paleoproxies are a powerful tool for understanding environmental and ecological conditions, and changes across different time periods. However, constructing a functioning paleoproxy requires a well-constrained and robustly tested model. This is challenging, especially if ecological traits are involved. In the current study we constructed an extended dataset to test the reliability of the derivation of leaf dry mass per unit area (LMA) from the thickness of fossil gymnosperm cuticle. Specifically, we tested if different leaf types (broad leaves, needles, scales), intraspecific variability in cuticle thickness, and growing conditions affect the functioning of the proxy. Taxonomic groups were analyzed to uncover the possible taxonomic influence on LMA, cuticle thickness and the LMA-CT relationship. Our results indicate that the cuticle thickness versus LMA relationship depends on multiple factors that can have various and incongruous effects on this relationship, depending especially on leaf type and growing conditions. We conclude that cuticle thickness measured from gymnosperm fossils could be used as a proxy for LMA in past ecosystems for some broad- and scale-leaved, but not needle-leaved gymnosperms. However, caution must be taken when comparing species from different environments or growth conditions.