By Eric Atkins
There are many techniques that can be used to offer insight to the past–a family of proxies that range from gas found in ice cores to fossilized feces. I have always been interested in fossils. The allure of something so ancient was introduced to me as a child. At the age of eight, I found a veiny imprint of a leaf in a chunk of sandstone. Fossils are a great tool for looking into the past. For nearly a century by paleoclimatologists and paleoecologists have been using leaf physiognomy as a proxy to deduce past climate environments. By looking at leaf mass per area, teeth area, number of teeth, and degree of blade dissection we can extrapolate information about past climates.
In recent years this type of proxy has evolved along with technology. Before the use of digital leaf physiognomy, paleoecologists used marginal leaf analysis. Marginal leaf analysis is the process of comparing the amount of smoothed edged leaves to the amount of toothed edged leaves. This ratio is then put into a linear function compared to temperature. With this technique you find that colder tree species produce teethed leaves and warmer species produce smooth leaves. The teethed leaves are able to photosynthesis sooner in spring than smooth leaves, however the drawback is teethed leaves are more prone to desiccation.
The technique that follows marginal leaf analysis was climate leaf analysis multivariate program or “CLAMP.” CLAMP uses 31 leaf characteristics including leaf marginal analysis to deduce mean average temperature. CLAMP primarily looks at compound, lobing, teeth, size, apex, base, length:wide ratio, and shape. Despite all of these data CLAMP produces mean average temperature that’s very similar to the results of marginal leaf analysis. This is due to some of the CLAMP characteristics being more qualitative rather than being quantitative. Digital leaf physiognomy overcomes these challenges with computer image analysis.
When it comes to proxies I believe the data becomes more significant with the more options you have. Paleoclimate has no shortage when it comes to usable proxies. Unfortunately digital leaf physiognomy, as a proxy, is limited to the fossil record, and fossils are rare. Test sites are limited to the are where the fossil is found so data on a regional scale is sporadic. However I do like the idea that as time progresses fossils are becoming more and more useful for discovering the past. I wonder what secrets my fossil holds.
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Royer DL, Meyerson LA, Robertson KM, Adams JM (2009) Phenotypic Plasticity of Leaf Shape along a Temperature Gradient in Acer rubrum. PLoS ONE 4(10): e7653. doi:10.1371/journal.pone.0007653
Wright, Ian J.. “Sensitivity of leaf size and shape to climate: global patterns and paleoclimatic applications.” New Phytologist (): 724739.
Uhl, Dieter, CHRISTOFER traiser, URSEL griesser, and THOMAS denk. “Fossil leaves as palaeoclimate proxies in the Palaeogene of Spitsbergen (Svalbard). Paleobotanica 47: 89107.