Leaf traits: A bridge between modern and fossil plant extinctions

Globally, 45% of flowering plant species are estimated to be threatened with extinction. Many factors are responsible for this figure, including climate change, that are pushing plant species towards their survival limits and increasing the likelihood of a sixth mass extinction event. Five mass extinction events have been identified in the fossil record that resulted in the collapse and reordering of entire ecosystems. However, it is thought that plants do not experience mass extinction on the same scale as animals due to the lack of observed extinctions across mass extinction boundaries. Plant extinction, both modern and in the fossil record, will be explored by studying leaf traits, which include the macromorphological, stomatal and chemical characteristics of a plant’s leaf. Although there is no consistent response, some fossil plant groups show shifts in leaf traits associated with plant survival across mass extinction events in the fossil record. However, only a tiny percentage of the diversity of plants has been preserved in the fossil record and understanding the taphonomic influences that filter which leaves become preserved in the fossil record is essential to interpreting the plant fossil record. The leaf traits of two modern floras, the Irish angiosperm flora and the Brazilian Fabaceae, were measured using herbarium specimens to assess the relationship between their leaf traits and extinction risk. Extinction risk was based on the IUCN Red List. Five species were analysed in a mesocosm study to assess the effect of the leaf chemical profile on decomposition across plant functional groups. The leaf traits of the Spitsbergen Cenozoic fossil flora were measured, along with a quantitative analysis of leaf traits observed in the fossil record literature, to examine the response of leaf traits in the fossil record. Results from the Irish angiosperm flora and the Brazilian Fabaceae showed no correlation between the leaf traits studied and extinction risk. Analysis of the chemical profile of different functional groups indicated that woody angiosperms and gymnosperms contain chemical compounds more resistant to decay than herbaceous angiosperms. This result was largely consistent when analysed across decomposition time and environment. The leaf traits preserved in the Spitsbergen Cenozoic fossil flora suggested a strong influence of life form on the leaf traits preserved. The new trait ‘Taphonomic Completeness Score’ indicated a poorer quality of fossil preservation for the herbaceous angiosperms compared to the woody angiosperms and gymnosperms. The quantitative analysis of leaf traits in the fossil record literature showed a lack of response of leaf size across mass extinction boundaries. The lack of a relationship between leaf traits and extinction risk in the modern floras suggests that climate change is not the primary driver of modern plant extinction. Likely, climate change acts in synergy with other drivers of extinction to push plants closer to the brink of extinction. Through preservation experiments and studies of the fossil record, it appears that herbaceous species possess a set of leaf traits, morphological and chemical, that make them less likely to be preserved in the fossil record. This bias against herbaceous species in the fossil record is important to interpretations of the fossil record as a high proportion of living herbaceous species are threatened with extinction. If this trend continued throughout Earth's history, the impact of mass extinctions on n herbaceous species may be underestimated.

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University of Galway

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Attribution-NonCommercial-NoDerivatives 4.0 International

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University of Galway Theses (PhD Theses)