Diversity of leaf architecture and its relationships with climate in extant and fossil plants

In this contribution, the diversity of functional leaf architecture and its association with climate is studied for extant woody dicot species of North America and Europe/South West Asia. The results are compared to fossil leaves of seventeen locations in Central Europe ranging from the Paleogene to Neogene. Four functional leaf traits, lobation, margin, type of primary and secondary venation, were selected as defining sixteen basic functional architectures (Trait Combination Types, TCT). Diversity of leaf architecture was studied as Shannon diversity (meaning frequency and evenness of TCT occurrence) as realized by site-specific species composition. The extant data indicate significant positive correlations of Shannon diversity of leaf architecture with temperature particularly for North America. After having reached a peak at about 18 degrees C MAT, functional diversity of leaf architecture then declines in North America with rising MAT. This decline occurs mostly for drier sites, probably because humidity becomes more influential at higher MAT, with low water availability driving selective pressure towards adaptive convergence. The data indicate that warm environments are able to accommodate a higher variety of leaf types than colder environments, unless water supply is scarce. The fossil data essentially agree with the recent data, with the exception of sites harbouring fossil leaves preserved in marine sediments. Here, the diversity of leaf architecture is lower which is probably due to taphonomic bias. Highest leaf architecture diversity was found for the Pliocene sites, although no distinct relationship with stratigraphic age could be identified. The results of this study indicate that diversity of leaf architecture may be a useful source of information for palaeoecology and palaeoclimate and merits further study.