Are small leaves somehow related to the phenomenon of ‘sclerophylly’ (tough, thickened leaves that seem to be adapted to dry, hot areas of Australia with poor soils)? Anecdotally this seems to be the case – as you head inland (where conditions would favour sclerophylly) you tend to get a predominance, but not an exclusivity, of smaller leaved plants.
Wade Tozer from Macquarie University spoke today at the Royal Botanic Gardens about his work (with Mark Westoby from the university, and Peter Weston and Darren Crayn from the Botanic Gardens Trust, the later now in Cairns) on this subject.
The short answer is possibly not. Wade devised a clever test using pairs of closely related species (based on their evolutionary trees) where one species had significantly wider leaves than the other. Using 25 of these pairs he looked for correlations with temperature, rainfall and soil, as well as various plant structural features. What he found was no proof of any relationship between leaf size and sclerophylly, or leaf size and the things that seem to encourage sclerophylly.
This doesn’t mean an interesting relationship doesn’t exist – it might be obscured by other patterns or the analysis might not be robust enough – but it certainly can’t be taken for granted.