EVOLUTIONARY COMPARATIVE ANALYSES OF THE RELATIONSHIP BETWEEN LEAF STRUCTURE AND FUNCTION

We have applied evolutionary comparative methods to control for phylog enetic relationships between species in order to determine if three re cently proposed relationships between leaf structure and function are upheld, using a large database comprising a wide range of species from tropical and temperate ecosystems. The three hypotheses tested were: (i) leaf thickness is positively correlated with total stomatal densit y (adaxial and abaxial values summed); (ii) amphistomatous leaves (sto mata present on the upper and lower surfaces) have a higher maximum st omatal conductance than hypostomatous (stomata on the lower surface on ly) leaves, and (iii) changes in the stomatal density on upper and low er leaf surfaces are regulated in a compensatory manner. The results s howed that, contrary to several mathematical modelling studies, thicke r leaves were not associated with more stomata either in species from lowland tropical rain forests or from central Europe. Amphistomatous l eaves had a higher maximum stomatal conductance, indicating that one a spect of previous modelling work is correctly underpinned after accoun ting for relatedness. Finally, we found no evidence that the stomatal densities on upper and lower leaf surfaces are closely regulated. Thes e three physiological traits are discussed with reference both to the modelling of leaf gas exchange and to plant function in relation to mi croclimate.