Structured Interspecific Determinants of Specific Leaf Area in 34 Species of Herbaceous Angiosperms

1. Specific leaf area is a variable implicated in a number of functional aspects of plant ecology, including gas exchange, relative growth rate and palatability. 2. Six leaf attributes (water content, lamina thickness, protruding vein thickness, dry weight, lamina area and specific leaf area) were determined on a collection of 572 leaves. The data set was derived from 194 individual plants, 34 species, 32 genera, 18 families and both classes of angiosperms. All species except one (a shrub) were herbaceous. All species were from sunny habitats. 3. Structured relationships between these six variables were obtained. These relationships consisted of path analyses, multiple regressions and major axis (allometric) regressions. 4. There was an interspecific allometric trend for leaf dry weight to increase more rapidly than leaf surface area, resulting in larger leaves having a lower specific leaf area. This trend did not exist at an intraspecific level. 5. Path analysis showed that the most important direct effect on specific leaf area was the water content of the leaf. Increasing water content resulted in a larger specific leaf area. The direct effect of lamina thickness was to decrease specific leaf area but lamina thickness itself was affected by leaf water content. 6. These effects on specific leaf area were mediated through variation in leaf dry weight. Leaf surface area appeared to be independent of the other five variables. 7. These results help to explain two unresolved general patterns in plant comparative ecology: (1) why net photosynthetic rate is not positively correlated with maximum relative growth rates and (2) why there is a negative correlation between maximum relative growth rate and seed size