Leaf epidermal cell size in trees: developmental possibilities stay wide with height growth in two angiosperm tree species.

Tree height varies across environments, with taller individuals found in cool, moist habitats and shorter trees in drier regions. Within species, trees can exhibit height variation due to environmental factors such as drought-induced dieback. A key question is what drives changes in leaf structure with increasing height-whether some trait values cannot be produced under the developmental conditions at treetops or whether differences arise because natural selection favors particular trait values at different canopy positions. Some hypotheses suggest that increasing height imposes "limits" on mature leaf traits, making some structural changes developmentally inevitable. However, selection could also favor structural changes within wide fields of developmentally possible trait configurations. We examined leaf epidermal cell size distributions in Bursera simaruba and Eucalyptus camaldulensis from seedlings to maximum tree heights in situations in which seedlings to adults were all exposed to full sun and thus had all "sun" leaves. We found that in general cell sizes increased, variance remained high, and distributions did not systematically shift with height. These results indicate that, rather than reflecting a developmental inability to produce certain leaf epidermal cell sizes at greater heights, the patterns we observed are better explained by selection simply favoring some cell sizes from among the many that development can produce.