Biomass investment in leaf lamina versus lamina support in relation to growth irradiance and leaf size in temperate deciduous trees

Foliar biomass investment in support and assimilative compartments was stud ied in four temperate deciduous tree species along a natural light gradient across the canopy. The species ranked according to shade tolerance as Betu la pendula Roth. < Populus tremula L. < Fraxinus excelsior L. < Tilia corda ta Mill. Long-term light conditions at sampling locations were characterize d as seasonal mean integrated quantum flux density (Q(int), mol m(-2) day(- 1)) estimated by a method combining hemispherical photography and light mea surements with quantum sensors. Leaf morphology was altered by Q(int) in al l species. Both lamina and petiole dry mass per lamina area (LMA and PMA, r espectively) increased with increasing Q(int). Shade-tolerant species had l ower LMA at low Q(int) than shade-intolerant species; however, PMA was not related to shade tolerance. Across species, the ratio of petiole dry mass t o lamina dry mass (PMR) varied from 0.07 to 0.21. It was independent of Q(i nt) in the simple-leaved species, but decreased with increasing Q(int) in t he compound-leaved F. excelsior, which also had the largest foliar biomass investment in petioles. Differences in leaf mass and area, ranging over fou r orders of magnitude, provided an explanation for the interspecific variab ility in PMR. Species with large leaves also had greater biomass investment s in foliar support than species with smaller leaves. This relationship was similar for both simple- and compound-leaved species. There was a negative relationship between PMR and petiole N concentration, suggesting that petioles had greater carbon assimilation rates and paid bac k a larger fraction of their construction cost in species with low PMR than in species with high PMR. This was probably the result of a negative relat ionship between PMR and petiole surface to volume ratio. Nevertheless, peti oles had lower concentrations of mineral nutrients than laminas. Across spe cies, the ratio of petiole N to lamina N varied from only 3 to 6%, demonstr ating that petiole costs are less in terms of nutrients than in terms of to tal biomass, and that the petiole contribution to carbon assimilation is di sproportionately lower than that of the lamina contribution.