Anatomical basis of the change in leaf mass per area and nitrogen investment with relative irradiance within the canopy of eight temperate tree species

Changes in leaf mass per area (LMA), nitrogen content on a mass-basis (N-m) and on an area basis (N-a) with relative irradiance were assessed in leaves of eight temperate species harvested at different depths in a canopy. Relative irradiance (GSF) at the points of leaf sampling was estimated by hemispheric photographs. There was a strong species-dependent positive relationship between LMA and GSF for all species. Shade-tolerant species such as Fagus sylvatica showed lower LMA for the same GSF than less tolerant species as Quercus pyrenaica or Quercus petraea. The only evergreen species in the study, Ilex aquifollium, had the highest LMA, independent of light environment, with minimum values much higher than the rest of the broad-leaved species studied. There was no relation between N and GSF for most species studied and only a very weak relation for the relative shade-intolerant species Q. pyrenaica. Within each species, the pattern of Na investment with regard to GSF was linked mainly to LMA. At the same relative irradiance, differences in N. among species were conditioned both by the LMA-GSF relationship and by the species N value. The lowest N-m value was measured in I. aquifollium (14.3 +/- 0.6 mg g(-1)); intermediate values in Crataegus monogyna (16.9 +/- 0.6 mg g(-1)) and Prunus avium (19.1 +/- 0.6 mg g(-1)) and higher values, all in a narrow range (21.3 +/- 0.6 to 23 +/- 0.6 mg g(-1)), were measured for the other five species. Changes in LMA with the relative irradiance were linked both to lamina thickness (LT) and to palisade/spongy parenchyma ratio (PP/SP). In the second case, the LMA changes may be related to an increase in lamina density as palisade parenchyma involves higher cell packing than spongy parenchyma. However, since PP/SP ratio showed a weak species-specific relationship with LMA, the increase in LT should be the main cause of LMA variation. (C) 2004 Elsevier SAS. All rights reserved.