Spatial distribution of leaf morphological and physiological characteristics in relation to local radiation regime within the canopies of 3-year-old Populus clones in coppice culture

Spatial distributions of leaf characteristics relevant to photosynthesis were compared within high-density coppice canopies of Populus spp. of contrasting genetic origin. We studied three clones representative of the range in growth potential, leaf morphology, coppice and canopy structure: Clone Hoogvorst (Hoo) (Populus trichocarpa Tom & Gray x Populus deltoides Bartr. & Marsh), Clone Fritzi Pauley (Fri) (Populus trichocarpa Tom & Gray) and Clone Wolterson (Wol) (Populus nigra L.). Leaf area index ranged from 2.7 (Fri and Wol) to 3.8 (Hoo). The clones exhibited large vertical variation in leaf area density (0.02-1.42 m(2) m(-3)). Leaf dry mass per unit leaf area (DMA) increased with increasing light in Clones Hoo and Fri, from about 56 g m(-2) at the bottom of the canopy to 162 g m(-2) at the top. In Clone Wol, DMA varied only from 65 to 100 g m(-2), with no consistent relationship with respect to light. Conversely, nitrogen concentration on a mass basis was nearly constant (around 1.3-2.1%) within the canopies of Clones Hoo and Fri, but increased strongly with light in Clone Wol, from 1.4% at the bottom of the canopy to 4.1% at the top. As a result, nitrogen per unit leaf area (N-A) increased with light in the canopies of all clones, from 0.9 g m(-2) at the bottom to 2.9 g m(-2) at the top. Although a single linear relationship described the dependence of maximum carboxylation rate (17-93 mumol CO2 m(-2) s(-1)) or electron transport capacity (45-186 mumol electrons m(-2) s(-1)) on N-A, for all clones, Clone Wol differed from Clones Hoo and Fri by exhibiting a higher dark respiration rate at low N-A (1.8 versus 0.8 mumol CO2 m(-2) s(-1)).