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

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Abstract
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 Torr. & Gray × Populus deltoides Bartr. & Marsh), Clone Fritzi Pauley (Fri) (Populus trichocarpa Torr. & 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 m2 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 (NA) 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 μmol CO2 m−2 s−1) or electron transport capacity (45–186 μmol electrons m−2 s−1) on NA, for all clones, Clone Wol differed from Clones Hoo and Fri by exhibiting a higher dark respiration rate at low NA (1.8 versus 0.8 μmol CO2 m−2 s−1).

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