Spatial distribution of leaf dry weight per area and leaf nitrogen concentration in relation to local radiation regime within an isolated tree crown.

摘要

To assess the spatial distribution of photosynthetic capacity within an isolated 20-year-old walnut tree (Juglans regia) crown, the distribution of relevant leaf characteristics was measured in field studies in a timber walnut orchard at Plauzat, France in summer 1996. Variations in leaf dry weight per area (Wa), and nitrogen concentration on a weight (Nw) and area basis (Na), were studied in relation to solar irradiance along one vertical and two horizontal transects, on July 30 and September 3. In addition, the concentrations of total nonstructural carbohydrates on a weight (TNCw) and area basis (TNCa) were measured on July 30. Concurrently, the spatial distribution of daily integrated leaf irradiance within the crown was simulated by a three-dimensional radiation transfer model over a one-week period before sampling at each date. Within the crown, high spatial heterogeneity was observed for Wa (from 50 to 140 g m-2), TNCa (from 4 to 17 g m-2) and Na (from 1.2 to 3.6 g m-2). Although TNCw and Nw were not correlated and only weakly correlated with daily leaf irradiance, respectively, Wa, TNCa and Na were strongly correlated with daily leaf irradiance. The relationship between observed Na and simulated daily leaf irradiance was used to assess the spatial distribution of Na within the crown at each date. Total leaf nitrogen in the foliage was estimated to be 339 g in late July and 317 g in early September. For the whole crown (i.e., 1729 current-year shoots), Na increased strongly with basal shoot diameter (an index of shoot vigour), highlighting the fact that large shoots were mainly located in sunlit locations and exhibited high photosynthetic capacity.