Shoot structure, canopy openness, and light interception in Norway spruce
We analysed changes in shoot structure with canopy openness in control (C) and irrigated–fertilized (IL) Picea abies trees. Canopy openness, which was used to characterize light availability, varied between 0·23 and 0·90 for C shoots and 0·07 to 0·75 for IL shoots. Needle width and thickness increased with canopy openness, but were similar for C and IL needles. The ratio of needle thickness to width remained fairly close to one throughout the canopy. This resulted in large values (3·0–4·0) of the ratio of total to projected needle area. The ratio of total to projected needle area did not correlate with canopy openness. Shoot silhouette to total needle area ratio (STAR) and specific needle area decreased with canopy openness. As a result, shoot silhouette area per unit needle mass was up to three times larger in shade shoots than in sun shoots. STAR and specific needle area were similar for C and IL shoots and needles. Needle nitrogen concentration varied between 0·7 and 1·1% (C), and between 1·0 and 1·6% (IL). On both plots, nitrogen content was linearly related to estimated relative light interceptance (RLI) (both expressed per needle area), but the ratio of nitrogen content to RLI increased with decreasing canopy openness.