Photosynthesis, Leaf Conductance, and Specific Leaf Weight in Long and Short Shoots of Populus 'Tristis #1' Grown Under Intensive Culture

Differences between second-order, midcrown, long and short shoots in photosynthetic rate, leaf conductance, and specific leaf weight were evaluated for a 4-year-old plantation of hybrid clone Populus 'Tristis #1' ( P. balsamifera X P. tristis ; NC 5260) grown under short rotation intensive culture (SRIC) in northern Wisconsin. Photosynthetic rates ( 14 CO 2 method) and abaxial leaf conductance were measured in situ for all leaves on short shoots and for tip, midshoot, and basal leaves on long shoots, at midday in July and August under natural diffuse light, sunflecks excluded (5-21 μE m -2 s -1 ; photosynthetic photon flux density) and at full insolation (800-1,850 μE m -2 s -1 photosynthetic photon flux density). Specific leaf weight of the same leaves was determined on the same day. Short-shoot photosynthetic rates per dm² and per g averaged 71 and 82 percent, respectively, of long-shoot rates. Long-shoot leaves also had higher specific leaf weights than short-shoot leaves. Differences in abaxial leaf conductance between long and short shoots were small and not significant. The lower average age of long-shoot leaves is probably an important reason for the greater photosynthetic rates of the long shoots. These results suggest that long and short shoots should be treated as separate populations when modelling net assimilation in SRIC poplars. Forest Sci. 28:737-744.