A simple economic analogy for growth in plants requires that the supply of compounds necessary for growth meet the demands of biosynthesis and maintenance. Recent investigations into size-related growth declines in trees have found evidence for hydraulic limitations to height growth. Many of these studies have assumed that lower carbon availability as a consequence of reduced hydraulic supply has caused the reduction in height growth, but the latter assumption has not been fully tested. We measured non-structural carbohydrate pools (NSC, sugars plus starches) in leaves and stems of Sequoia sempervirens trees in three height classes (30m, 70m, and 110m) and across two seasons (autumn and spring) to test the hypothesis that carbon supply to the treetop decreases with height.
Results/Conclusions

Leaf starch and soluble sugar levels ranged from less than 0.1 to 6% and 8 to 13% dry mass, respectively, while stem starch and sugar levels ranged from 4 to 9% and 3 to 10% across both seasons. We found that NSC pools do change with height and that patterns across trees of different heights vary by season.  In autumn, 30m treetops had lower NSC levels than 70m trees (P = 0.07) but they and 70m trees did not differ from 110m trees. Treetop NSC in the 30m and 70m trees increased from autumn to spring, primarily due to increased starch levels, but did not change in 110m trees (30m, P= 0.007; 70m, P = 0.03; 110m, P = 0.70).  As a result, treetop NSC values in spring were higher in 30m and 70m trees than 110m trees (30m v 110m, P = 0.06; 30m v 70m, P = 0.03; 30m v 70m,, P = 0.70;). Treetop stem NSC pools followed the same trends as leaves but were generally non-significant. These results provide support for the hypothesis that carbon supply to the treetop decreases with tree height, particularly in the spring when growth demand for carbohydrates is high.