Carbon allocation in trees is an important determinant of growth and survival, but it remains poorly understood. One prominent theory predicts that allocation priority is determined by proximity to the canopy, with closer tissues receiving greater access to recent photosynthates. Such a top-down strategy could cause roots to disproportionately suffer C shortages, with implications for how trees—particularly tall trees—experience stress. We tested this theory in tall (~22 m) and short (~12 m) aspen stands in Alberta during 2014, which suffer from both gradual, age-dependent and sudden, stress-induced declines that are hypothesized to affect the roots before the canopy. We measured root growth and nonstructural carbohydrates (NSC) in the canopy, at multiple locations down the bole, and in the roots. If allocation is top-down, we predicted that 1) tissues farther from the canopy would have lower NSC concentrations early in the growing season and take longer to reach their seasonal NSC maxima, 2) in tall stands—with the potential for greater carbon demand from their longer boles— it would take longer for root NSC to reach seasonal maxima and for root growth to begin, and 3) root growth and NSC would be lower in tall stands.
NSC concentrations peaked earlier within the canopy than below the canopy, but there was no difference in the timing of seasonal NSC maxima in phloem tissue along the bole. However, sapwood NSC increased with height along the bole throughout the growing season. NSC was also consistently higher in phloem tissue within the canopy than below the canopy. Short stands generally had higher NSC levels aboveground than tall stands. In contrast, tall stands reached maximum fine root NSC earlier in the growing season and had higher coarse root NSC concentrations throughout the year. There was no difference in the timing or amount of root growth between stands.
We found only limited evidence for top-down allocation. Root systems of tall stands do not appear to be more C-limited, despite the potential for longer boles to act as large C sinks. Changes in allocation that cause earlier budset and reduce NSC stem storage in taller trees may ensure an adequate C supply to the root systems.