Temporal variation in storage of nonstructural carbohydrates along a height gradient in Douglas-fir trees

Background/Question/Methods

The accumulation of nonstructural carbohydrates (NSCs) is used to evaluate the balance of carbon sources and sinks in plants and as an indicator of the extent to which growth is limited by photosynthesis vs. other physiological processes governing sink strength. Height above the ground influences the water relations of plant tissues in such a way as to function as a surrogate for an aridity gradient. As such, assessing NSCs along a height gradient provides insight into how changes in climate conditions may influence source-sink relationships and overall growth in trees. Relationships between height, storage of NSC and shoot development provide insight into the role that photosynthesis plays in constraining growth under conditions of water stress. We analyzed concentrations of starch, sucrose, glucose and fructose in branch wood, foliage and stem sapwood of Douglas-fir along a height gradient of ~ 2 to ~ 55 m. NSC analyses were conducted on samples collected during different phenological stages over the course of 17 months in 2009 and 2010. Shoot extension, mid-day shoot water potential, and shoot osmotic potential were also measured.

Results/Conclusions

Mean concentrations of NSC were highest in the tallest height class and lowest in the lowest height class, and height-related trends in NSC were most pronounced in branch wood. Fluctuations in branch NSC followed a height-related trend such that the mean variation in branch NSC throughout the 17 month sampling period was 78%, 68%, 64% and 47% in the 2, 18, 35 and 55 m height classes, respectively. Branch NSC increased with height-related declines in midday leaf water potential (r²= 0.67; p = 0.0001), shoot osmotic potential (r²= 0.92; p < 0.000001) and shoot extension (r²= 0.77; p = 0.0004). Temporal fluctuations in branch NSC also declined with decreasing midday leaf water potential (r²= 0.49; p = 0.012), shoot osmotic potential (r²= 0.76; p = 0.0002) and shoot extension (r²= 0.65; p = 0.0026). The lack of direct evidence for carbon availability limiting growth under conditions of moderate water stress suggests the role of other mechanisms not directly related to photosynthesis. Constraints on turgor-driven cell expansion and mobilization of photosynthate represent alternative mechanisms for reduced growth that are directly related to water stress.