Forest carbon storage in the upper Great Lakes displays considerable interannual variability, which is largely climate dependant. Annual net ecosystem production (NEP) at the University of Michigan Biological Station (UMBS) in northern lower Michigan varied over 100 % from 1999-2003 (Gough et al., 2007 b). Meteorological and ecological measurements of NEP show that forest growth lags behind canopy photosynthesis, indicating that late season C uptake is stored over winter and applied to spring growth. The objective of this study was to quantify the relationship between late season carbohydrate storage and growth the following year. We tracked seasonal changes of non-structural carbohydrates (starch and soluble sugars) in leaves, twigs, stems, and coarse roots of the dominant canopy species bigtooth aspen (Populus grandidentata Michx.) and northern red oak (Quercus rubra L.) in 2005 and 2006.
Seasonal variation of storage carbohydrates in both species was related to canopy photosynthesis, reproductive events, and leaf phenology. In 2005, we observed an inverse correlation between spring twig starch and soluble sugar concentrations in Q. rubra, suggesting a conversion of starch to soluble sugars. During leaf out, Q. rubra experienced a >50 % and >20 % depletion of starch in coarse roots and stems, respectively. It was this early season reallocation of C stored in reserve organs that fueled spring growth, leaf production, and acorn swelling.
Likewise, in 2005, P. grandidentata exhibited an inverse correlation between stem starch concentrations and twig soluble sugar and starch concentrations, suggesting a reallocation to spring growth and leaf production. In 2006, P. grandidentata root bark exhibited a strong depletion of soluble sugars during the period of biomass growth. Furthermore, in both 2005 and 2006, starch concentrations in P. grandidentata root bark were significantly lower prior to the growing season compared to after the initiation of growth suggesting a seasonal reloading during the photosynthetic period. A gradual accumulation of daytime soluble sugars in leaf tissue of Q. rubra and P. grandidentata over the growing season also was observed. Furthermore, interannual variation in tree growth appears correlated with non-structural carbohydrate concentrations at the end of the previous growing season.