Sink limitation in trees is often inferred from the size of the nonstructural carbohydrate pool, based on the assumption that storage is a passive process that does not compete with other sinks within the plant for carbon. If carbon is actively partitioned to growth, then higher concentrations of carbohydrates may not be an indicator of sink limitation. Furthermore, if storage is active, trees may be able to acclimate to carbon-limiting conditions or disturbances by storing more at the expense of growth. To test these ideas, we hand-defoliated six adult trees of Quercus velutina in the New Jersey Pinelands. Defoliated trees were paired with controls of similar size, and circumference growth and starch concentrations were measured throughout the growing season. Wood starch content was measured from tree cores, and results are reported in percent dry weight.
Results/Conclusions
As expected, both growth during the remainder of the growing season and starch concentrations at the end of the season were significantly lower for defoliated trees. Circumference growth was positively correlated with the increase in starch content, which is consistent with active partitioning of carbon to storage at the expense of additional growth. Defoliated trees displayed a smaller increment of growth per increment starch content, but this was not significant. The results are consistent with the idea that carbon-limiting events may lead to a shift in allocation, favoring reserves at the expense of growth. We conclude that tree growth may remain carbon limited when starch content is relatively high, even in nutrient poor sites like the Pine Barrens. Furthermore, the ability of trees to actively maintain reserves, along with the possibility that reserve allocation is plastic, complicates the use of NSC as indicator of sink limitation.