Print PageHigh levels of nonstructural carbohydrates (NSC) in trees are often used as indicators of sink limitation to growth. This assumes that most storage is passive, but carbon may be stored actively, before growth is carbon saturated. In fact, trees could actively reduce growth in order to increase storage. If so, common patterns of increased NSC and reduced growth used to infer sink limitation are also consistent with carbon limitation. However, because we know little about what controls storage allocation, it remains unclear what NSC levels indicate about the nature of limitation. To investigate the effects of carbon limitation on the relationship between storage and growth, we conducted defoliation experiments with mature Quercus velutina in the New Jersey Pinelands and saplings in a common garden.
Results/Conclusions
Mature trees were completely defoliated in June 2010, and at the end of 2011, defoliated trees still had lower circumference growth and stem starch levels (percent dry weight). However, the increase in starch levels over the growing season (storage increment) was not significantly different between defoliated and undefoliated trees. Furthermore, there was a significant inverse relationship between May starch levels and storage increment. In the common garden experiment, half-defoliated saplings had reduced growth by several measures at one and four months following defoliation, but experienced no change in starch levels. Together, these results suggest that storage may become more important when carbon availability is low, causing trees to allocate relatively more carbon to storage at the expense of growth. We suggest that maintaining or even increasing storage under carbon-limiting conditions is an important adaptation for reducing the chance of carbon starvation, with the result that growth becomes even more carbon limited. If so, increased NSC and lower growth rates may be more indicative of carbon limitation than of sink limitation.
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