Towards a better understanding of nonstructural carbohydrate storage and carbon limitation in trees

Nonstructural carbohydrates (NSC) are an important component of the carbon storage pool in trees and commonly used to assess whether tree growth under various conditions is carbon limited. However, we know very little about what causes variation in NSC levels and allocation among trees. While NSCs may accumulate passively when growth is carbon saturated, their storage may be active, competing with growth for carbon. If storage is mostly an active process, trees may maintain high NSC levels at the expense of potential growth, making NSC levels poor indicators of carbon saturated growth. To determine how growth and NSC allocation are related, we compared variation in the seasonal changes in NSC levels (NSC increment) with variation in radial growth in mature, canopy black oaks in the New Jersey Pine Barrens. Using experimental defoliation of saplings in a common garden and mature trees in the field, we also explored whether allocation shifts that favor storage reduce growth. Growth and NSC increment were positively correlated, indicating that storage was active, not the result of carbon-saturated growth. Defoliation reduced sapling growth, but increased the proportion of biomass allocated to NSCs. In adults, defoliation reduced growth for three years. However, while aboveground NSC levels may not have completely recovered by the end of the third year, belowground levels recovered after two years due to greater NSC allocation. Defoliation also changed the relationship between NSC increment and growth in the two years following defoliation, causing a shift in allocation that favored storage. The positive relationship between growth and NSC increment indicated that the allocation shift was not due to sink limitation. We conclude that storage is a high priority in these trees. In addition, carbon limiting events like defoliation can greatly alter carbon allocation by increasing the priority of storage, potentially in response to greater threats of carbon starvation. Our results indicate that carbon limitation to growth may occur not only because of decreased carbon availability but because of allocation shifts that reduce the priority of growth relative to other processes