Nonstructural carbon age and lateral mixing in the stem wood of tropical trees

Trees store most of the nonstructural carbon (NSC, mainly sugars and starch) in the stem wood to support metabolism and growth. For many species, NSC concentration decreases radially with sapwood depth. Spatial distribution of NCS also varies with wood anatomical traits, as NSC is dispersed in the wood living-fibers or concentrated  in parenchyma cells. These distributions may  influence temporal changes in NSC and are related to mortality and growth. Observed relations between NSC storage and wood traits thus are associated with differences in the time scales of NSC cycling in  trees and in the ability of trees to survive and recover from stressful conditions such as drought or mechanical damage.  

Here we focus on the following questions: i) what are the radial mobilization rates of NSC across the sapwood for tropical trees with different NSC spatial distributions? and ii) how old is the NSC stored in these trees and the NSC that they access for respiration and wood growth?

To answer these questions we measured NSC content through a radial path in the sapwood (from bark to pith) in eight tropical tree species, four fiber and four parenchyma-storing species, in a seasonal dry forest in Brazil during 2019. We measured the ¹⁴C in the soluble carbon extracted from wood segments corresponding to two depth ranges of each radial path (0-2cm and 2-4cm) and in the respired CO₂ from 6cm wood core segments. We estimated the age of the wood by counting tree rings and measuring the ¹⁴C in the cellulose.  

We found significant seasonal changes in the starch content at different sapwood depths in all trees evaluated, indicating that NSC is metabolically active across all depths where starch is stored. Radiocarbon data indicate that fiber-storing trees retained NSC in the wood for decades. In some cases NSC was even older than the wood that contained it, indicating the mixing of old NSC coming from deeper layers of wood. Irrespective of the stored NSC ages, trees always used younger NSC for respiration, indicating that our water extraction includes both reserves being used for metabolism and older C that may be less available at the moment of sampling. However, trees accessed older NSC when they faced stressful conditions, e.g. when in negative C balance, requiring a larger contribution of the old stored NSC to support respiration. Thus, tree species with low mortality and slow growth such as fiber-storing species may remobilize older NSC from deeper layers of wood to survive stressful conditions for longer time than parenchyma-storing species.

These findings highlight the diversity of NSC storage and remobilization strategies in tropical trees. These strategies have important implications for our understanding not only of how trees will respond to future climatic changes but also about the mechanism of carbon cycling in tropical trees and ecosystems.