Environmental plasticity in total NSC stores unveiled when tree size is considered
While partitioning of stores between sugar and starch is plastic, we found total NSC stores to be the same, on average, between the two gardens. This is particularly interesting as the timing of leaf out and growth rates of trees significantly differ between the two sites. In Clatskanie, leaves come out earlier than Corvallis, potentially giving the Clatskanie trees a greater opportunity to produce carbon compared to the Corvallis trees(Evans et al., 2014). This carbon advantage is reflected in the diameters of trees at both sites, where trees in Clatskanie are almost double the size of trees in Corvallis (Figure 5). However, both sites average the same total NSC storage concentrations. Without the growth data, total NSC stores appear to have no environmentally driven plasticity, however with the growth data, the lack of difference between the two sites could actually be indicative of environmental plasticity in a tradeoff between the growth and storage. Trees growing in more variable or extreme conditions may “bet-hedge” by storing more NSC at the expense of other uses, such as growth (Sala, Woodruff, & Meinzer, 2012; Wiley & Helliker, 2012). This particular pattern has been shown in Arabidopsis, where plants favor storage over growth when photosynthetic productivity declines (Gibon et al., 2009; Smith & Stitt, 2007). Our results here suggest that this may be similar in the case in black cottonwoods, where genotypes grown in the continental garden (Corvallis), with larger temperature and precipitation extremes, maintain the same concentration of stores, but down-regulate growth to compensate. Thus, proportionally more energy is allocated to storage at the expense of growth in the more stressful environment.