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.