Carbon dynamics in trees: feast or famine?

Abstract

Research on the degree to which carbon (C) availability limits growth in trees, as well as recent trends in climate change and concurrent increases in drought-related tree mortality, have led to a renewed focus on the physiological mechanisms associated with tree growth responses to current and future climate. This has led to some dispute over the role of stored non-structural C compounds as indicators of a tree's current demands for photosynthate. Much of the uncertainty surrounding this issue could be resolved by developing a better understanding of the potential functions of non-structural C stored within trees. In addition to functioning as a buffer to reconcile temporal asynchrony between C demand and supply, the storage of non-structural C compounds may be under greater regulation than commonly recognized. We propose that in the face of environmental stochasticity, large, long-lived trees may require larger C investments in storage pools as safety margins than previously recognized, and that an important function of these pools may be to maintain hydraulic transport, particularly during episodes of severe stress. If so, survival and long-term growth in trees remain a function of C availability. Given that drought, freeze-thaw events and increasing tree height all impose additional constraints on vascular transport, the common trend of an increase in non-structural carbohydrate concentrations with tree size, drought or cold is consistent with our hypothesis. If the regulated maintenance of relatively large constitutive stored C pools in trees serves to maintain hydraulic integrity, then the minimum thresholds are expected to vary depending on the specific tissues, species, environment, growth form and habit. Much research is needed to elucidate the extent to which allocation of C to storage in trees is a passive vs. an active process, the specific functions of stored C pools, and the factors that drive active C allocation to storage.

Effect of Tree Spacing on Tree Level Volume Growth, Morphology, and Wood Properties in a 25-Year-Old Pinus banksiana Plantation in the Boreal Forest of Quebec

“…The longer living crown increased the overall needle biomass and global photosynthesis rate of the individual tree, thereby producing more assimilates during the growing season [29]. Assimilates can be stored during non-photosynthetic periods and used for tree growth during the vegetation period and this can explain the tree volume increment [57]. A positive relationship between stem and branch diameter was obtained to support this statement.…”

Consistent limitation of growth by high temperature and low precipitation from range core to southern edge of European beech indicates widespread vulnerability to changing climate

“…As older trees have been shown to increase investment in reproduction relative to growth (Genet et al 2010), the effect of masting on tree growth may increase with tree age (Hacket-Pain et al 2015b). In contrast, older trees allocate more carbon to storage and have larger overall stores of carbon (Genet et al 2010;Sala et al 2012), implying that carbohydrate depletion during unfavourable conditions should have a smaller effect on growth the following year in older (and larger) trees (the carryover mechanism).…”

Drought and shade deplete nonstructural carbohydrate reserves in seedlings of five temperate tree species

“…2009; McDowell 2011; Sala et al. 2012). A synthesis of mortality mechanisms suggests two related hypotheses of drought‐induced mortality: carbon starvation and hydraulic failure (McDowell et al.…”

“…On the other hand, drought could decrease a tree's carbon demand by downregulating growth, leading to a sink limitation, and in some cases, increased allocation to storage (Sala et al. 2012). When NSC has been measured directly, results have provided mixed support for the role of NSC reserve mobilization, including examples of NSC depletion (Galiano et al.…”