A re‐evaluation of carbon storage in trees lends greater support for carbon limitation to growth

We still have a poor understanding of what limits tree growth under many conditions. In general, tree growth can be limited by the availability of carbon within the plant (carbon limitation) or by the tree’s ability to use the available carbon it has because of nutrient shortage, environmental conditions, or developmental constraints (sink limitation). A growing body of literature suggests that current tree growth is sink limited under most conditions, with much of the evidence inferred from trees’ high levels of nonstructural carbon (NSC). These inferences rely on the assumption that carbon storage is passive and that NSC does not accumulate when growth is carbon limited (Bansal Lacointe et al., 2004; Genet et al., 2010). We argue that carbon starvation – even if it does not occur every year or even every decade in the life of a tree – is crucial to understanding carbon storage and carbon limitation in trees. We define carbon starvation, modified after McDowell (2011), as the outcome of insufficient carbon supply from current photosynthesis and available storage to sustain the metabolism, regeneration, or defense necessary for immediate survival (Fig. 1). Episodes of carbon starvation (past and present) may have led to the evolution of conservative allocation strategies favoring greater reserve formation at the expense of potential growth. Therefore, an increase in carbon storage and decrease in growth could be a plastic or evolutionary response to carbon-limiting conditions – not a result of sink limitation. To illustrate this point, we reexamine two phenomena relevant to the active debates regarding carbon limitation: tree growth at alpine treeline and under water stress. Scheme for carbon supply and allocation to three groups of plant functions. The order of groups indicates carbon allocation priorities, with competition for carbon occurring among processes in the same group. The order within a group does not indicate priority, however it is assumed that some priority structure does exist and can change. Group I includes carbon needs essential to immediate survival. Carbon starvation occurs when the amount of available carbon is less than that required by Group I processes. Group II includes processes which are not immediately necessary for survival, but may affect survival or fitness in the long run. For a given tree, the allocation hierarchy is determined by its life-history strategy as well as inter- and intra-annual plasticity in response to environmental cues or carbon status. Growth (or any process) can become more carbon limited by (1) a decrease in available carbon, (2) an increase in Group I requirements, or (3) increased allocation to other processes (such as active storage) within Group II. Even though active storage can contribute to the available carbon pool, it may only do so selectively for particular uses. Finally, if Group II processes become carbon saturated (sink limited), only then will carbon be allocated to Group III, which includes overflow functions, such as passive storage. We believe that passive storage is rare, and argue that most nonstructural carbon is actively stored, which can occur when growth is still carbon limited. Carbon storage, defined here as formation of nonstructural carbon compounds that ‘can be mobilized in the future to support biosynthesis’, can be either passive or active (Chapin et al., 1990). Passive storage forms an excess pool when carbon supply is greater than carbon demand of other processes, such as growth (Fig. 1). Active storage, or reserve formation, on the other hand, occurs when carbon is directed into storage at the expense of growth or other processes. Reserve formation allows a tree to prepare for (1) predictable asynchrony of supply and demand for carbon (e.g. early spring growth for deciduous trees) and (2) unpredictable emergencies such as disturbances that affect carbon gain or need (Chapin et al., 1990). The potential for active reserves is often ignored when inferring the nature of growth limitation. When comparing trees in different sites or treatments, sink limitation is incorrectly inferred from one of two main patterns: (1) higher NSC content together with lower growth rates (Hoch Würth et al., 2005; Palacio et al., 2008; Sala Shi et al., 2008). The rationale for inferring sink limitation from the first pattern is that NSC pool size is the difference between carbon gain (source) and carbon demand (sink) within the plant (Körner, 2003). This assumption is true if all storage is passive, but not if it is active (Lacointe et al., 2004; Bansal Genet et al., 2010). Reduced growth and increased storage is also consistent with carbon limitation if (a) populations vary genetically in the amount of active reserve formation or (b) reserve formation responds plastically to environmental or internal cues. The rationale for inference (2) is that if growth is annually carbon limited during periods when demand exceeds supply, NSC should approach zero during these periods, because growth could only be carbon limited if all storage is depleted (Hoch et al., 2003; Körner, 2003). But this complete, yearly drawdown will only occur if all reserves are stored because of annual asynchrony of supply and demand (i.e. earmarked for a particular time of year or frequent event). Reserves, though, can be formed to avoid running out of carbon during emergencies, which may be unpredictable and infrequent, and trees may actively maintain a baseline level of NSC, growing only after storage requirements are met. Growth in trees with higher NSC levels may be more carbon limited than in those with lower NSC levels. Increased storage is often associated with survival (Canham et al., 1999; Gleason Myers although see Piper et al., 2009), recovery from disturbance (Breda et al., 2006), or resistance to biotic attack (Dunn et al., 1987). If storage can be active and increase the chance of survival, then increasing storage could be an adaptive response to an increased threat of carbon starvation. Populations that experience severe episodes of carbon starvation may experience stronger selection for increased reserves, leading to more conservative allocation strategies. However, these conservative populations, while less prone to carbon starvation, are more likely to experience carbon limitation to growth because less carbon is available after reserves are formed. Therefore higher NSC levels may indicate, and cause, greater carbon limitation. It has been argued that large reserves cannot be beneficial given both the large opportunity costs of not growing, and also that maintaining high photosynthesis rates necessary to develop such reserves is costly, in terms of nutrients, and risky, in terms of cavitation (Sala Körner, 2003). Additionally, if carbon starvation is infrequent but relatively predictable by a reliable cue, plasticity of reserve allocation may be selected for. For example, when carbon uptake is low, a tree will take longer to accumulate enough carbon to recover from disturbance or defend against biotic attack unless it has sufficient reserves; therefore storing more when carbon uptake declines could help a plant survive or recover faster. Non-carbon status cues (i.e. temperature or water potential) could also be reliable indicators of increasing threat of starvation or damage, against which an increase in storage could defend. In Arabidopsis thaliana, there is accumulating evidence that plants actively shift carbon allocation between storage and growth, favoring storage when carbon uptake declines (Smith Gibon et al., 2009). Growth slows under salt stress and cold temperatures in part due to higher levels of DELLA proteins, which down-regulate growth (Achard et al., 2006, 2008). Mutants without these proteins grow faster and have lower survival than wildtype plants under salt stress, indicating that growth is actively, and potentially adaptively, suppressed. Suppression may allow plants to switch to a more conservative strategy of carbon use in response to environmental cues to mitigate the effects of harsh conditions (Smith alternatively, if carbon supply were limiting, NSC concentrations would be lower at treeline (Hoch et al., 2002). Treeline trees often grow slower and have higher NSC concentrations than trees at lower altitudes, presumably supporting the growth limitation hypothesis (Hoch et al., 2002; Hoch Shi et al., 2008). However, because increased storage may be adaptive and actively controlled (Sveinbjörnsson, 2000; Bansal Susiluoto et al., 2007) and when deciduous trees were given increased CO2 (Handa et al., 2005). In the same CO2 enrichment experiment, the evergreen species did not increase growth under higher CO2 availability alone, but shoot elongation was greater when defoliated under elevated CO2. These results suggest that temperature cannot be directly limiting growth if trees can increase growth when temperature does not change, and that even for the evergreen species, growth in response to disturbance or damage seems to be carbon limited. Finally, maximum photosynthetic rates at treeline and lower altitudes or latitudes may not differ (Vowinckel et al., 1975; Sveinbjörnsson, 1983; Körner, 1999). If the higher NSC levels at treeline were passively accumulating, photosynthetic down-regulation should occur, reducing assimilation rates (Bansal Ryan Ryan et al., 2006). The evidence supporting sink limitation under water stress is often very similar to that used to support sink limitation at the treeline. Because of turgor-limited cell expansion, low water availability may directly affect growth more than photosynthesis, leading to a passive buildup of excess NSC within the tree (Körner, 2003). Consistent with this hypothesis, tropical tree species during the dry season have lower growth rates and higher NSC content than during the wet season (Würth et al., 2005), droughted seedlings decrease growth while either showing no change in NSC (Sanz-Pérez et al., 2009) or an increase (Galvez et al., 2011), and tall trees often have lower branch elongation rates and higher NSC concentrations than shorter trees (Sala Woodruff McDowell et al., 2008). Reduced carbon uptake and storage during a drought may also lead to greater canopy dieback the following season (Breda et al., 2006), reducing carbon uptake and storage even more, leading eventually to starvation (Breda et al., 2006; Galiano et al., 2011). Additionally, declining photosynthesis could reduce defense allocation, reducing a tree’s ability to avoid or recover from biotic attack (McDowell, 2011). Increasing reserves at the onset of water stress may allow a plant to avoid these fates, but at the cost of reducing growth. Even if carbon starvation via NSC depletion is rare, increasing storage could still be an adaptation to drought. For example, carbon starvation could occur via phloem malfunction, before all reserves are depleted (Sala et al., 2010). But increased storage in all plant organs could potentially lengthen the time a plant could survive on ‘local’ carbon without long-distance transport. In which case, mortality by phloem malfunction could still select for increased storage under water stress. Furthermore, even if carbon starvation isn’t a current cause of mortality, increased storage may still be adaptive. If during a population’s evolution, starvation occurred, trees may have evolved to avoid this type of mortality, leaving other ways for trees to die (e.g. hydraulic failure). The adaptive nature of increased storage is supported by the divergent response of NSC content to drought between more and less drought-tolerant tree species: the more drought tolerant increase NSC content while the less tolerant show the opposite response (Regier et al., 2009; Piper, 2011). We believe that increasing carbon storage and decreasing growth is an active response, either plastic or evolutionary, to reduce the risk of starvation—not a result of sink limitation. If true, we are left with the counter-intuitive conclusion that tree populations with the highest NSC concentrations may be the most carbon limited in terms of growth; this puts into question many recent conclusions about what limits tree growth. In fact, the rejection of the carbon storage pool as purely passive leads us to conclude that we may have greatly underestimated the importance and frequency of carbon limitation to growth in trees. Our knowledge of NSC dynamics in trees remains limited, however, and much more work is needed to determine the nature of storage and whether some species or populations favor storage more than others. To determine if storage is active, we may need first to determine where and when growth is carbon limited. Increasing carbon uptake experimentally or relying on natural variation may be the surest way of determining limitation; if growth increases with greater carbon uptake, all else being equal, the tree was carbon limited – and any storage that occurred was active. To test if certain populations store more than others (e.g. cold, fire, or pest tolerant vs intolerant, late vs early-successional), common garden experiments could be used to compare NSC concentrations or pool sizes relative to growth rates. Furthermore, as genetic work with Arabidopsis has demonstrated the importance of negative growth regulators, measuring DELLA protein expression may be useful in understanding when growth is constrained by environmental conditions or actively reduced by the plant. We thank Brenda Casper and three anonymous referees for their very helpful comments.