Carbon allocation, the process by which plants invest carbon into stored reserves and structures such as new leaves, stem tissue and roots, has implications for topics as varied as drought tolerance, carbon sequestration and crop yield. An upcoming special issue of Tree Physiology addresses the issue of carbon allocation in a series of articles ranging from very general reviews of modeling approaches to the very specific, such as a study of the transport of 13C through young loblolly pine trees over a 3-week period. While the specific studies each warrant discussion, three articles should be of interest to anyone involved in plant biology.
Carbon allocation represents a poorly understood process with a proliferation of modeling approaches. The article by Franklin et al. contends that this is because carbon allocation represents not a single process, but several interacting ones. This review focuses on guiding principles in carbon allocation models, placing them in the broader classes of empirical, allometric, functional balance, eco-evolutionary, and thermodynamic models. Guiding much of the article is a discussion of when more complicated models are required to answer questions of interest. For example, empirical allometry does not address the plastic responses to environmental changes that are critical to assessing the effects of climate change.
Likewise, within the category of eco-evolutionary models, some explicitly address competition, such as approaches of game-theoretic maximization (King 1993) and adaptive dynamics (Dybzinski et al. 2011), while others only focus on the optimal response of individuals according to a fitness proxy (Franklin et al. 2009). The authors discuss how an individual optimal response may incorporate one dimension of competition implicitly by choice of a fitness proxy, e.g. height as a proxy for light competition. Since competition has more than one dimension in many systems, as when plants compete for both light and nutrients, the case is made that this is often an insufficient representation of competition. In addition to providing an excellent guide for modelers interested in specific types of questions surrounding allocation, this article provides guidance for empiricists who wish to generate appropriate data to impact the development of these models.
Commenting on this review, Mäkelä (2012) classifies carbon allocation models as mechanistic (bottom-up), decision rule (top-down) and those that address system dynamics as a whole. Mäkelä makes an excellent point in noting that any top-down model must be regarded as eco-evolutionary, as trees can hardly be said to make decisions in any other manner. Reshuffling categories of models is not a mere exercise in pigeonholing, however; it highlights another set of challenges facing these models.
While the prospect of understanding long-term carbon allocation from mechanistic principles no doubt represents an attractive goal in this area of research, the complexity of these models due to the number of interacting processes makes them difficult to parameterize and best suited for short-term questions. On the other hand, whole system approaches such as adaptive dynamics may also suffer from excessive complexity arising from tracking the dynamics of a structured population with plastic responses. Decision rule based models represent a range of simplifying assumptions, but involve decisions on which traits to accept as adaptive and which are taken as constraints. In the end, both Mäkelä and Franklin et al. agree that the choice of model is dependent on the questions being asked and all models must be carefully tested against observations.
Guiding principles and modeling approaches aside, the review by Sala, Woodruff and Meinzer highlights recent research into the timing of carbon supply and demand in tree species. Of particular interest are studies that indicate stored carbohydrates are not simply ‘passive overflow reservoirs,’ but may be actively competing with growth for carbohydrates. These authors discuss why large trees may be dependent on the large safety margins provided by carbohydrate reserves to maintain hydraulic transport during droughts and how droughts may also impact the long distance transport of stored reserves. This means that the location and availability of reserves become an issue under water stress. Indeed, stored carbon may become sequestered in xylem by embolism events and become inaccessible altogether. This review highlights how commonly held assumptions, such as the passive nature of carbohydrate storage, are being revisited and why carbon allocation processes remain an area of active inquiry in tree physiology research.