Tree Physiology: Carbon allocation special issue

Tree Physiology cover for Carbon Allocation of Trees and Forests issueCarbon 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.

Eric Ward.

Post Doctoral Researcher, North Carolina State University, PINEMAP

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