## 2.19.2. Carbon Allocation for Maintenance Respiration Costs[¶](#carbon-allocation-for-maintenance-respiration-costs "Permalink to this headline")
-------------------------------------------------------------------------------------------------------------------------------------------------

Allocation of available carbon on each time step is prioritized, with first priority given to the demand for carbon to support maintenance respiration of live tissues (section 13.7). Second priority is to replenish the internal plant carbon pool that supports maintenance respiration during times when maintenance respiration exceeds photosynthesis (e.g. at night, during winter for perennial vegetation, or during periods of drought stress) (Sprugel et al., 1995). Third priority is to support growth of new tissues, including allocation to storage pools from which new growth will be displayed in subsequent time steps.

The total maintenance respiration demand (\\(CF\_{mr}\\), gC m\-2 s\-1) is calculated as a function of tissue mass and nitrogen concentration, and temperature (section 13.7) The carbon supply to support this demand is composed of fluxes allocated from carbon assimilated in the current timestep (\\(CF\_{GPP,mr}\\), gC m\-2 s\-1 and from a storage pool that is drawn down when total demand exceeds photosynthesis ( \\(CF\_{xs,mr}\\), gC m\-2 s\-1):

(2.19.1)[¶](#equation-19-1 "Permalink to this equation")\\\[CF\_{mr} =CF\_{GPP,mr} +CF\_{xs,mr}\\\]

(2.19.2)[¶](#equation-19-2 "Permalink to this equation")\\\[\\begin{split}CF\_{GPP,mr} =\\\_ \\left\\{\\begin{array}{l} {CF\_{mr} \\qquad \\qquad {\\rm for\\; }CF\_{mr} \\le CF\_{GPP} } \\\\ {CF\_{GPP} \\qquad {\\rm for\\; }CF\_{mr} >CF\_{GPP} } \\end{array}\\right.\\end{split}\\\]

(2.19.3)[¶](#equation-19-3 "Permalink to this equation")\\\[\\begin{split}CF\_{xs,mr} =\\\_ \\left\\{\\begin{array}{l} {0\\qquad \\qquad \\qquad {\\rm for\\; }CF\_{mr} \\le CF\_{GPP} } \\\\ {CF\_{mr} -CF\_{GPP} \\qquad {\\rm for\\; }CF\_{mr} >CF\_{GPP} } \\end{array}\\right.\\end{split}\\\]

The storage pool that supplies carbon for maintenance respiration in excess of current \\(CF\_{GPP}\\) ( \\(CS\_{xs}\\), gC m\-2) is permitted to run a deficit (negative state), and the magnitude of this deficit determines an allocation demand which gradually replenishes \\(CS\_{xs}\\). The logic for allowing a negative state for this pool is to eliminate the need to know in advance what the total maintenance respiration demand will be for a particular combination of climate and plant type. Using the deficit approach, the allocation to alleviate the deficit increases as the deficit increases, until the supply of carbon into the pool balances the demand for carbon leaving the pool in a quasi-steady state, with variability driven by the seasonal cycle, climate variation, disturbance, and internal dynamics of the plant-litter-soil system. In cases where the combination of climate and plant type are not suitable to sustained growth, the deficit in this pool increases until the available carbon is being allocated mostly to alleviate the deficit, and new growth approaches zero. The allocation flux to \\(CS\_{xs}\\) (\\(CF\_{GPP,xs}\\), gC m\-2 s\-1) is given as

(2.19.4)[¶](#equation-19-4 "Permalink to this equation")\\\[\\begin{split}CF\_{GPP,xs,pot} =\\left\\{\\begin{array}{l} {0\\qquad \\qquad \\qquad {\\rm for\\; }CS\_{xs} \\ge 0} \\\\ {-CS\_{xs} /(86400\\tau \_{xs} )\\qquad {\\rm for\\; }CS\_{xs} <0} \\end{array}\\right.\\end{split}\\\]

(2.19.5)[¶](#equation-19-5 "Permalink to this equation")\\\[\\begin{split}CF\_{GPP,xs} =\\left\\{\\begin{array}{l} {CF\_{GPP,xs,pot} \\qquad \\qquad \\qquad {\\rm for\\; }CF\_{GPP,xs,pot} \\le CF\_{GPP} -CF\_{GPP,mr} } \\\\ {\\max (CF\_{GPP} -CF\_{GPP,mr} ,0)\\qquad {\\rm for\\; }CF\_{GPP,xs,pot} >CF\_{GPP} -CF\_{GPP,mr} } \\end{array}\\right.\\end{split}\\\]

where \\(\\tau\_{xs}\\) is the time constant (currently set to 30 days) controlling the rate of replenishment of \\(CS\_{xs}\\).

Note that these two top-priority carbon allocation fluxes (\\(CF\_{GPP,mr}\\) and \\(CF\_{GPP,xs}\\)) are not stoichiometrically associated with any nitrogen fluxes.

