### 2.20.3.1. 14.3.1 Seasonal-Deciduous Onset Trigger[¶](#seasonal-deciduous-onset-trigger "Permalink to this headline")

The onset trigger for the seasonal-deciduous phenology algorithm is based on an accumulated growing-degree-day approach (White et al., 1997). The growing-degree-day summation (\\({GDD}\_{sum}\\)) is initiated ( \\({GDD}\_{sum} = 0\\)) when the phenological state is dormant and the model timestep crosses the winter solstice. Once these conditions are met, \\({GDD}\_{sum}\\) is updated on each timestep as

(2.20.46)[¶](#equation-zeqnnum510730 "Permalink to this equation")\\\[\\begin{split}GDD\_{sum}^{n} =\\left\\{\\begin{array}{l} {GDD\_{sum}^{n-1} +\\left(T\_{s,3} -TKFRZ\\right)f\_{day} \\qquad {\\rm for\\; }T\_{s,3} >TKFRZ} \\\\ {GDD\_{sum}^{n-1} \\qquad \\qquad \\qquad {\\rm for\\; }T\_{s,3} \\le TKFRZ} \\end{array}\\right.\\end{split}\\\]

where \\({T}\_{s,3}\\) (K) is the temperature of the third soil layer, and \\(f\_{day} ={\\Delta t\\mathord{\\left/ {\\vphantom {\\Delta t 86400}} \\right.} 86400}\\). The onset period is initiated if \\(GDD\_{sum} >GDD\_{sum\\\_ crit}\\), where

(2.20.47)[¶](#equation-zeqnnum598907 "Permalink to this equation")\\\[GDD\_{sum\\\_ crit} =\\exp \\left(4.8+0.13{\\kern 1pt} \\left(T\_{2m,ann\\\_ avg} -TKFRZ\\right)\\right)\\\]

and where \\({T}\_{2m,ann\\\_avg}\\) (K) is the annual average of the 2m air temperature, and TKFRZ is the freezing point of water (273.15 K). The following control variables are set when a new onset growth period is initiated:

(2.20.48)[¶](#equation-20-48 "Permalink to this equation")\\\[GDD\_{sum} =0\\\]

(2.20.49)[¶](#equation-20-49 "Permalink to this equation")\\\[t\_{onset} =86400\\cdot n\_{days\\\_ on} ,\\\]

where \\({n}\_{days\\\_on}\\) is set to a constant value of 30 days. Fluxes from storage into transfer pools occur in the timestep when a new onset growth period is initiated. Carbon fluxes are:

(2.20.50)[¶](#equation-zeqnnum904388 "Permalink to this equation")\\\[CF\_{leaf\\\_ stor,leaf\\\_ xfer} ={f\_{stor,xfer} CS\_{leaf\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{leaf\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.51)[¶](#equation-20-51 "Permalink to this equation")\\\[CF\_{froot\\\_ stor,froot\\\_ xfer} ={f\_{stor,xfer} CS\_{froot\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{froot\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.52)[¶](#equation-20-52 "Permalink to this equation")\\\[CF\_{livestem\\\_ stor,livestem\\\_ xfer} ={f\_{stor,xfer} CS\_{livestem\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{livestem\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.53)[¶](#equation-20-53 "Permalink to this equation")\\\[CF\_{deadstem\\\_ stor,deadstem\\\_ xfer} ={f\_{stor,xfer} CS\_{deadstem\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{deadstem\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.54)[¶](#equation-20-54 "Permalink to this equation")\\\[CF\_{livecroot\\\_ stor,livecroot\\\_ xfer} ={f\_{stor,xfer} CS\_{livecroot\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{livecroot\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.55)[¶](#equation-20-55 "Permalink to this equation")\\\[CF\_{deadcroot\\\_ stor,deadcroot\\\_ xfer} ={f\_{stor,xfer} CS\_{deadcroot\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{deadcroot\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.56)[¶](#equation-zeqnnum195642 "Permalink to this equation")\\\[CF\_{gresp\\\_ stor,gresp\\\_ xfer} ={f\_{stor,xfer} CS\_{gresp\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} CS\_{gresp\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

and the associated nitrogen fluxes are:

(2.20.57)[¶](#equation-zeqnnum812152 "Permalink to this equation")\\\[NF\_{leaf\\\_ stor,leaf\\\_ xfer} ={f\_{stor,xfer} NS\_{leaf\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} NS\_{leaf\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.58)[¶](#equation-20-58 "Permalink to this equation")\\\[NF\_{froot\\\_ stor,froot\\\_ xfer} ={f\_{stor,xfer} NS\_{froot\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} NS\_{froot\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.59)[¶](#equation-20-59 "Permalink to this equation")\\\[NF\_{livestem\\\_ stor,livestem\\\_ xfer} ={f\_{stor,xfer} NS\_{livestem\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} NS\_{livestem\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.60)[¶](#equation-20-60 "Permalink to this equation")\\\[NF\_{deadstem\\\_ stor,deadstem\\\_ xfer} ={f\_{stor,xfer} NS\_{deadstem\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} NS\_{deadstem\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.61)[¶](#equation-20-61 "Permalink to this equation")\\\[NF\_{livecroot\\\_ stor,livecroot\\\_ xfer} ={f\_{stor,xfer} NS\_{livecroot\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} NS\_{livecroot\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

(2.20.62)[¶](#equation-zeqnnum605338 "Permalink to this equation")\\\[NF\_{deadcroot\\\_ stor,deadcroot\\\_ xfer} ={f\_{stor,xfer} NS\_{deadcroot\\\_ stor} \\mathord{\\left/ {\\vphantom {f\_{stor,xfer} NS\_{deadcroot\\\_ stor} \\Delta t}} \\right.} \\Delta t}\\\]

where \\({f}\_{stor,xfer}\\) is the fraction of current storage pool moved into the transfer pool for display over the incipient onset period. This fraction is set to 0.5, based on the observation that seasonal deciduous trees are capable of replacing their canopies from storage reserves in the event of a severe early-season disturbance such as frost damage or defoliation due to insect herbivory.

If the onset criterion (\\({GDD}\_{sum} > {GDD}\_{sum\\\_crit}\\)) is not met before the summer solstice, then \\({GDD}\_{sum}\\) is set to 0.0 and the growing-degree-day accumulation will not start again until the following winter solstice. This mechanism prevents the initiation of very short growing seasons late in the summer in cold climates. The onset counter is decremented on each time step after initiation of the onset period, until it reaches zero, signaling the end of the onset period:

(2.20.63)[¶](#equation-20-63 "Permalink to this equation")\\\[t\_{onfset}^{n} =t\_{onfset}^{n-1} -\\Delta t\\\]

