*a*′ | unit conversion coefficient | kg kPa^{−1} mol^{−1} | | function of air density and temperature |

*A*_{az} | hydraulically effective cross-sectional area of branch | m^{2} | | |

*A*_{b} | cross-sectional area of a branch | m^{2} | | on a plane normal to the length |

*A*_{can} | ground projection area of tree | m^{2} | 5.56 | density of 1800 trees/Ha, based on *Ewers et al.* [2001] |

*A*_{m} | scaling coefficient for effective cross-sectional area | m^{2} | 0.01 | minimal stem area |

*A*_{tot} | total cross-sectional area of a conductive system | m^{2} | | |

*A*_{z} | cross-sectional area of branch element | m^{2} | | on a plane normal to the length |

*A*_{zo} | cross-sectional area at tree base | m^{2} | 0.68 | |

*b*′ | minimal conductivity parameter of stomatal conductance model | mol m^{−2} s | 0.01 | *Leuning* [1995] |

*C* | water capacity of xylem | kg m Pa^{−1} | | |

*C*_{tot} | tree-level capacity | kg m Pa^{−1} | | |

*C*_{1} | empirical curve fitting coefficient-cavitation pressure | Pa | 4.8 × 10^{−6} | *Picea abies* [*Mayr et al.*, 2003] |

*c*_{2} | empirical curve fitting coefficient for conductance | | 3.5 | *Picea abies* [*Chuang et al.*, 2005] |

*c*_{3} | empirical curve fitting coefficient for steepness of stomatal response | | 8 | *Cruiziat et al.* [2002] |

*c*_{4} | empirical “thinning” coefficient | | 0.004 | fitted between stem base and top diameters of 25 and 0.5 cm |

*C*_{a} | CO_{2} concentration in air | ppm | 380 | measured October 2001 |

*C*_{i} | intracellular CO_{2} concentration | ppm | | |

*D*_{0} | scaling parameter of *VPD* in the stomatal conductance model | Pa | 3000 | *Leuning* [1995] |

*EB* | “extra branch” coefficient in the scaling law that governs branch cross-sectional area | | range 1–3 in steps of 0.2 | in the “natural tree” *EB* = 1.75 [*Oren et al.*, 1986] |

*ED* | “exponent order” coefficient in the scaling of conductivity and cross-sectional area | | range 1.4–2.8 in steps of 0.2 | in the “natural tree” *ED* = 2.44 [*Cruiziat et al.*, 2002] |

*e*_{m} | maximal quantum efficiency | | 0.08 | *Leuning* [1995] |

*E*_{l,max} | maximal leaf transpiration rate | kg m^{−2} s | | |

*E*_{V} | transpirational water sink | kg s^{−1} | | |

*E*_{V,max} | maximal potential transpiration | kg s^{−1} | | no hydrologic limitations |

*E*_{sf} | sap flux | kg s^{−1} | | |

*g* | gravitational acceleration | m s^{−2} | 9.807 | |

*k* | xylem conductivity to water | m^{2} s | | |

*k*_{b} | boundary-layer-air water conductance | μmol m^{−2} s^{−1} | | function of wind speed |

*k*_{l} | leaf to air water conductance | μmol m^{−2} s^{−1} | | |

*k*_{max} | maximal xylem conductance | m^{2} s | 1.36 × 10^{−8} | |

*l* | hydraulic path length | m | | |

*l*_{i} | hydraulic path length from the ground to point *i* | m | | |

*LAD* | leaf area density | m^{2} m^{−1} | | |

*LAI* | total leaf area index | m_{leaf}^{2} m_{ground}^{−2} | 6 | *Ewers et al.* [2001] |

*lb* | length along a side branch | m | | |

*m* | fitting parameter of stomatal conductance model | | 4 | *Leuning* [1995] |

*m*_{c} | Michaelis constant for CO_{2} fixation | ppm | 300 | *Leuning* [1995] |

*m*_{c} | Michaelis constant for O_{2} inhibition | ppm | 300,000 | *Leuning* [1995] |

*NB* | number of side branches at each split | | 4 | |

*N*_{el} | number of branch elements | | 114 | |

*o*_{i} | O_{2} mol fraction | ppm | 210,000 | *Leuning* [1995] |

*p* | empirical curve fitting coefficient for water content | | 20 | |

*PAR* | photosynthetic active radiation | μmol m^{−2} s^{−1} | measured | Duke Forest, October 2001 |

*R*_{l} | leaf resistance in 2R model | Pa s kg^{−1} | | |

*R*_{l,min} | minimal leaf resistance | Pa s kg^{−1} | | assuming no hydraulic limitations |

*R*_{p} | total “per tree” resistance (leaf + wood) in RC model | Pa s kg^{−1} | | |

*R*_{s} | wood resistance in 2R model | Pa s kg^{−1} | | stem + branches |

*S*_{nk} | total water sink | kg s^{−1} | | |

*S*_{tot} | total tree storage | kg | | |

*t* | time | s | | |

*V*_{c,max} | maximal carboxylation capacity of Rubisco | μmol m^{−2} s^{−1} | 59 | *Leuning* [1995] |

*V*_{d} | dark respiration | μmol m^{−2} s^{−1} | 0.015*V*_{c,max} | *Leuning* [1995] |

*VPD* | vapor pressure deficit in air | kPa | measured | Duke Forest, October 2001 |

*z* | vertical height | m | | |

Φ | water pressure | Pa | | |

Φ_{a} | water pressure air | Pa | | equivalent liquid phase pressure |

Φ_{l} | water pressure in leaves petiole, end of branches | Pa | | |

Φ_{s} | water pressure at base of stem | Pa | −783.77 | = Ψ_{s}, “soil” pressure |

Φ_{tot} | “tree level” pressure | Pa | | volume averaged |

Φ_{0} | empirical parameter for water pressure of dry xylem | Pa | 5.74 × 10^{8} | |

Φ_{σ} | empirical critical pressure where stomata are half close | Pa | 200,000 | *Cruiziat et al.* [2002] |

Γ* | compensation point for CO_{2} in the absence of dark respiration | ppm | 60 | *Leuning* [1995] |

Ψ | total water potential | Pa | | |

Ψ_{s} | water potential at base of stem | Pa | −783.77 | represents soil at near saturation |

α | zenith angle of branch element | deg | | |

*B*_{1} | Farquhar coefficient for transpiration | μmol m^{−2} s^{−1} | | *Farquhar et al.* [1980] |

*B*_{2} | Farquhar coefficient for respiration | ppm | | *Farquhar et al.* [1980] |

β_{p} | leaf absorptivity for *PAR* | | 0.8 | *Leuning* [1995] |

θ | water content | kg m^{−3} | | |

θ_{sat} | water content at saturation | kg m^{−3} | 573.5 | *Picea abies* [*Chuang et al.*, 2005] |

ρ | water density | kg m^{−3} | 999 | |

τ | capacitor time constant | s | | |