Mortality for Mid-Rotation and Old-Growth White spruce
For white spruce with DBH>4, annual survival probability (Ps) is predicted using a logistic model.
Pslogistic = exp(c) / (1+exp(c))
Where: c = b0 + b1*DBH + b2*DBH2 + b3*DI + b4*SWBAGT2
DBH is in cm
DI is the current diameter increment (cm/y)
SWBAGT is the basal area in thicker spruce (m2/ha)
b0 = 4.01, b1 = 0.0563, b2 = -0.00109, b3 = 6.74, b4 = -0.000484
For very large, old trees and stands there are two additional mortality penalties, designed to account for wind damage and increased mortality in heavily stocked stands. These features were not captured in the logistic model above, and are based on the work of Yang (2002). These are active when the MAFlage is selected.
1) If the quadratic mean DBH of the white spruce in the stand (QMDsw) exceed 28cm and the current tree is thicker than the QMDsw, survival of the tree is reduced by a factor proportional to the square of the amount QMDsw exceeds 28:
Z = (QMDsw - 28) ^ 2 / 100
SRF1 = (1 + Z * QMDsw / DBH) / (1 + Z)
2) If the stand basal area exceeds 55 m2/ha in mixed stands (BAsw/BAtotal<0.8), or 50 m2/ha in white spruce dominated stands, survival of all white spruce trees is reduced
SRF2 = ((BAtotal - 55) / 80) if mixedwood
SRF2 = ((BAtotal - 50) / 80) if spruce-dominated
If unmodified by the criteria above, all SRFx=1. The final annual survival probability, applied to the tree expansion factor is:
Pslogistic = exp(c) / (1+exp(c)) * SRF1*SRF2
Note: Sw is not subjected to the MaxDensity (Self-thinning) line.