Offset vs. Age

When the Patchworks model starts up it reads the initial age of each stand from the inputs dataset, and uses this to set the initial OFFSET value for the stand. The OFFSET attributes describes how old the stand is relative to the 'track' that it is following. There are independent offset values for the managed (MANAGEDOFFSET) and unmanaged (UNMANAGEDOFFSET) portions of the stand.

In most situations the offset value is synonymous with stand age. A stand with offset 5 usually represents a 5-year old stand, and attribute values are looked up along the yield curves at an offset of 5. For stands that are older than a 'stand-replacing successional event' the link between age and offset becomes less clear. As mentioned previously, Patchworks incorporates succession as an endemic factor within the tracks that describe stand growth and development. Tracks describe the growth, development and management opportunities for stands over very long time horizons.

For example, if a stand should be forecast to breakup and renew at age 140, then the Matrix Builder tool will compose yield curves that adjust appropriately at offset 140. At offset 141 and beyond, the stand will have attributes that reflect the post-successional condition. Similarly, treatments and responses for the post-successional are also adjusted accordingly and adding at appropriate future offsets. From the simulation model point of view all stand attributes are being forecast correctly both before and after the succession event. However, a stand that has an offset of 141 will be 141-years from the last human-caused disturbance, but may reflect structural characteristics of a stand with a younger age.

In summary, offset refers to the number of years since the previous human-caused disturbance that is used to look up stand attributes in a track. If succession events have not occurred the offset value may have the same meaning as stand age. If the stand condition is old and succession has been applied using a breakup and renewal type of rule, the offset will still describe the time since the last treatment, and might not have the same value as the age of the new replacement stand.