Knowledge reference for national forest assessments - observations and measurements
6. Temporal vs. permanent observations
Observation units, viz. sample plots, are either measured once in an inventory (temporary plots) or repeatedly in subsequent inventories (permanent plots). In the latter case, exactly the same plot is being visited again. Permanent plots are established to allow for an efficient estimation of changes.
Plots need to be registered and marked so that they can be found at later occasions. This is usually done by visible or invisible markers in the field, by a sketch map, distance and bearing measurements to reference objects and/or by GPS measurements. Also, to be able to generate ¿time series¿ of tree measurements, the individual trees need to be recorded in a manner that later measurements can be attributed to exactly the same individual tree. This is either done by markers on the tree, like numbered aluminum tags (with the disadvantage that the can be easily removed by someone), or by recording the tree position relative to a reference system (e.g. distance and bearing to plot center of circular plots; length, side and perpendicular distance from the central track of a strip plot).
When tree positions are recorded, a next step would be to also spatially record / map other features like stand boundaries, dead wood, etc. so that at the end we have a mapped plot design.
Establishing plots that are supposed to be permanent requires a thorough documentation and permanence in data management and also in project management is imperative.