MAI vs CAI: picking the right rotation age
Every thinning crew foreman has hit this at the gate: the growth and yield tables say a stand is ready, but the timber itself doesn't look ready. That gap usually comes down to mixing up two different growth curves, mean annual increment and current annual increment.
What MAI and CAI actually measure
Mean annual increment (MAI) is average volume growth per year since planting: total standing volume divided by age. Current annual increment (CAI) is growth added in the most recent year alone. Early on, CAI runs well above MAI, because young trees pack on wood fast relative to the modest volume already standing. Growth slows as the trees mature, and at some point the average catches up with the current rate.
Where the two curves cross
The point where CAI drops below MAI marks the biological rotation age, the age that maximizes wood produced per hectare per year. Past that crossing, the average is still climbing, but more slowly each year, meaning the trees are adding wood below their own lifetime pace. That's the textbook answer to when to cut, but it only accounts for wood. It says nothing about stumpage price, haul distance, carrying cost, or what the next crop could do on that same ground while you wait.
Biological rotation age vs economic rotation age
Biological rotation age maximizes wood volume per hectare per year. The economic answer usually lands earlier, because money has a time value and standing timber doesn't earn interest. A forester weighing the economic case asks when the next dollar of added growth, plus any price bump for bigger logs, stops beating what that capital could earn if the block were cut, sold, and replanted today. Land expectation value, site index, and local log markets all pull that number around. On most privately held ground, the economic cut comes several years ahead of the biological one, before age-class balance or mill demand even enter the picture.
On paper this is tidy. In the field, growth and yield tables assume one trajectory, and real stands drift from it block by block, depending on site quality, stocking density, and whatever the weather did the last few seasons. A block planted the same year and species as its neighbor can sit years ahead or behind on that same curve, and the only way to know, traditionally, has been sending a crew out to run plots.
Tracking growth without walking every compartment
This is where a stand-level growth index earns its keep. Instead of re-measuring plots, an annual satellite pass over each compartment gives a repeatable signal for that ground, tracked against its own expected curve. It flags which blocks are running ahead of schedule, closer to their rotation point, and which are lagging and need another season before thinning makes sense. You can see how that scheduling works on Plantation Monitor's compartment tracking, built from annual multispectral passes instead of a crew walking transects to eyeball crown closure.
None of this replaces the price sheet or a land expectation value spreadsheet. What it replaces is the guesswork over which blocks are even worth running those numbers on this season. Knowing that block 14 is tracking ahead of its curve and block 9 is lagging turns an estate-wide question into a short list.
MAI and CAI give you the theory for an even-aged stand sitting near its crossing point. They don't tell you whether this particular block, on its particular site, has actually gotten there. That's the read you still need, block by block, season by season, before crews get scheduled.
If scheduling thinning and clearfell against a rotation plan is the recurring headache at your gate, Plantation Monitor is built to settle that question compartment by compartment.