LEGGETT, Calif. — Braced against a steep slope, Robert Hrubes cinched his measuring tape around the trunk of one tree after another, barking out diameters like an auctioneer announcing bids. “Twelve point two!” “Fourteen point one!”

Mr. Hrubes’s task, a far cry from forestry of the past, was to calculate how much carbon could be stored within the tanoak, madrone and redwood trees in that plot. Every year or so, other foresters will return to make sure the trees are still standing and doing their job.

Such audits will be crucial as California embarks on its grand experiment in reining in climate change. On Jan. 1, it will become the first state in the nation to charge industries across the economy for the greenhouse gases they emit. Under the system, known as “cap and trade,” the state will set an overall ceiling on those emissions and assign allowable emission amounts for individual polluters. A portion of these so-called allowances will be allocated to utilities, manufacturers and others; the remainder will be auctioned off.

Over time, the number of allowances issued by the state will be reduced, which should force a reduction in emissions.