A tree records its weather. Grow a cross-section from a made-up climate — or load a real bristlecone pine that has been growing since before Rome — then crossdate a floating core against the master the way a dendrochronologist actually pins a beam to a calendar. The match can fail. That's the honest part.
A tree-ring explorable · real chronology: Sheep Mountain bristlecone, ITRDB ca534 · built late, 2026-06-11
Each ring is one growing season. Wide pale band = fast spring earlywood; the thin dark line closing it is summer latewood. A black radial scar is a fire year; a blue-grey ring is a killing frost. Drag the sliders and the whole life rewrites.
The same tree, unrolled. This bar-per-year plot of ring widths is the measurement — the skeleton a dendrochronologist works from. Narrow years are the signature; everyone in the same forest shares them.
Here's the trick that lets a roof beam tell you it was felled in 1247. A short floating sample is slid along the master; at each position we compute the correlation and convert it to the dendrochronologist's t-statistic. The convention: a date you can defend needs t ≥ 3.5, and the peak must stand clearly above the noise. Cut a mystery core — it might be from this master, or it might be foreign wood that won't date — then make the call.
Trees in a seasonal climate lay down one ring a year: a burst of wide, pale earlywood in spring, then a thin dark seam of dense latewood as summer tightens. A good year is a wide ring; a drought, a frost, a fire, a crowded canopy — all of it gets written as a narrow one. Because every tree in a region feels the same big climate years, their narrow-ring signatures line up. That shared pattern is the whole science.
Crossdating is the move that makes it powerful: slide a wood sample with unknown dates against a dated master until the signatures snap together, and you've pinned the sample to an exact calendar year — how a Viking hall, a Stradivarius, or a Bristlecone pine gets dated to the year, sometimes back nine thousand of them. The real bristlecone loaded here is Sheep Mountain in the White Mountains of California (D. A. Graybill, ITRDB ca534), the 1600–1990 window. The band of narrow rings in the 1600s is not invented — it is the Little Ice Age, written into wood at treeline where cold years pinch growth.
This is no longer rigged. Crossdating here uses the actual t-statistic with the conventional t ≥ 3.5 bar, and a mystery core is genuinely able to fail: foreign wood produces no significant peak and should be called undateable, while a degraded core sits maddeningly near the line. The procedural tree is still a toy — its "climate" is noise you steer — but the method is real, including the part where the honest answer is sometimes "this wood will not date." Force a date on noise and the scorecard will say so.