Read a climate
out of wood
A tree records its weather. Grow a cross-section from a made-up climate — or load a real bristlecone master — 391 years of measured ring widths from Sheep Mountain, 1600 to 1990 — then crossdate a floating core against the master the way a dendrochronologist actually pins a beam to a calendar. The match can fail — foreign wood produces no significant peak and should be called undateable.
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.
How to read it
A tree adds one ring every year — fat in good years, thin in bad ones — so a trunk becomes a barcode of the weather it lived through. Crossdating slides an undated core against a known chronology until the thin-year patterns snap into place. Sometimes the wood is from the wrong forest and never lines up. The instrument keeps that answer too.
The chronology
The same tree, unrolled — a bar-per-year plot of ring widths. Narrow years are the signature; everyone in the same forest shares them. Flip to the skeleton plot, the notation Douglass invented in the 1900s: only the anomalously narrow years get a tick, tall for the sharpest — the pure signal a crossdater's eye actually matches on.
Crossdate the core
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.
What you're looking at
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.
Crossdating here uses the actual t-statistic against the conventional t ≥ 3.5 bar, and a mystery core can genuinely 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 answer is sometimes "this wood will not date." Force a date on noise and the scorecard will say so.