Reading the cross-section: rings, sapwood and heartwood

Why start with a slice of the stem

A board is a slice through a living tissue, and much of its behaviour was decided before the saw touched it. Most hard numbers in the portal's wood-structure articles come from North American sources (the USDA Wood Handbook and US extension guides). European values come from German, Swedish, Czech and other studies, and each figure below carries its region. Figures are examples, not specifications.

From the bark to the pith

Moving inwards you meet the bark, then the cambium, a very thin growing layer that makes new wood cells inwards and bark cells outwards. In temperate climates it works in seasonal pulses, so each growing year leaves a growth ring (Virginia Cooperative Extension, North America). Inside the cambium lies the sapwood, the living outer band, then the older heartwood, and at the centre the pith, where the tree began.

Earlywood and latewood

Within one ring the earlywood, formed first, has large-diameter cells with thin walls. The latewood, formed at the end of the season, has narrow cells with thick walls. Latewood is denser, so ring structure translates directly into density (Lesson 3).

How sharply one zone changes into the other differs by species. The Czech Timber Atlas (Europe) describes the transition as gradual in spruce, semi-gradual in fir, abrupt in pine and very abrupt in larch and Douglas fir.

Sapwood and heartwood

Heartwood is the older inner zone, where extractives accumulate that affect colour, smell and decay resistance. Two cautions from the Wood Handbook (older edition, North America):

• The change from sapwood to heartwood does not add or remove cells, and basic strength is essentially unaffected. Heartwood is not stronger for being heartwood.
• None of the sapwood of any species is resistant to decay (Lesson 7).

Sapwood width varies. LWF Bayern (Germany) reports European larch sapwood mostly under 2 cm, often under 1 cm in old high-altitude trees. Pine shows a clear colour contrast between heartwood and sapwood, while in spruce the two look alike once dried, so heartwood is hard to identify by eye (Swedish Wood, Sweden).

The water difference

Moisture content (MC) here is on the oven-dry basis. Sapwood and heartwood arrive at the mill with different amounts of water.

• Sweden: Scots pine sapwood 107 %, Norway spruce sapwood 95 %, Norway spruce heartwood 35 % (Hajian and others, 2023).
• North America (Wood Handbook): Sitka spruce 41 % heartwood against 142 % sapwood; loblolly pine 33 % against 110 %.
• Hardwoods are less regular: white oak 64 % against 78 %, white ash almost equal at 46 % and 44 %.

Worked example (own arithmetic). A spruce log with basic specific gravity 0.40 carries 0.40 x 1000 x 1.20 = 480 kg of water per m3 of green volume in sapwood at 120 % MC, but only 0.40 x 1000 x 0.35 = 140 kg in heartwood at 35 %. A kiln charge mixing both starts with a wide spread, so the portal's drying article suggests sorting by sapwood share, or at least controlling on the wettest pieces.

False heartwood: beech red heart

Do not confuse true heartwood with the coloured core of beech. Red heart is a brown-red false heartwood that forms where oxygen enters the stem centre of older trees (Knoke, 2002, abstract). It is not decay in itself but often accompanies it. In a 2010 Romanian study (Transilvania University of Brasov; nine beech trees from one felling area), red heartwood occupied about 34 % of 65.638 m3 of round wood, varying from 5 % to 48 % between logs, and the author estimated that downgrading cut round-wood revenue by 40 to 50 %. This small sample shows the mechanism, not a European average.

What it means at the saw and in the kiln

• At the saw. The log end shows where the pith sits, how wide the sapwood is and whether there is a coloured core. Keep sapwood out of boards meant for exposed use.
• In the kiln. Softwood charges start with a wide moisture spread, so control on the wettest pieces. Dry red-heart beech separately where you can (the portal's inference; see Lesson 2 on tyloses).

Key points

• A ring is one growing season: thin-walled, light earlywood then thick-walled, dense latewood.
• The earlywood-latewood transition is gradual in spruce and very abrupt in larch and Douglas fir (Europe).
• Heartwood is not stronger than sapwood; it differs in extractives, colour and durability.
• Softwood sapwood is much wetter than heartwood (Sweden: pine sapwood 107 %, spruce heartwood 35 %); hardwoods are less regular.
• Beech red heart is false heartwood: a colour and drying problem, not a strength feature.