Noun
The alburnum, or part of the wood of any exogenous tree next to the bark, being that portion of the tree through which the sap flows most freely; -- distinguished from heartwood.
Source: Webster's dictionaryConsequently, the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the inner heartwood. Source: Internet
In species which show a distinct difference between heartwood and sapwood the natural color of heartwood is usually darker than that of the sapwood, and very frequently the contrast is conspicuous (see section of yew log above). Source: Internet
As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume. Source: Internet
Tabebuia is distinguished from Handroanthus by wood that is not especially hard or heavy, and not abruptly divided into heartwood and sapwood. Source: Internet
Heartwood and sapwood A section of a Yew branch showing 27 annual growth rings, pale sapwood, dark heartwood, and pith (center dark spot). Source: Internet
Hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. Source: Internet