The classical pipe model theory introduced by Shinozaki, Yoda, Hozumi,
and Kira (1964a,b) is used to estimate tree sapwood area. It rests on the
conceptual idea that leaves are supported by pipes, serving as vascular
passages. The simple pipe model of plant form does not predict the heart
wood area and is thus not suitable for stem diameter estimates below the
tree crown. For this reason, Shinozaki, Yoda, Hozumi, and Kira (1964a)
verbally described an extended pipe model theory of tree form that in
principle accounts for the accumulation of disused pipes from discarded
branches and leaves. However, this pipe model theory of tree form is difficult
to apply in practice as lost branches and leaves are rarely known.
Here, we synthesize the pipe model theory of plant form with a recently
developed theory of branch thinning that quantify discarded branches and
leaves. This allows us to develop a new stem model of tree profiles from
breast height up to the top of the tree. We test the stem model perfor
mance on empirical data from four tree species across three continents.
We find that the stem model accurately describes heartwood and sapwood
profiles of all tested tree species (calibration; R2: 84-99 %). Furthermore,
once calibrated to a tree species, the stem model predicts heartwood and
sapwood profiles of conspecific trees in similar growing environments based
only on the age and height of a tree (cross-validation / prediction; R2:
62-98 %).