tree-to-wood product conversion studies. More recently, allometric exponents, or the
comparison of different allometric exponents, have also been used to gain insight on
the effects of climate on stem form. These three modelling approaches are however
empirically based. Several theories have been proposed to explain the effects of
various factors on stem form, with the hydraulic and biomechanic theories being the
most widely used, and often opposed. Both theories, when simplified to their
simplest expression, underline the importance of crown dimensions in determining
tree form. Nevertheless, these theories cannot explain all of the variations observed
empirically. In the case study of balsam fir, climatic variables such as total summer
precipitation and mean winter temperature are slightly more important in explaining
tree taper, when compared to average wind speed. This signifies that the proposed
theories, be it either hydraulic or biomechanic, should be hybridized with physiological processes in order to account for all the empirical observations.
1 Introduction
Foresters have long been interested in the shape of the main stem of a tree (Pressler
1865; Metzger 1893). The reason of this interest is that tree volume and biomass is
dependant on tree form since the main stem accounts for over 60% of the total
aboveground biomass of a tree with a diameter at breast height larger than 10 cm
(Lambert et al. 2005). With green accounting and biomass estimation on large tracks
of land, small variations in tree form can imply large biases (Weiskittel et al. 2015).
Moreover, sampling height, or lack of homogeneity in sampling height, can strongly
influence the past climate reconstruction when using tree ring chronologies due to
variations in the vertical radial growth distribution (Autin et al. 2015).
The shape of the main stem can be quantified by its form or its taper. Stem form
refers to geometrical shape of the stem. Generally, the tree is divided into three
sections: the lower part of the tree that resembles a neiloid, the central part follows a
paraboloid and the upper part which is often designated as a conoid. Stem taper on
the other hand refers to the change in diameter between two points and the distance
between these. Empirical results have long highlighted the relationship between
crown size and stem taper (Larson 1963).
Besides the role of stem form in forest mensuration, forest ecology has always
tried to understand to what degree taper is influenced by environmental factors. It is
well known that auxins and other hormones such as indole-3-acetic acid (IAA),
gibberellic acid and cytokinin initiate secondary growth in trees (Nilsson et al. 2008;
Immanen et al. 2016). IAA is produced in the apical meristems (Zhao 2010) and
diffused through the stem of the tree, thus initiating its radial growth.
Physiological processes can explain how the taper of a tree varies. However, these
do not readily provide reasons why a tree would need to have more or less taper. The
two most important theories used to explain such variations are based either on
hydraulic or mechanical constraints. These two theories rely on the functions of the
296
R. Schneider
comparison of different allometric exponents, have also been used to gain insight on
the effects of climate on stem form. These three modelling approaches are however
empirically based. Several theories have been proposed to explain the effects of
various factors on stem form, with the hydraulic and biomechanic theories being the
most widely used, and often opposed. Both theories, when simplified to their
simplest expression, underline the importance of crown dimensions in determining
tree form. Nevertheless, these theories cannot explain all of the variations observed
empirically. In the case study of balsam fir, climatic variables such as total summer
precipitation and mean winter temperature are slightly more important in explaining
tree taper, when compared to average wind speed. This signifies that the proposed
theories, be it either hydraulic or biomechanic, should be hybridized with physiological processes in order to account for all the empirical observations.
1 Introduction
Foresters have long been interested in the shape of the main stem of a tree (Pressler
1865; Metzger 1893). The reason of this interest is that tree volume and biomass is
dependant on tree form since the main stem accounts for over 60% of the total
aboveground biomass of a tree with a diameter at breast height larger than 10 cm
(Lambert et al. 2005). With green accounting and biomass estimation on large tracks
of land, small variations in tree form can imply large biases (Weiskittel et al. 2015).
Moreover, sampling height, or lack of homogeneity in sampling height, can strongly
influence the past climate reconstruction when using tree ring chronologies due to
variations in the vertical radial growth distribution (Autin et al. 2015).
The shape of the main stem can be quantified by its form or its taper. Stem form
refers to geometrical shape of the stem. Generally, the tree is divided into three
sections: the lower part of the tree that resembles a neiloid, the central part follows a
paraboloid and the upper part which is often designated as a conoid. Stem taper on
the other hand refers to the change in diameter between two points and the distance
between these. Empirical results have long highlighted the relationship between
crown size and stem taper (Larson 1963).
Besides the role of stem form in forest mensuration, forest ecology has always
tried to understand to what degree taper is influenced by environmental factors. It is
well known that auxins and other hormones such as indole-3-acetic acid (IAA),
gibberellic acid and cytokinin initiate secondary growth in trees (Nilsson et al. 2008;
Immanen et al. 2016). IAA is produced in the apical meristems (Zhao 2010) and
diffused through the stem of the tree, thus initiating its radial growth.
Physiological processes can explain how the taper of a tree varies. However, these
do not readily provide reasons why a tree would need to have more or less taper. The
two most important theories used to explain such variations are based either on
hydraulic or mechanical constraints. These two theories rely on the functions of the
296
R. Schneider