(Klos et al. 2007; Schneider et al. 2013) and have been attributed to differences in
climatic conditions.
Studies on the influence of different climatic variables on stem shape have shown
that temperature and precipitations can both result in trees with different forms. Jack
pine (Pinus banksiana) tree form was found to vary with the growing season degreedays (Morris and Forslund 1992). On the other hand, Nigh and Smith (2012)
reported that mean annual precipitation and Julian date of the first frost were the
best climatic variables to explain taper, where taper increased with higher values of
these two variables. They hypothesized that the tree crowns were under the control
of these variables, and thus that both climatic variables had an indirect effect on tree
shape through changes in crown length. Differences in stand dynamics and wind
patterns were also suggested as possible reasons for observing shifts in stem form.
Lee et al. (2006) linked changes in the taper in the lower part of Korean red pine
(Pinus densiflora) to the amount of precipitations late in the growing season whereas
shifts in tree form in the upper part of the stem were related to temperature,
precipitation and wind during the growing season. Crown dynamics, and their effect
on tree biomechanics were used to explain these observations.
The effect of climatic variables varies between species. The variations in stem
form with growing season temperature were found to be more important in conifers
when compared to broadleaved species (Franceschini et al. 2016). This result was
interpreted using differences in primary and secondary growth rates between these
clades, as broadleaved species generally have less apical dominance than conifers
(Millet 2012). A biomechanical explanation could also be put forth, whereby
broadleaved species, due to their architecture, are less prone to wind pressure
through the damping effect (James et al. 2006; Eloy 2011). Differences in stem
form between species have also been linked to certain functional traits such as shade
and waterlogging tolerance (Schneider et al. 2018). Thus, species with higher shade
or waterlogging tolerances will decrease their taper, leading to an increase of up to
6% in tree volume when actual and projected climatic variables are compared. It was
hypothesised that these observations were explained by hydraulic limitations.
Wind has been found to greatly effect tree form. Guyed trees show lower radial
increment at breast height when compared to unguyed trees (Moore et al. 2014). In
fact, trees under less wind stress demonstrate a shift in their allocation patterns
(Meng et al. 2006), as guyed trees invest more in height growth than radial growth.
In other words, radial growth at given point in the stem is closely related to the
bending moment above that point (Meng et al. 2006). Gardiner et al. (2016) presents
a review on the effects of wind on plant growth and adaptation through a biomechanical perspective.
5.2 Untangling the Effect of Wind and Climate on Stem Form
It is however difficult to untangle the confounding effects of wind and other climatic
variables in order to evaluate which effect, wind or climate, is the most important in
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R. Schneider
climatic conditions.
Studies on the influence of different climatic variables on stem shape have shown
that temperature and precipitations can both result in trees with different forms. Jack
pine (Pinus banksiana) tree form was found to vary with the growing season degreedays (Morris and Forslund 1992). On the other hand, Nigh and Smith (2012)
reported that mean annual precipitation and Julian date of the first frost were the
best climatic variables to explain taper, where taper increased with higher values of
these two variables. They hypothesized that the tree crowns were under the control
of these variables, and thus that both climatic variables had an indirect effect on tree
shape through changes in crown length. Differences in stand dynamics and wind
patterns were also suggested as possible reasons for observing shifts in stem form.
Lee et al. (2006) linked changes in the taper in the lower part of Korean red pine
(Pinus densiflora) to the amount of precipitations late in the growing season whereas
shifts in tree form in the upper part of the stem were related to temperature,
precipitation and wind during the growing season. Crown dynamics, and their effect
on tree biomechanics were used to explain these observations.
The effect of climatic variables varies between species. The variations in stem
form with growing season temperature were found to be more important in conifers
when compared to broadleaved species (Franceschini et al. 2016). This result was
interpreted using differences in primary and secondary growth rates between these
clades, as broadleaved species generally have less apical dominance than conifers
(Millet 2012). A biomechanical explanation could also be put forth, whereby
broadleaved species, due to their architecture, are less prone to wind pressure
through the damping effect (James et al. 2006; Eloy 2011). Differences in stem
form between species have also been linked to certain functional traits such as shade
and waterlogging tolerance (Schneider et al. 2018). Thus, species with higher shade
or waterlogging tolerances will decrease their taper, leading to an increase of up to
6% in tree volume when actual and projected climatic variables are compared. It was
hypothesised that these observations were explained by hydraulic limitations.
Wind has been found to greatly effect tree form. Guyed trees show lower radial
increment at breast height when compared to unguyed trees (Moore et al. 2014). In
fact, trees under less wind stress demonstrate a shift in their allocation patterns
(Meng et al. 2006), as guyed trees invest more in height growth than radial growth.
In other words, radial growth at given point in the stem is closely related to the
bending moment above that point (Meng et al. 2006). Gardiner et al. (2016) presents
a review on the effects of wind on plant growth and adaptation through a biomechanical perspective.
5.2 Untangling the Effect of Wind and Climate on Stem Form
It is however difficult to untangle the confounding effects of wind and other climatic
variables in order to evaluate which effect, wind or climate, is the most important in
306
R. Schneider