sap conduction. Cavitation increases in hot and/or dry conditions (Rennenberg et al.
2006) or in cold conditions (Arias et al. 2017). If cavitation is important enough, it
can lead to the death of the tree (Allen et al. 2010). Thus, trees must adapt themselves
to reduce the effects of cavitation. The changes in stem form with the various
climatic variables studied might be explained by such adaptations. For example, if
a tree is growing in an area with cold winters, it might shift its radial growth
distribution towards the top of the tree, in order to have more sapwood. This safety
factor would thus produce trees with less taper. In the same manner, trees with a
wetter climate will not have the need to produce as much conducting area as
cavitation due to water stress is less likely. The trees would thus have more taper.
These explanations remain hypothetical and would need to be further validated.
6 Conclusion
The debate has been ongoing in order to evaluate if tree shape is determined by either
mechanical or hydraulic constraints. The stem of a tree has to ensure mechanical
stability in order for the tree to support the crown all the while maintaining hydraulic
conductivity. In conifers, the tracheid cells in the stem perform both functions
whereas broadleaved species are composed of fibres and vessels, the prior contributing to biomechanical stability and the latter to sap conductivity. Some studies have
found that hydraulic constraints are more important, while others reported that
biomechanical theories better explain tree growth and form (Farnsworth and
Gardingen 1995; Domec and Gartner 2002; Eloy et al. 2017).
Others have tried to propose optimization approaches between the different
functions of a tree (Farnsworth and Niklas 1995), relying on the underlying objective
that a tree optimizes its volume occupancy (West et al. 1999, 2009). Nevertheless,
these approaches oppose hydraulic to biomechanical functions. These functions are
however intimately related because often the same cells carry both functions out in
conifers or both types of cells (i.e. conducting and support) are present in
broadleaved species. Thus, optimization processes opposing hydraulic to biomechanical might not be necessary, since if one function is secured, the other should
also be ensured.
Furthermore, the results presented here indicate that climatic variables are slightly
more important in determining tree form, when compared to average wind speed. It
could be that variations in wind are more important than wind speed per
se. However, these two methods of quantifying wind could not be disentangled
with the data on hand. Nevertheless, the results indicate that other processes must
also be considered in developing theories of stem form. For hydraulic theories, there
is a need to better understand the relationships between sapwood and used pipe area,
or revisit the pipe model theory in order to better relate the state variables with
measurable metrics found in a tree. In biomechanical approaches, some physiological considerations must be included, such as adaptations to cavitation and their
implications on stem wood production. In other words, instead of opposing
Understanding the Factors Influencing Stem Form with Modelling Tools
311
2006) or in cold conditions (Arias et al. 2017). If cavitation is important enough, it
can lead to the death of the tree (Allen et al. 2010). Thus, trees must adapt themselves
to reduce the effects of cavitation. The changes in stem form with the various
climatic variables studied might be explained by such adaptations. For example, if
a tree is growing in an area with cold winters, it might shift its radial growth
distribution towards the top of the tree, in order to have more sapwood. This safety
factor would thus produce trees with less taper. In the same manner, trees with a
wetter climate will not have the need to produce as much conducting area as
cavitation due to water stress is less likely. The trees would thus have more taper.
These explanations remain hypothetical and would need to be further validated.
6 Conclusion
The debate has been ongoing in order to evaluate if tree shape is determined by either
mechanical or hydraulic constraints. The stem of a tree has to ensure mechanical
stability in order for the tree to support the crown all the while maintaining hydraulic
conductivity. In conifers, the tracheid cells in the stem perform both functions
whereas broadleaved species are composed of fibres and vessels, the prior contributing to biomechanical stability and the latter to sap conductivity. Some studies have
found that hydraulic constraints are more important, while others reported that
biomechanical theories better explain tree growth and form (Farnsworth and
Gardingen 1995; Domec and Gartner 2002; Eloy et al. 2017).
Others have tried to propose optimization approaches between the different
functions of a tree (Farnsworth and Niklas 1995), relying on the underlying objective
that a tree optimizes its volume occupancy (West et al. 1999, 2009). Nevertheless,
these approaches oppose hydraulic to biomechanical functions. These functions are
however intimately related because often the same cells carry both functions out in
conifers or both types of cells (i.e. conducting and support) are present in
broadleaved species. Thus, optimization processes opposing hydraulic to biomechanical might not be necessary, since if one function is secured, the other should
also be ensured.
Furthermore, the results presented here indicate that climatic variables are slightly
more important in determining tree form, when compared to average wind speed. It
could be that variations in wind are more important than wind speed per
se. However, these two methods of quantifying wind could not be disentangled
with the data on hand. Nevertheless, the results indicate that other processes must
also be considered in developing theories of stem form. For hydraulic theories, there
is a need to better understand the relationships between sapwood and used pipe area,
or revisit the pipe model theory in order to better relate the state variables with
measurable metrics found in a tree. In biomechanical approaches, some physiological considerations must be included, such as adaptations to cavitation and their
implications on stem wood production. In other words, instead of opposing
Understanding the Factors Influencing Stem Form with Modelling Tools
311