Discussion
Thinning increased the structural complexity of individual Scots pine
trees confirming our hypothesis H1. Crown projection area showed a
positive relationship (mean correlation coefficient >0.6)
with structural complexity whereas tree height did not, leading to
partially accepting the hypothesis about associations between structural
complexity and horizontal and vertical measures of Scots pine trees
(H2). Finally, the hypothesis H3 is also partly accepted as there was
practically no relationship between structural complexity and
benefit-to-cost ratio, tree growth, or the light availability but crown
and growth attributes were, however, significant when estimating
structural complexity. Forest management, and thinning especially,
affected structural complexity of individual Scot pine trees. Stem,
crown, and growth variables were found significant predictors for it
indicating them as drivers for structural complexity.
Crown dimensions affected structural complexity more than stem
attributes (i.e. DBH, tree height, and volume) which is similar findings
to Seidel et al. (2019b) who studied the relationship between structural
complexity and stem and crown attributes of four deciduous species (i.e.Fagus sylvatica L., Fraxinus excelsior L., Acer
pseudoplatanus L., and Carpinus betulus L.). Thinning affected
stem or growth attributes, benefit-to-cost ratio, and light availability
of Scots pine trees, as either moderate or intensive thinning resulted
in differing values for these attributes compared to Scots pine trees
without a thinning treatment (i.e. control plots).
There was no relationship (R2<0.2 and
correlation coefficient <0.1, not significant) between
structural complexity and benefit-to-cost ratio indicating that it does
not affect structural complexity of Scots pine trees. This is
contradictory to the findings by Seidel et al. (2019a) who found
R2 of at least 0.25 between structural complexity and
cost-to-benefit ratio for 76 deciduous trees (i.e. 46 Fagus
sylvatica L., 25 Fraxinus excelsior L., and 5 Acer
pseudoplatanus L.). Their study site was a mixed, unmanaged deciduous
forest and there were ~150 trees/ha whereas in our study
the tree density per ha was at least twice of that. Thus, it can be
assumed that there was more space for the deciduous trees to grow.
Additionally, tree form in general is different between conifers and
deciduous trees due to the difference in their shoot growth, in other
words the degree of apical dominance differs being strong for conifers
and weaker for deciduous trees (Kozlowski 1964). This can produce
differences in structural complexity and its relationship to
benefit-to-cost ratio.
Light availability (measured through competition) did not affect
structural complexity of Scots pine trees and there was no relationship
between them. However, larger variation in light availability was found
in plots without thinning treatments, which is expected as it has shown
in previous studies that thinning decreases competition (Mäkinen &
Isomäki 2004, Juchheim et al. 2017, Jacobs et al. 2020). Seidel et al.
(2019a) reported decreasing structural complexity when competition
increased but their study only included 93 read oak (Quercus
rubra L.) trees from ten different sites compared to our
~740 Scot pine trees. Similar to the results of
benefit-to-cost ratio discussed above, deciduous trees have a different
branching pattern compared to conifers. Future research should focus on
the response of structural complexity to light availability of different
species with enough trees to clearly understand their relationship.
Ehbrecht et al. (2017) studied stand structural complexity in mixed
stands with even-aged and uneven-aged forest management. They discovered
that stand structural complexity differed between tree species and
even-aged coniferous stands had less complex structure compared toFagus sylvatica (L.) stands. We only studied one tree species,
namely Scots pine, but found that structural complexity varied between
trees with different management history, although all the plots were
even-aged and single layer that could have decreased their structural
complexity.
Forest structural complexity has been widely studied (Camarretta et al.
2020, McElhinny et al. 2005, Ishii et al. 2004) but there is less
research on tree structural complexity, which was the focus of this
study. TLS can provide traditional tree attributes (i.e. DBH, volume,
crown dimensions) for characterizing tree structure but its potential
can also be expanded for new attributes. Here, we only utilized box
dimension to measure structural complexity, which can be considered as a
weakness of the study as we cannot compare these results to other
measures. However, the box dimension was chosen because it was easy to
calculate, it has shown its potential in characterizing individual tree
structure (Seidel et al. 2019a, b) , and because no other measures for
individual tree structural complexity was found in the literature. As it
was shown, there was a relationship between box dimension and stem,
crown, and growth attributes indicating that these attributes can
explain structural complexity if individual trees. None of them,
however, considers the entire 3D tree structure to which the box
dimension brings added value. The strength of the study is that the
study design allowed us to compare forest management practices without
confounding factors that could have been a challenge in uneven-aged or
mixed-species stands.
Forest management in general, and thinning in particular, controls the
between-tree competition especially by removing part of shadowing canopy
mass to enhance the growth of remaining trees (White 1980). Although
there is a strong economic incentive in forest management for wood
production (Puettman et al. 2015), there is an increasing understanding
how forest management can also be applied for supporting diversity of
forest structure (Bergeron et al. 2002, Kuuluvainen 2009), biodiversity
(Fedrowitz et al. 2014), resilience (Messier et al. 2013), and carbon
uptake (Hardiman et al. 2011). Additionally, there is an increasing
respect towards recreational opportunities and landscape amenities
(Butler & Leatherberry 2004, Hugosson & Ingemarson 2004, Urquhart &
Cortney 2011). Therefore, structural diversity has been identified as a
silvicultural principle that could be addressed with non-conventional
forest management practices (Puettmann et al. 2015). Furthermore, forest
structural variety can be used as a measure for biodiversity and
structurally complex forests enhance carbon uptake (Gough et al. 2019),
which are important ecosystem services provided by forests. Thus, it can
be expected that structural diversity can also be beneficial for
functional and species diversity of forests.