DISCUSSION
Even if we did not find direct evidence of sexual dimorphism in B.
cordata , our results suggests that plant-sex affects arthropod
communities through both top-down and bottom-up mechanisms; and that
this effect has a strong temporal component linked to the plant
phenology. We suggest that seasonal variation on plant sex- arthropod
interactions might be particularly important for plants that experience
seasonal environments and that accounting for plant seasonality effects
has the potential to reconcile previous debates about plant-sex
preference and performance.
Sexual dimorphism inB uddleia
cordata and its implications on sex-biased herbivory
Differences between sexes (i.e., sexual dimorphism) are common in
dioecious plants. These differences that are the result of sex-specific
gene expression and physiological trade-offs affect the arthropod
communities associated with plants of different sexes (Mooney and Singer
2001, Nell et al. 2018). But contrary to our initial expectation, we did
not find sex-related differences in the two plant traits that we
evaluated: leaf thickness and water content (Fig. 1). These results are
consistent with the observations by Moreira et al. (2019) who found no
sex-related differences in water content, and in nitrogen and
phosphorous concentrations in B. cordata during a study conducted
in September 2017. Nevertheless, we observed a strong temporal variation
in the water content of B. cordata ’s leaves that is consistent
with the highly seasonal climate of the preserve, having the highest
leaf water content during the rainy season in August, and the lowest
leaf water content during the dry season in February (Fig. 1). It is
important to highlight that while our results do not suggest sexual
dimorphism, there are multiple important axis of variation that we did
not evaluate. For example, other studies have found differences in the
secondary metabolites content of B. cordata’ s males and females
(Moreira et al. 2019). Furthermore, the female-biased sex-ratio that we
found in B. cordata ’s population at PSAER is consistent with
differences in life-history traits between individuals of different sex.
The response of arthropods to plant sexual dimorphism has been studied
during the last century mostly in the context of herbivore preference
and performance, showing a great support for the hypothesis of
sex-biased herbivory (Danell et al. 1985, Boecklen et al. 1990, Hjaltén
1992, Agren et al. 1999, Cornelissen and Stiling 2005, Granados-Sánchez
et al. 2008, Kabir et al. 2014). This hypothesis suggests that male
plants experience higher herbivory than female plants due to male plants
allocating more resources on vegetative growth, while female plants
allocate more resources into reproduction and defensive traits
(Cornelissen and Stiling 2005, Ribeiro-Mendes et al. 2012). For example,
in a review, Agren et al. (1999) showed that in 25 out of 39 studies
male plants were preferred over female plants by herbivores, and
herbivore densities were higher in male plants than in females during
field experiments. Nevertheless, Sargent and McKeough (2022) in a recent
meta-analysis, that included 58 new observations of herbivory and 41 of
secondary chemistry compared to the original database used by
Cornelissen and Stiling (2005), found evidence that a male-sex bias in
herbivory is not the rule. Our results did not show differences in
herbivory rates between plants of different sex, and thus constitute
another example against the herbivory-bias hypothesis. Nevertheless, it
is important to notice that our quantification method for herbivory
might not adequately capture the damaged produced by sap-sucking
insects, such as Hemiptera, and this group was the second most important
group of herbivores in our sample.
Overall, recent studies contrast with the previously proposed pattern of
male-preference in sex-biased herbivory, by either finding no
differences in herbivory or female-preference examples. Even if we
accept the expectation of female plants allocating more resources on
reproduction compared to male plants, there are many compensatory
mechanisms that may reduce the effect of a higher reproductive resource
assignment on vegetative traits (Delph 1999). Furthermore, in some
scenarios, we might expect male-plant reproductive allocation to be
higher than in female plants, for example in wind-pollinated plants with
a large production of nitrogen-rich pollen (Delph et al. 1993, Harris
and Pannell 2008). In fact, there is evidence of cases in which male
plants show higher concentration of secondary metabolites associated
with defense against herbivores (Hjaltén 1992, Bañuelos et al. 2004,
Zhang et al. 2019, Yang et al. 2020); including our study systemB. cordata (Moreira et al. 2019). Finally, even if we did not
find sex-bias in leaf water content and thickness, we found a strong
seasonal pattern observed in these metrics that raises the question of
whether other variables that show sex-dependence (like the secondary
compounds recorded by Moreira et al. 2019) would also display a temporal
variation pattern. A seasonal dependent pattern of plant traits could
explain some of the contradictory results regarding which plant sex
displays more defensive traits found in the literature (i.e., Sargent
and McKeough 2022). It could also suggest that the effect of plant sex
on arthropod communities varies through time, and it highlights the need
for conducting studies in dioecious plants, especially those
experiencing seasonal environments, while accounting for seasonal
variation.