A lot of variability in the reproductive and vegetative output across populations and years can be envisioned from this study. This is not surprising, especially in a masting species like F. ornus
in which low-producing years alternate with mast years. Despite this temporal and spatial variability, most of the estimates of sexual dimorphism point in the direction of males growing more than hermaphrodites, as expected under the hypothesis of lower cost of male reproduction. This fact suggests that gender-associated costs of reproduction in F. ornus
are present in good and bad years and populations. This finding is consistent with many studies from dioecious species, although some others have failed to find such differences (Delph, 1999
; Obeso, 2002
; Delph et al., 2005
Interspecific differences in the gender-associated costs of reproduction may come from different selective pressures in each species. For example, males from some species may be strongly selected for massive pollen production. Zahoueh et al. (1991)
showed that males of Pistacia terebinthus
allocated 8 times more biomass to flowers but 1·5–3 times less biomass to overall reproduction than females. Similarly, the female reproductive effort of Simmondsia chinensis
was less than that of males unless seed set exceeded 30 % (Wallace and Rundel, 1979
). Under these circumstances of selection for massive pollen production, reproductive effort for females might not be necessarily greater than that of males (Verdú and García-Fayos, 1998
). At first glance, one could think that males of F. ornus
might be strongly selected for massive pollen production to compensate for the loss of female function relative to hermaphrodites, as the theory of androdiecy predicts (Pannell, 2002a
). Indeed, it was found here that males produce significantly more flowers than hermaphrodites, but the magnitude of such difference was rather small (see below for a discussion on the comparative magnitudes of the gender effect sizes). It is known that F. ornus
males do not compensate for the loss of the female function with a massive pollen production, but through the higher siring ability of the pollen grains (Verdú et al., 2004
Reproductive costs might not necessarily be different between genders if the gender carrying the greater reproductive burden develops some compensatory mechanism, for example, to expand the photosynthetic period and thus to increase the resource intake (Tuomi et al., 1983
). This is not the case for F. ornus
because gender-associated differences in the length of photosynthetic period have not been found. Although different compensation mechanisms may exist, like photosynthesis of fruits, nutrient resorption from senescent structures, sex-differential age at maturity and longevity (Obeso, 2002
; Verdú, 2004
), these do not seem to be equilibrating the costs of reproduction between genders because the present results clearly show that males consistently grow more quickly than hermaphrodites across years and at the extremes of the species range.
In some cases, the failure to detect gender-associated costs of reproduction may come from a lack of statistical power rather than a reduced effect of gender on reproductive and vegetative traits (see the P
-values of the ANOVA traditional tests in Table ). This situation is particularly evident when the distribution of effect sizes that are non-significant under traditional statistical tests is not centred around zero (Osenberg et al., 1999
). In the absence of gender effects, the expected distribution of data would be 50 % negative and 50 % positive values. A single small positive mean may fail to reject the null hypothesis because of low statistical power. However, the meta-analysis may accumulate the evidence pointing towards the same direction by averaging many small positive means and then reject the null hypothesis. For example, Verdú (2004)
failed to detect significant differences in vegetative growth between both genders of F. ornus
in spite of the trend of males growing more than hermaphrodites. Now, meta-analytical tools have allowed us not only to reject the null hypothesis (i.e. males and hermaphrodites do not differ in vegetative and reproductive parameters) but also to quantify how large the difference between genders is.
In the context of behavioral sciences, Cohen (1988)
defined three cut points that allow the separation of small (d
=0·2–0·5), medium (d
=0·5–0·8) and large (d
>0·8) effect sizes. These are standardized mean difference effect sizes that can be converted to correlation coefficients (Verdú and Traveset, 2004
) corresponding to percentages of the explained variance of 1–6·24 %, 6·25–16 % and >16 %, respectively. The usual values of explained variance in ecological and evolutionary meta-analyses only range from 2·5 to 5·4 % (Møller and Jennions, 2002
). The effect sizes shown here for reproductive and vegetative characters are d
=−0·31 and d
=−0·45 respectively, which accounts for 2·3 % and 4·8 % of the variance respectively. These magnitudes are small in the traditional Cohen's scale, but in an ecological context the effects of gender should be considered as relatively small for reproductive traits but medium to high for vegetative characters.
Contrasting ecological conditions may produce intraspecific variation in the gender-associated costs of reproduction. Some authors have stated that harsh conditions may promote gender differences because the gender with greater reproductive costs is more affected by such conditions (Dawson and Bliss, 1989
; Dawson and Ehleringer, 1993
; Dawson and Geber, 1999
; Delph, 1999
; Obeso, 2002
). No evidence has been found in F. ornus
for such a prediction because the gender-associated differences observed in vegetative and reproductive characters were independent of the stress conditions like short frost-free periods or high moisture deficits. Other factors like herbivory, soil characteristics, competition, etc. could also be acting as stressors; it would be interesting for future studies to relate these factors with sexual dimorphism in reproduction and growth.
In summary, the lower costs of reproduction for males allows them to grow more quickly than hermaphrodites, although such differences in sex-specific reproductive costs are not magnified under stressful conditions.