Changes in sex ratios of a dioecious grass with grazing intensity: the interplay between gender traits, neighbour interactions and spatial patterns

Abstract

Summary1. Dioecious plants frequently face sex-related resource allocation trade-offs. Differential traits of male and female plants can have important consequences that influence their direct and indirect interactions with neighbours and herbivores. 2. Gender differences in traits have been considered to be inherently linked to sex-biased ratios along environmental stress gradients. However, the mechanisms behind the variation in sex ratios with increasing biotic stress remain elusive. 3. We linked the results of process-based field experiments with fine-scale spatial patterns of naturally established plants (both sexes) at different grazing intensities. We demonstrated that slowgrowing females were better defended against insects and small mammals but were more susceptible than males to competition from neighbours. 4. In large-herbivore exclosures, we found that sex-biased spatial patterns at the microsite scale can be predicted from gender-specific traits, with females more segregated from competitors (unpalatable grasses) than males. Traits related to growth/defence trade-off in females, coupled with their higher competition costs than males, suggest that gender dimorphism in traits can directly influence species interactions and spatial organization. 5. In the presence of large herbivores, female individuals were preferentially browsed despite their larger investment in antiherbivore compounds. The greater distance of females from unpalatable competitors was the main determinant of female-biased consumption by domestic herbivores. Females growing close to neighbours successfully eluded browsing but also faced competition. Scaling up to a population level, population sex ratios drift from female-to male-bias with increasing domestic grazing intensity. 6. Synthesis: The sexual dimorphism in traits of a dioecious bunchgrass results in differential herbivory by natural enemies, differential competitive ability and differential spatial distribution of sexes. Domestic grazers disrupt this balance by shifting species interactions and their spatial organization. Changes in dioecious plant populations towards male-biased sex ratios with grazing intensity could not be predicted solely by gender-specific traits.

Fine‐scale root community structure and below‐ground responses to grazing show independence from above‐ground patterns

“…Preferred and non‐preferred species abundance and spatial distribution are markedly affected mainly under intensively grazed sites (Cipriotti & Aguiar ; Graff et al. , ; Golluscio et al. ; Oñatibia & Aguiar ).…”

“…For example, total stock of grass aboveground biomass is lower under intensively grazed conditions compared to those ungrazed and moderately grazed, while grass above-ground production is twice as high in moderately grazed paddocks as in those without grazing and intensively grazed pastures (Oñatibia & Aguiar 2016). Preferred and non-preferred species abundance and spatial distribution are markedly affected mainly under intensively grazed sites (Cipriotti & Aguiar 2005;Graff et al 2007Graff et al , 2013Golluscio et al 2009;Oñatibia & Aguiar 2016). Also, soil organic C and N stocks may decrease with grazing intensity (Golluscio et al 2009) and forage provision and C and N stocks in plant biomass are maximized in moderately grazed areas (Oñatibia et al 2015).…”

Competitive interactions are mediated in a sex‐specific manner by arbuscular mycorrhiza in Antennaria dioica

“…Few studies have evaluated sexual dimorphism in competitive abilities between the sexes of dioecious species (Table S1), and most of them only investigated intraspecific interactions (but see Graff et al . ; Sánchez‐Vilas et al . ).…”

Impact of grazing intensities on reproduction patterns of elm trees (Ulmus pumila) in degraded sandy lands in China

“…Effects of grazing on plant reproduction cause concern (Gu et al, 2017;Perez-Llorca & Vilas, 2019). Grazing could influence the reproduction of plants by altering sex ratio, reproductive biomass, and biomass allocation (Graff, Rositano & Aguiar, 2013;Niu et al, 2009;Niu et al, 2012;Ren, Zheng & Bai, 2009). Most of these studies focus on grass species.…”