Abstract
Male and female plants of dioecious species often experience differential herbivory, possibly due to differences in defences such as secondary metabolite composition or nutritional quality. These plant sex effects on herbivory have been extensively studied for plant individuals, but not for stands/populations. For mobile herbivores, such as deer, stands may be a more relevant scale to study than individual plants. We predicted that male Salix viminalis plants should be subject to more extensive roe deer (Capreolus capreolus) browsing than female plants due to weaker defence in male plants. Furthermore, we expected that mixed-sex stands should experience more damage than monosexual stands due to positive effects of diet mixing on browsing by generalists. We tested for differences in roe deer browsing in plots that were either monosexual male or female, or a mix of male and female plants in a replicated field experiment. Roe deer browsing was estimated after one growth season with heavy herbivory. We also measured plant secondary metabolite concentrations and nitrogen content in leaves from all experimental clones to test the assumption that the sexes differed in defence or nutrients. Mixed-sex plots were more extensively browsed than monosexual plots. However, there was no difference in browsing between male and female plant individuals within mixed-sex plots or between monosexual plots. Plant secondary metabolite profiles differed between male and female plants, while nitrogen content did not. Our findings suggest that the diversified plant secondary metabolite contents of mixed-sex plots may have led to more extensive herbivory. Higher browsing of plant sex mixes may impact both natural and commercial S. viminalis stands with different sex ratios.
Highlights
Sex-biased herbivory is common for plants and has long received attention
In addition to leaf chemistry, shoot tissue plant secondary metabolite concentrations often vary between sexes of Salix spp.; total phenol concentration is higher in female S. lasiolepsis (Price et al, 1989) and salicortin concentrations have been found to be highest in stems of females of both S. rigida (Elmqvist et al, 1991) and S. myrsinifolia (Nybakken and Julkunen-Tiitto, 2013)
Male and female plants did not differ in total concentrations of either phenolic acids (χ2 = 1.92, P = 0.17, d.f. = 1, Fig. 3a) or lignans (χ2 = 0.29, P = 0.59, d.f. = 1, Fig. 3b)
Summary
Sex-biased herbivory is common for plants and has long received attention (reviewed by Cornelissen and Stiling, 2005). Reported studies of sex-biased herbivory have mainly dealt with effects on individual plants while effects over larger spatial scales, such as stands or populations, are not well-understood. Investigating plant sex effects at larger spatial scales is important, because plant sex ratios are often skewed and vary among species, populations, and stands, ranging from strongly female- to strongly male-biased (Barrett et al, 2010). Sex-biased abundances are at least in part dependent on genetic factors in several dioecious species (Alström-Rapaport et al, 1997; Åhman, 1997; Barrett et al, 2010). Sex-biased abundances can be explained partly by a higher mortality among plants of either sex (Lloyd, 1974), possibly due to herbivory (Elmqvist and Gardfjell, 1988)
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