Investigating the interaction between ungulate grazing and resource effects on Vaccinium myrtillus populations with integral projection models

Dense ungulate populations in forest accompanied by high grazing intensities have the potential to affect plant population dynamics, and such herbivory effects on populations are hypothesised to differ along environmental gradients. We investigated red deer grazing and resource interaction effects on the performance and dynamics of the functionally important boreal shrub Vaccinium myrtillus using integral projection models (IPMs). We sampled data from 900 V. myrtillus ramets in 30 plots in two consecutive years across the boreo-nemoral pine forest on the island Svanøy, western Norway. The plots spanned two environmental gradients: a red deer grazing intensity gradient (assessed by Cervus elaphus faecal pellets), and a relative resource gradient (DCA-ordination of species composition). The use of IPMs enabled projections of population growth rate (λ) using continuous plant size instead of forcing stage division upon the demographic data. We used the environmental gradients as continuous variables to explain the dynamics of V. myrtillus populations and found that both increasing grazing intensity and resource levels negatively affected λ of the V. myrtillus populations. Interestingly, these factors interacted: the negative effects of grazing were strongest in the resource-rich vegetation, and higher resource levels reduced λ more strongly than at low resource levels when grazing intensities became higher. Populations with λ > 1 were projected if the grazing intensity was less than or equal to the mean grazing intensity on the island, and indicated that V. myrtillus is relatively tolerant of grazing. Variance decomposing showed that the decrease of λ along the grazing gradient, both at low and high resource levels, was largely caused by reductions in plant growth. The use of IPMs together with important environmental gradients offered novel possibilities to study the synthesised effect of different factors on plant population dynamics. Here, we show that the population response of an abundant boreal shrub to ungulate grazing depends on resource level.