Intraspecific variation in nutritional traits of neighbouring plants generates a continuum of associational effects

Aims: When deciding whether or not to eat a plant, herbivores are influenced by the nutritional value of potential foods, but also indirectly by neighbouring plants (associational effects). We aimed to investigate how the abundance and nutritional quality of neighbours of balsam firs (Abies balsamea) affects browsing on balsam firs by white-tailed deer. We sought to distinguish the effects of conspecific and heterospecific neighbour abundance, and to evaluate whether intraspecific variation in nutritional traits produces associational effects. Location: Anticosti Island, QC, Canada. Methods: We measured the abundance of stems in 4-m2 plots centred on a focal balsam fir and evaluated nutritional value (nitrogen content and digestibility) of focal firs and neighbouring plants. We used generalized linear models to explain browsing on firs as a function of the neighbouring stem abundance (conspecific and heterospecific), and of the nutritional value of firs and neighbouring stems. Results: Fir abundance only affected browsing of firs after accounting for fir nutritional value, e.g., browsing in plots of low fir digestibility decreased as the number of fir stems increased (resource dilution effect). The associational effects of heterospecific neighbour abundance, especially birch stem and shoot abundance, were also contingent on neighbour nutritional value. For example, at low birch digestibility, browsing on the focal fir increased with the number of neighbouring birches. Browsing on fir also increased as the number of spruce stems of high nutritional value increased. Conclusions: By taking into account the abundance of all species, we could discriminate between conspecific effects and associational effects caused by heterospecifics. Our study suggests that the strength of associational effects varies in a continuous fashion in response to relative nutritional value. We propose that future studies should not only consider the identity of neighbours but also the nutritional traits of plant communities.