Spatial correlations between browsing on balsam fir by white‐tailed deer and the nutritional value of neighboring winter forage

Abstract

Associational effects, that is, the influence of neighboring plants on herbivory suffered by a plant, are an outcome of forage selection. Although forage selection is a hierarchical process, few studies have investigated associational effects at multiple spatial scales. Because the nutritional quality of plants can be spatially structured, it might differently influence associational effects across multiple scales. Our objective was to determine the radius of influence of neighbor density and nutritional quality on balsam fir (Abies balsamea) herbivory by white‐tailed deer (Odocoileus virginianus) in winter. We quantified browsing rates on fir and the density and quality of neighboring trees in a series of 10‐year‐old cutovers on Anticosti Island (Canada). We used cross‐correlations to investigate relationships between browsing rates and the density and nutritional quality of neighboring trees at distances up to 1,000 m. Balsam fir and white spruce (Picea glauca) fiber content and dry matter in vitro true digestibility were correlated with fir browsing rate at the finest extra‐patch scale (across distance of up to 50 m) and between cutover areas (300–400 m). These correlations suggest associational effects, that is, low nutritional quality of neighbors reduces the likelihood of fir herbivory (associational defense). Our results may indicate associational effects mediated by intraspecific variation in plant quality and suggest that these effects could occur at scales from tens to hundreds of meters. Understanding associational effects could inform strategies for restoration or conservation; for example, planting of fir among existing natural regeneration could be concentrated in areas of low nutritional quality.

Defensive chemicals of neighboring plants limit visits of herbivorous insects: Associational resistance within a plant population

“…Both density and associational effects can be applied to intraspecific interactions within a population (e.g., the effects of neighbor plants that are of the same or different phenotypes and/or genotypes, respectively). The spatial heterogeneity in a plant community or population influences abundance and foraging behavior of herbivores, thereby determining the damage level of plants (Champagne, Moore, Côté, & Tremblay, ; Stastny & Agrawal, ; Underwood et al, ). Whether the associational effect is beneficial (i.e., associational resistance) or detrimental (i.e., associational susceptibility) from the perspective of plants (Barbosa et al, ) is likely to be determined by the foraging behaviors of herbivores, depending on a fine‐scale assortment of plants with different palatabilities.…”

“…Most previous studies have considered only associational effects involved in interspecific interactions, such as plant species diversity (e.g., Andow, 1991). Similar arguments on associational effects would also apply to intraspecific interaction, that is, interactions among conspecific plants with different phenotypes and/or genotypes (Champagne et al, 2018;Coverdale, Goheen, Palmer, & Pringle, 2018). For example, within a population of Solidago altissima, genotypic diversity of co-occurring plants decreased herbivory on the focal plants (i.e., associational resistance), although the mechanism was unclear (Genung, Crutsinger, Bailey, Schweitzer, & Sanders, 2012).…”

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

“…We measured deer browsing and stem abundance in 2014 in the cutover zones of one of these management enclosures (size: 1.4 km 2 ), where 56% of the surface was clear‐cut and fenced in 2000, and the remaining 34% is residual, unharvested forest (see map in Appendix , originally published in Champagne et al, 2018b). The residual forest was composed of mature balsam fir stands, more than 80 years old.…”

“…We used a stratified systematic sampling design incorporating random non‐aligned points (Jensen, 2005), a design fully described in Champagne et al (2018b), to position 125 random points throughout the study area (Figure of Appendix ). We centred a circular plot of 4 m 2 (diameter = 1.13 m) on the closest balsam fir (at least 0.25 m high) for each random point (hereafter focal fir; mean height of focal firs = 1.3 m, SD = 0.7).…”

“…In a natural environment, Moore et al (2010) demonstrated that rates of tree use by koalas ( Phascolarctos cinerus ) were influenced both by the size and nutritional value of heterospecific and conspecific neighbours. For mammals, few other studies have evaluated how intraspecific variation in heterospecific neighbours influences the strength and direction of associational effects (but see Miller et al, 2007; Champagne et al, 2018b).…”