Prescribed fire and partial overstory removal alter an acorn–rodent conditional mutualism

Abstract

In eastern North America, oak (Quercus) regeneration failure has spurred management using silvicultural approaches better aligned with the autecology of oaks. In particular, shelterwood harvests can create favorable intermediate light conditions for oak establishment and prescribed fire is predicted (by the oak–fire hypothesis) to favor oak regeneration. These approaches substantially modify forest structure and may affect crucial trophic interactions including the conditional mutualism between oaks and granivorous rodents that scatterhoard acorns, which shifts along a continuum from antagonistic to mutualistic depending on external factors. We investigated how overwinter survival and dispersal of northern red oak (Quercus rubra) acorns were influenced by location within or outside of group shelterwood harvests (small canopy gaps created throughout an intact forest stand) with and without prescribed fire. We conducted two concurrent experiments to test (1) dispersal and survival of acorns presented on the forest floor and (2) acorn pilferage rates from caches that mimic squirrel scatterhoards in shelterwood gap/group interiors, edges, and the uncut forest matrix in burned and unburned forest stands. In both experiments, acorn survival was generally higher in burned than unburned stands. Acorn survival from forest floor presentations was higher in the unharvested forest matrix than harvest gap interiors; however, there was no effect of proximity to harvest gaps on survival of cached acorns. Survival of cached acorns was associated with understory vegetative cover (−), coarse woody debris cover (−), and distance to nearest tree (+), but uncorrelated with canopy cover above the cache. Our results suggest that reduced understory cover following prescribed fire may increase perceived habitat riskiness for granivores resulting in higher acorn survival up to 2 yr post‐fire. These findings unify the oak–fire and oak–granivore conditional mutualism hypotheses, and suggest that the environmental conditions following prescribed fire and group shelterwood harvests may shift the oak–granivore conditional mutualism in a direction beneficial for oak regeneration.

Small mammal personalities generate context dependence in the seed dispersal mutualism

“…To understand how mutualistic interactions shape communities and ecosystems, we must identify the mechanisms that underlie their functioning. Synzoochory, for example, has been termed a “conditional mutualism” ( 22 , 23 ) since outcomes vary depending on several biotic and abiotic factors, such as the ratio of seeds to scatter hoarders, the composition of seeds available, soil moisture, fire events, tree density, and seasonal temperature and precipitation ( 22 , 24 – 27 ). If individual dispersers differ in their propensities to contribute in mutualistic or antagonistic manners, this would mean that certain individuals may be ultimately more important to the seed dispersal mutualism [i.e., keystone individuals ( 28 )].…”

Tree encroachment impacts on seed predator selection and seedling establishment in degraded pine woodlands

“…Microhabitat structure is another factor known to influence seed predator behavior (Orrock et al, 2004). In general, microhabitats with vegetation cover are thought to provide favorable environments for seed predators at risk of predation from larger predators (Matos & Orrock, 2010; Greenler et al, 2019). Also, variability in timing and frequency of prescribed fire affects the distribution of plant types and seed production (Lashley et al, 2014) and the phenology of fruiting (Lashley et al, 2015) which may contribute to seed depredation risks by influencing seed availability.…”