Acorn dispersal location and predation risk beneath nurse plants in a Californian oak woodland

Highlights

• Over 91% of acorns planted under shrubs were removed within 62 days.
• Acorn predation by small mammals is greatest at the center of shrubs.
• In open areas without dense shrub cover, acorn predation was lower and slower.
• Acorns along shrub edges may be safest where large and small herbivores coexist.

Oak (Quercus spp.) regeneration is threatened by high rates of acorn predation by small granivorous mammals. While the microhabitat created by nurse plant shrubs is favorable for the growth of oak seedlings and saplings, as they are protected from ungulate herbivores and intense sunlight, it is also beneficial for the small mammals hiding from their predators. For nurse plants to be practical management tools for sustainable oak restoration, acorn predation should be minimized. We investigated how the position of acorns beneath nurse plant shrubs, small mammal predator presence, and the surrounding vegetation structure influence acorn removal by small granivorous mammals. We deployed 225 acorns beneath 45 shrubs and used camera traps and weekly surveys to monitor acorn predation. Over 62 days, 91.6% of acorns were removed, with significantly higher removal rates nearer the shrub center, where small mammals face the least predation risk. Surrounding shrub cover positively predicted acorn removal risk, while predator activity had a strong negative effect. Our findings align with the ecological concept of a "landscape of fear" and demonstrate the spatial heterogeneity in acorn fate beneath shrubs. Despite the protective benefits of nurse plants for oak seedlings and saplings, planting acorns near shrub centers may be counterproductive due to the increased risk of granivory. Instead, oak restoration efforts may consider sowing acorns along the shrub edges and implementing rodent deterrents. This study highlights the importance of wielding animal behavior and microsite dynamics for the conservation of declining oak ecosystems.