Odour guided predation on acorns by small rodents during direct seeding

Costs related to planting of seedlings and plant material is a major economic obstacle that may impede successful large-scale forest restoration. One way to reduce this cost is to use direct seeding instead of planting. Nut-bearing trees such as oaks have seeds with large nutrient storage that results in competitive seedlings. However, the main problem of direct seeding of oak is the high predation rates on buried acorns by small rodents, which use their well-developed olfactory sense to locate buried seeds. In this thesis, I therefore, investigated different aspects of rodent predation on acorns and how they use their olfactory sense to locate these seeds. In a review, I synthesised current knowledge about the mechanisms in rodent predation on seeds and acorns. I also snap-trapped rodents to investigate if they use odours from the acorn nut or the shell to find acorns. The headspace of the two seed parts was analysed by using combined gas chromatography and mass spectrometry (GC-MS). Finally, in a laboratory study, I developed a two-choice behavioural assay to quantify lab-bred bank voles’ odour preference. I tested if they were attracted to the odour of acorns and their common laboratory diet, and also analysed the odourants emanating from these two foods. The review showed that a wide variety of protection methods have been tested. Physical protection was among the most successful methods but lacked possibilities for largescale implementation due to high costs. Few studies have addressed how rodents use odours to find acorns although this knowledge could be used to interfere with rodents’ foraging. In my field study, odours from both the nut and the shell were attractive to forest rodents, but the nut was more attractive when cover from avian predation was higher, indicating that rodents may use a trade-off with accuracy for protection when foraging. The seed parts’ headspace showed that they shared many compounds. When quantifying bank vole odour preference by the time spent in a choice arm, short time intervals performed best. Furthermore, naïve bank voles were as attracted to acorns as to their common laboratory food, indicating that bank voles have an innate preference for acorn odour. In conclusion, this thesis show that rodents are well-adapted to locate acorns using olfaction. The preference of odourants can be tested in field or laboratory, and since rodents rely heavily on smell, odour manipulations have the potential to provide costefficient protection to direct-seeded acorn