Tamm Review: Direct seeding to restore oak (Quercus spp.) forests and woodlands
The scale of opportunity to implement forest restoration exceeds billions of ha worldwide, and a part of this area in northern temperate regions includes formerly oak (Quercus spp.)-dominated ecosystems. The cost of achieving restoration is often high, and tends to increase with severity of ecosystem degradation. Therefore, it is important to develop cost-efficient regeneration practices to support forest restoration. Direct seeding of acorns, one of the earliest artificial forest regeneration techniques developed, offers several benefits including costs that can be about a third of planting oak seedlings. However, direct seeding is presently a more uncertain practice for establishing oak forests and woodlands than is planting seedlings. Much of this uncertainty can be a result of acorn depredation by rodents even though other factors may also limit oak establishment. This review provides a synthesis of foundational knowledge, particularly from Europe and North America, pertaining to the biology and ecology of acorn production, acorn germination, early seedling growth, and acorn depredation by granivorous rodents. We build on this knowledge to review research on operational direct seeding practices and four basic strategies for acorn depredation control (chemical, ecological, physical and silvicultural). We suggest that environmentally sound approaches to minimize acorn depredation and increase seedling establishment currently exists. For example, seeding operations probably hold higher chances for success if scheduled during years of high masting, if applied on open sites (areas with little shrub and tree cover), and if acorn lots are sized and sown at an appropriate depth. Several gaps in our knowledge limit the development of improved operational practices for broader success. We lack information on basic acorn biology and proper handling and storage practices for many of the oaks around the world. And, we need research to support development of new techniques or combinations of strategies and techniques that minimize acorn depredation under a wide range of site conditions. Nevertheless, our synthesis suggests that direct seeding will remain a cost-effective and environmentally sound practice for oak forest and woodland establishment, and it will likely gain expanded use as future research informs innovation that increases seed availability, decreases acorn depredation, and raises predictability of oak seedling establishment.