New approach to asisted migration
Earth’s climate has undergone various changes throughout history. Estimates indicate that around 50 million years ago, the global mean temperature was approximately 13 °C higher than in the late 20th century [1]. What sets the present-day changes apart is the unprecedented rate at which they are occurring. Even optimistic projections foresee an accelerated warming that took millions of years to reverse in the past [2].
Climate change leads to conditions like increased temperatures and extended periods of drought, which put trees under stress and elevate their vulnerability to diseases and pests. While trees can adapt to some extent through phenotypic plasticity, these adaptations are often insufficient in counteracting the rapid environmental shifts [3].
One way how species response to climate change is by migration to areas with more suitable conditions. Nevertheless most tree seeds and pollen are typically confined to a limited range, often a few tens of meters from the parent tree. To adequately adapt to fast-paced climate changes, some models suggest that the migration rate of tree species may need to be up to a hundred times faster than what has been historically observed [3].
Therefore, human-aided conservation strategies such as “assisted migration” have been gaining traction to align with climate change’s rapid pace. This strategy involves deliberately relocating tree populations to areas selected for their projected favorable climate conditions.
The use of distant seeds or seedlings in assisted migration has sparked debates in science and forestry due to its complex challenges. Four key challenges that limit wider utilization of assisted migration include: current trends in forestry, promoting closer to nature forestry, phytosanitary risks associated with plant translocation, economic and expertise challenges for local nurseries, and legal barriers to the transportation and utilization of seeds and plant material across borders and regions.
We propose a novel approach to assisted migration, centered around the deliberate transfer of pollen rather than seeds or seedlings. Since pollen cannot be directly sown into soil, it becomes essential to utilize a specific platform. Seed orchards, clonal tree plantations dedicated to producing genetically improved forest reproductive material, can serve as an effective platform for gathering distant pollen. This pollen can then be hybridized within local seed orchards during the peak receptivity of female strobili.
Seed orchards offer several advantages. The relatively low height of trees simplifies pollination and the collection of both pollen and seeds, making the process cost-effective. Another advantage is the dense network of seed orchards in Europe, enabling the selection of the most suitable pollen source based on predicted climate scenarios for the reforestation site. Additionally, seed orchards consist of the best locally adapted trees, ensuring that half of the genetic information passed on to the offspring originates locally. This aspect aligns with the principles of modern forestry, emphasizing the conservation of local genotypes and potentially encouraging a broader adoption of the assisted migration strategy.
Seeds produced in local seed orchards will be sown in a nearby nursery, allowing local conditions to influence gene expression through epigenetic mechanisms [5]. This modulation fosters better adaptation to the local environment. Moreover, this approach supports smaller states with weaker economies by aiding local nurseries that might otherwise face competition from larger and more economically powerful states. The seedlings cultivated by these local nurseries will then be planted in nearby reforestation sites, further enhancing their local adaptation.
We believe that this innovative approach addresses numerous concerns raised by both scientists and forest managers. Some of these concerns are explored in this blog post. One remaining concern, pertains to phytosanitary risks associated with plant translocation. Considering that there are few known pathogens of pollen, we posit that the transfer of pollen across borders is relatively safe.
Sources:
[1]: Caballero, R. & Huber, M. State-dependent climate sensitivity in past warm climates and its implications for future climate projections. Proceedings of the National Academy of Sciences 110, 14162–14167 (2013).
[2]: Burke, K. D. et al. Pliocene and Eocene provide best analogs for near-future climates. Proceedings of the National Academy of Sciences 115, 13288–13293 (2018).
[3]: Sáenz-Romero, C. et al. Assisted migration of forest populations for adapting trees to climate change. Revista Chapingo serie ciencias forestales y del ambiente 22, 303–323 (2016).
[4]: Williams, M. I. & Dumroese, R. K. Preparing for Climate Change: Forestry and Assisted Migration. J For 111, 287–297 (2013).
[5]: Franks, S. J. & Hoffmann, A. A. Genetics of climate change adaptation. Annu Rev Genet 46, 185–208 (2012).
