Unlock Climate Resilient Forests with Assisted Tree Migration

Climate Change and Forest Resilience

European forests, covering around 35% of the land, are crucial in mitigating climate change by removing carbon dioxide from the atmosphere. However, climate change poses a serious threat to these forests. If strategic actions are not taken, the ability of these forests to act as carbon sinks could be significantly reduced, impacting the global effort to limit temperature rise.

To combat this, it’s essential to adopt measures to enhance the resilience of forests. One such measure is assisted migration, which involves moving tree species and their seed provenances to areas where they are more likely to thrive under future climate conditions. This approach can help maintain and even increase the carbon sequestration capacity of forests.

Considering the limited natural migration capacity of many tree species, assisted migration becomes even more critical. Restricted gene flow and the rapid pace of climate change mean that many tree populations could become maladapted, leading to reduced ecosystem services. Thus, transferring tree species to suitable habitats is a proactive strategy.

Assisted migration can be categorized into two types: ‘assisted gene flow’, involving moving seed provenances within their current ranges, and ‘assisted species migration’, moving them beyond their present ranges. Both strategies aim to enhance the adaptive capacity and resilience of forests.

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Implementation of Assisted Migration

In our study, we examined seven major European tree species using data from extensive provenance trials. By analyzing shifts in species distributions and population transfer models, we assessed the impact of assisted migration on the annual carbon sequestration (CS) of European forests. Our focus was on identifying the most suitable species and seed provenances for future climates.

We followed a two-step approach:

• Selected the best-suited species for each location using species distribution models (SDMs).
• Identified the best-fitting seed provenances for these species by modeling their annual CS.

We developed universal response functions (URFs) that account for environmental and genetic variations in above-ground CS. This approach helps estimate the carbon sequestration potential of forest regrowth up to 40 years, providing meaningful insights for assisted migration applications.

To ensure effective implementation, we created species-specific seed provenance clusters (SPC) that group climatically and geographically similar seed sources. This helps align local adaptations with future climate conditions, optimizing the selection of seed provenances for reforestation efforts.

Shifts in Forest Composition

Our models indicate significant shifts in the climatic suitability of tree species due to climate change. Conifers like P. abies and A. alba, dominant in Central and Northern Europe, are expected to see a decline in suitable habitats. Meanwhile, broadleaved species like F. sylvatica and Q. robur are likely to become more prevalent.

The impact of climate change on suitable habitats varies, with the highest uncertainty observed under the RCP 8.5 scenario. Despite these uncertainties, the rankings of species in terms of suitable cultivation areas remain relatively constant, emphasizing the need for strategic planning in forest management.

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The scenarios for assisted migration focus on optimizing the CS potential of the regenerated forests. Our models suggest that planting non-local SPC suitable for projected climate conditions can significantly enhance the carbon sequestration capacity of forests.

Local seed provenances may offer optimal CS only in limited areas. For instance, in contemporary climate, local provenances provide higher CS in only 3–4% of the total suitable area for P. abies and P. sylvestris. This highlights the importance of selecting adapted seed provenances for future reforestation efforts.

Maintaining Carbon Sequestration

Changing tree species and selecting the right seed provenances can drastically affect the annual CS of forest regrowth. If we rely on local seed provenances, the shift from conifers to deciduous trees could decrease overall CS. However, using adapted seed provenances could significantly boost CS, even under challenging climate scenarios.

For instance, planting the most productive SPC for the respective climate could increase annual CS to 66.78 TgC under contemporary climate and 59.61 TgC under the RCP 4.5 scenario. Even under the pessimistic RCP 8.5 scenario, assisted migration could enhance CS to 48.18 TgC.

Under contemporary climate, conifers contribute substantially to CS in regrowing forests. However, under climate change scenarios, their contribution is expected to decline, especially if local seed sources are used. Conversely, the contribution of broadleaved species to CS is projected to rise.

Overall, all species show gains in annual CS when adapted seed provenances are planted. For conifers, assisted migration results in a 150–200% gain in annual CS under the RCP 4.5 scenario, compared to local seed provenances. Broadleaved species also experience significant gains, highlighting the effectiveness of assisted migration in maintaining forest carbon sequestration.

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Addressing Limitations and Conflicts

While our study provides valuable insights, it is not without limitations. The available data and models have constraints, and our focus was limited to seven main tree species common to central and northern Europe. This represents two-thirds of Europe’s forests but does not include rare or scattered species.

Another potential limitation is the inclusion of non-autochthonous seed sources. Historical management and seed transfers may have introduced non-native provenances, which could affect the accuracy of our models. However, we mitigate this by grouping provenances into large-scale SPC to reduce uncertainties.

Our analysis also excludes Mediterranean forests, which face similar adaptation challenges due to climate change. Despite these limitations, our recommendations for assisted migration are grounded in extensive empirical data and align with the overarching scientific consensus on the need for adapted seed provenances.

In conclusion, assisted migration offers a promising strategy to maintain and enhance the carbon sequestration capacity of European forests under climate change. By selecting the right tree species and seed provenances, we can ensure the resilience of these vital ecosystems and contribute to global climate mitigation efforts.