Assisted migration of trees between populations as a climate change mitigation strategy for Canada's boreal forest

各国一致认为，要将全球平均气温上升幅度控制在工业化前水平以上2摄氏度以内，就必须大幅减少全球温室气体净排放。与森林有关的缓解活动在这种努力中发挥着重要作用。值得注意的是，人类辅助的树木个体在物种范围内种群之间的移动（即辅助的基因流动）有可能通过加速树木的生长速度和增强树木对环境和气候压力的抵抗力和恢复力来增加大气中的碳清除。近年来，越来越强调利用这种活动作为一种适应措施，以维持受气候变化不利影响地区的森林健康。虽然辅助基因流动有助于稳定这些森林的健康，但它也可能有助于缓解，这取决于对森林碳的影响。然而，这种与森林有关的缓解潜力存在不确定性，因为气候变化可能带来负面反馈，即如果迁移的种源仍然暴露在它们不适应的气候压力下，辅助基因流动就会带来风险。本研究的目的是评估辅助基因流对加拿大北部广泛分布的针叶树物种的长期地上净初级生产力（NPP）的影响。这将通过创新地使用普通花园网络的树木生态学采样来实现，并采用基于过程的建模框架，在该框架中测试辅助基因流对长期未来NPP的影响。具体而言，这项工作将(1)对NPP的演变及其对物种、种群、树木寿命和普通花园过去气候条件的响应进行回顾性分析，(2)发现树木种群中存在的局部适应迹象，(3)评估在未来暴露于气候变化的情况下，辅助基因流的缓解潜力。待验证的假设是，年NPP及其对气候的响应在不同种群之间表现出高度差异，表明地方适应和缓解潜力。我们认为森林缓解潜力是相对于以当地种源管理森林的基线而言，通过引入非本地种源而增强的NPP。根据2015年签署的《巴黎协定》，加拿大需要制定深入的目标，该协定要求所有国家在本世纪下半叶实现全球人为温室气体排放和消除之间的平衡。加拿大的森林面积大，是采伐木材产品的主要生产国，这意味着必须探索与森林有关的缓解活动的潜力，包括辅助基因流动。这项研究将提供基于科学的信息，加拿大各司法管辖区（如省、地区）将能够利用这些信息做出有关缓解的知情决定和政策。

Countries have agreed that deep global net greenhouse gas (GHG) emission reductions are required to hold the increase in global average temperature to less than 2°C above pre-industrial levels. Forest-related mitigation activities have a significant role in such efforts. Notably, human-assisted movements of tree individuals between populations within species ranges (i.e. assisted gene flow) have the potential to increase atmospheric carbon removal by accelerating the rate of tree growth and increasing tree resistance and resilience to environmental and climatic stresses. In recent years, emphasis has strengthened around the utility of such activity as an adaptive measure so to maintain the health of forests in areas negatively impacted by climate change. While assisted gene flow could help to stabilize the health of these forests, it might also contribute to mitigation depending on the implications for forest carbon. Yet, there is uncertainty in this forest-related mitigation potential because a changing climate can pose negative feedback, i.e. assisted gene flow carries the risk if moved provenances remain exposed to climatic stresses to which they were not adapted.     The objectives for this research are to assess the implication of assisted gene flow on the long-term aboveground net primary productivity (NPP) of widespread conifer species in boreal Canada. This will be done by innovative use of dendroecological sampling of a network of common gardens with a process-based modeling framework in which the effects of assisted gene flow on long-term future NPP are tested. Specifically, the work will (1) Develop a retrospective analysis of the evolving NPP and its response to past climate conditions across species, populations, tree lifespan, and common gardens, (2) Find signs of local adaptation present in tree populations and (3) Assess the mitigation potential of assisted gene flow under future exposure to climate change. The hypothesis to be tested is that annual NPP, and its response to climate, exhibits a high degree of variation among populations, suggesting local adaptation and potential for mitigation. We consider forest mitigation potential as an enhancement of NPP that would result from introduction of non-local provenances, relative to a baseline where forests are managed for local provenances.     Canada will need to establish deep targets in keeping with the 2015 Paris Agreement that commits all countries to an aim of achieving a balance between global anthropogenic GHG emissions and removals in the second half of this century. Canada's large forest and its position as a major producer of harvested wood products mean that it is important to explore the potential of forest-related mitigation activities including assisted gene flow. This research will provide science based information that Canadian jurisdictions (e.g. provinces, territories) will be able to use for making informed decisions and policies concerning mitigation.