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

各国同意，必须大幅削减全球温室气体净排放量，才能将全球平均气温的升幅控制在比工业化前水平高出不到2摄氏度的水平。与森林有关的缓解活动在这些努力中发挥着重要作用。值得注意的是，人类辅助的树种范围内种群之间的个体移动(即辅助基因流动)有可能通过加快树木生长速度和增强树木对环境和气候压力的抵抗力和复原力来增加大气中的碳清除。近年来，重点加强了将这类活动作为一种适应措施的效用，以维持受气候变化不利影响地区的森林健康。虽然辅助基因流动可以帮助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，因为气候变化可能会带来负面反馈，即如果迁移的种源继续暴露在它们不适应的气候压力下，辅助基因流动具有风险。*这项研究的目标是评估辅助基因流动对加拿大北部广泛分布的针叶树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树枝生态采样普通花园网络和一个基于过程的建模框架来实现，在该框架中测试了辅助基因流动对长期未来净初级生产力的影响。具体地说，这项工作将(1)对物种、种群、树木寿命和普通花园中不断演变的NPP及其对过去气候条件的反应进行回顾分析，(2)发现树木种群中存在的局部适应迹象，(3)评估未来气候变化下辅助基因流动的缓解潜力。需要检验的假设是，年度净初级生产力及其对气候的反应在不同种群之间表现出高度的差异，表明了当地的适应和缓解的潜力。我们认为森林缓解潜力是引入非本地种源所带来的净生产总值的提高，相对于为本地种源管理森林的基线。*加拿大将需要根据2015年《巴黎协定》制定深入的目标，该协定要求所有国家致力于在本世纪下半叶实现全球人为温室气体排放和清除量之间的平衡。加拿大的大片森林及其作为木材采伐产品主要生产国的地位意味着，必须探索与森林有关的缓解活动的潜力，包括协助基因流动。这项研究将提供以科学为基础的信息，加拿大司法管辖区(如省、地区)将能够利用这些信息做出关于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 2C 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.******