Treeline advances in a changing climate: understanding how climate affects reproduction and early life stages in Black spruce

NERC: Jessie Foest: NE/S00713X/1北方高纬度生态系统的变暖速度比地球上任何其他生态系统都要快。这种变暖强烈地影响了以前温度有限的系统中的植被，如亚北极苔原。树线，即森林和无树苔原之间的过渡带，预计会向北移动。然而，一项关于树线进展的荟萃分析发现，自1900年以来，只有一半的被调查树线有所进展。为什么气候变暖与某些种群的林木线扩张有关，而与其他种群无关，这一问题仍未得到解决。了解树木线生态系统将如何应对气候变化是至关重要的。首先，没有树木的苔原地区的土壤储存了地球上三分之一的土壤碳。当树木侵入这些地区时，它们会改变土壤条件（例如干燥、透气的土壤），并促进进一步的植被转移。这可能会释放土壤中储存的碳。此外，由于系统反照率的变化，气候反馈循环可能会出现，其中树木覆盖的增加可能会导致进一步变暖。树木的繁殖可能有助于解释为什么树木线对变暖的反应滞后。最近对13个亚北极树线过渡带的生殖行为进行的全球综合报告显示，这些种群中产生的可存活种子数量很少。在植物中，种子落到森林地面后的早期生命阶段（即发芽和育苗）的特点是死亡率高。因此，低水平的种子产量可能导致幼苗招募完全失败。因此，随着气候变暖，一些种群可能无法向极地迁移。然而，我们对亚北极森林种子生产和种子活力如何变化的了解很少。在这个项目中，我们将研究气候如何控制黑云杉（Picea mariana）的种子生产和生存能力，黑云杉是亚北极西北部主要的树线形成物种。此外，拟议的研究将用于建立一个长期的实验，以验证我们的假设，即黑云杉树线推进的能力是由它们产生足够的可存活种子的能力决定的。寄主群体及其网络在2006-2011年间收集的快照数据显示，春季生长度日数（日平均气温在50 - 50℃之间的日数）、较高的夏季降水水平和较高的树木同密度增加了各种树线形成物种的种子产量。与水分胁迫相关的生长期日数和降水量较低的连续年份与种子产量较低有关。种子活力受花期气候条件、林分和种子特性的影响。在这些快速变化的生态系统中继续和扩大数据收集将有助于确定种子生产的驱动因素是否正在发生变化。这种不断发展的认识需要准确地模拟和预测森林冻土带系统中未来的种子可用性。

NERC : Jessie Foest : NE/S00713X/1Northern high-latitude ecosystems are warming faster than any other ecosystem on Earth. Such warming strongly affects the vegetation in previously temperature-limited systems such as the sub-arctic tundra. Treelines, the ecotones between forests and the tree-less tundra, are expected to respond by shifting northwards. However, a meta-analysis on treeline advances found that only half of the examined treelines had advanced since 1900. Why warming is associated with treeline advances in some but not other populations remains unresolved.It is crucial to understand how treeline ecosystems will respond to climate change. Firstly, the soils in tree-less tundra regions store a third of the Earth's soil carbon. When trees encroach on these areas, they change soil conditions (e.g. drying, aerating soils) and facilitate further vegetation shifts. This may release the stored carbon from the soil. Additionally, climate feedback loops may arise because of changes in the system's albedo, where an increase in tree cover may drive further warming.Tree reproduction may help explain why treelines show lagged responses to warming. A recent global synthesis of reproductive behaviour across 13 sub-arctic treeline ecotones reported that low numbers of viable seeds were produced in these populations. In plants, the early life stages that occur after seeds fall to the forest floor (i.e. germination and seedling establishment) are characterised by high mortality rates. Low levels of seed production may thus result in complete failure of seedling recruitment. Consequently, some populations might be unable to advance poleward with climate warming. However, we have a poor understanding of how seed production and seed viability vary in sub-arctic forests. In this project we will investigate how climate controls seed production and viability in black spruce (Picea mariana), the dominant treeline-forming species in the north-west sub-arctic. Moreover, the proposed research will be used to establish a longer-term experiment to test our hypothesis that the capacity for black spruce treeline advance is determined by their ability to produce sufficient viable seeds.Snapshot data collected by the hosting group and their network between 2006-2011 revealed that the amount of spring-time growing degree days (days with daily mean temperatures >5 degree C), higher levels of summer precipitation and higher conspecific densities of trees increase seed production in a variety of treeline-forming species. Consecutive years with more growing degree days and low precipitation levels, variables linked to moisture stress, were associated with low seed production. Seed viability was affected by climatic conditions during flowering, as well as by stand and seed characteristics. Continuing and expanding data collection in these quickly changing ecosystems will help determine if the drivers of seed production are changing. Such an evolving understanding is required to accurately model and predict future seed availability in forest tundra systems.