(Table 2) Survivorship of transplanted seedlings of Picea glauca inside and outside three tree islands in 1994, 1995, 2009 and after 15 years

The northern boundary of boreal forest and the ranges of tree species are expected to shift northward in response to climate warming, which will result in a decrease in the albedo of areas currently covered by tundra vegetation, an increase in terrestrial carbon sequestration, and an alteration of biodiversity in the current Low Arctic. Central to the prediction of forest expansion is an increase in the reproductive capacity and establishment of individual trees. We assessed cone production, seed viability, and transplanted seedling success of Picea glauca (Moench.) Voss. (white spruce) in the early 1990s and again in the late 2000s at four forest stand sites and eight tree island sites (clonal populations beyond present treeline) in the Mackenzie Delta region of the Northwest Territories, Canada. Over the past 20 years, average temperatures in this region have increased by 0.9 °C. This area has the northernmost forest-tundra ecotone in North America and is one of the few circumpolar regions where the northern limit of conifer trees reaches the Arctic Ocean. We found that cone production and seed viability did not change between the two periods of examination and that both variables decreased northward across the forest-tundra ecotone. Nevertheless, white spruce individuals at the northern limit of the forest-tundra ecotone produced viable seeds. Furthermore, transplanted seedlings were able to survive in the northernmost sites for 15 years, but there were no signs of natural regeneration. These results indicate that if climatic conditions continue to ameliorate, reproductive output will likely increase, but seedling establishment and forest expansion within the forest-tundra of this region is unlikely to occur without the availability of suitable recruitment sites. Processes that affect the availability of recruitment sites are likely to be important elsewhere in the circumpolar ecotone, and should be incorporated into models and predictions of climate change and its effects on the northern forest-tundra ecotone.

受气候变暖影响，北方针叶林（boreal forest）的北界与树木物种分布范围预计将向北迁移，这将导致当前苔原植被覆盖区域的反照率（albedo）降低、陆地碳汇（terrestrial carbon sequestration）提升，并改变当前低北极区域的生物多样性。森林扩张预测的核心在于树木个体繁殖能力与定植能力的提升。
我们分别于20世纪90年代初与2000年代末，对加拿大西北地区麦肯齐三角洲区域的4个林分样地与8个树岛样地（当前林线以外的克隆种群）的白云杉（white spruce，Picea glauca (Moench.) Voss.）的球果产量、种子活力以及移栽幼苗存活率进行了评估。过去20年间，该区域平均气温上升了0.9℃。该区域拥有北美最北端的林线-苔原交错带（forest-tundra ecotone），且是少数针叶林北界可达北冰洋的环北极区域之一。
研究发现，两次调查期间球果产量与种子活力均未发生显著变化，且这两项指标均随跨林线-苔原交错带向北逐渐降低。尽管如此，林线-苔原交错带北界的白云杉个体仍可产生具有活力的种子。此外，移栽幼苗可在最北端样地存活15年，但未观测到自然更新迹象。
上述结果表明，若气候条件持续改善，树木繁殖产量或可提升，但该区域林线-苔原带内若无适宜的定植位点，幼苗定植与森林扩张将难以实现。影响定植位点可获得性的过程，在环北极交错带的其他区域或同样关键，因此应将其纳入气候变化及其对北方林线-苔原交错带影响的模型与预测当中。