NERC : Jessie Foest : NE/S00713X/1
Northern 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.