Phosphorus as a Colimiting Nutrient with Nitrogen of Birch Plant Growth Across the Arctic

Nitrogen is widely cited as the principal growth limiting nutrient for plants in moist, mesic and dry arctic tundra ecosystems. As such, the widespread increases in deciduous shrubs that have been observed across the Arctic as the climate has warmed over the past 30 years, are generally attributed to enhanced nitrogen supply due to temperature-induced increases in soil organic matter decomposition.

A recent factorial nitrogen-phosphorus (N x P) chronic addition experiment in the Canadian low Arctic demonstrated that birch shrub growth was as limited by the availability of P as by N. How widespread is this N P colimitation of birch growth, and might variation in the extent of N relative to P limitation be a primary determinant of niche space and therefore species coexistence in birch hummock tundra plant communities? In this talk I will address the following specific questions: a) Can birch leaf N:P be used as a reliable indicator of the relative importance of N and P as growth limiting nutrients? b) What is the pattern of birch leaf N:P ratios across the Arctic? c) Do species that grow alongside birch differ in leaf N:P in ways that suggest differing relative limitation?

With the assistance of many colleagues, I collected 150 birch leaf and underlying soil samples from 50 different locations across the North American and European Arctic. I used the factorial experiment described above to test the suitability of leaf N:P ratio as an indicator of relative nutrient limitation for all species in birch hummock tundra communities, and then interpreted the patterns of birch leaf N:P across the Arctic to characterise the spatial extent of NP co-limitation.

The results strongly suggest that birch shrub growth is colimited by N and P availability across its range. This conclusion is important because the controls on P availability in soils are as much abiotic (e.g. pH) as biotic (e.g. decomposition), implying that the impacts of climate warming on P supply to plants may be quite different to those of N supply. These results will be discussed in terms of plant community responses to greenhouse warming, as well as ongoing change in unmanipulated tundra vegetation, all with a view to better predicting the likely outcomes of climate warming on plant growth and community structure in the low Arctic.