Birch shrub expansion across the Low Arctic: Are leaf nutrient concentrations a reliable indicator of the relative importance of nitrogen and phosphorus as growth-limiting nutrients?

Background/Question/Methods

Birch shrub cover has been expanding across the Low Arctic over the past 35 years – especially in coastal regions where evidence of climate warming is unequivocal. Understanding the mechanisms explaining this species’ rise to dominance is critical to predicting future tundra vegetation community structure. Since soil nitrogen (N) supply is widely cited as the principal tundra plant growth-limiting nutrient, the birch response has generally been attributed to temperature-induced increases in soil organic matter decomposition. Nevertheless, recent global meta-analyses suggest that phosphorus (P) colimitation of plant growth is much more widespread than previously thought, but evidence from the Arctic is exceedingly rare. In this talk, I will synthesize data from a long-term factorial N and P annual addition experiment consisting of two levels of each nutrient – one well in excess of, and the other matching, annual plant demand. I will address the following specific questions: a) Do multiple-level fertilisation studies support the use of birch leaf N:P as a reliable indicator of the relative importance of N and P as growth-limiting nutrients? b) Do these experimental results correspond with our earlier unpublished leaf N:P ratio data from 150 different birch locations across the North American and European Low Arctic?

Results/Conclusions

The factorial experiment results clearly demonstrate that birch leaf N:P ratios are highly sensitive to even low-level additions of each nutrient. This conclusion is important because the low-level additions probably realistically simulate anticipated increases in nutrient availability as tundra soils warm due to climate change. Furthermore, good correspondence between the experimental and the regional birch leaf nutrient concentration data strongly suggests that colimitation of birch shrub growth by both N and P is as common as N-only limitation across the Low Arctic. This conclusion is important because the controls on soil P availability are as much abiotic (e.g. pH) as biotic (e.g. decomposition rate), implying that the impacts of climate warming on P supply to plants may be quite different to those of N supply. Together, these results will be discussed in terms of decadal patterns of birch shrub growth in unmanipulated tundra vegetation across the Daring Lake landscape – a Canadian, interior continental, low Arctic site where the 25 year climate record indicates little evidence of past warming. I will conclude by incorporating these results into a conceptual framework aimed at investigating and better predicting the likely outcomes of climate warming on birch plant growth across the Low Arctic.