Strong N and P Co-Limitation Constrains Community Composition and Productivity of Post-Fire Tundra

As a result of climate warming, wildfire is an emerging pressure on Arctic tundra ecosystems. As a result of wildfire, some communities experience a shift in composition while others gradually return to their pre-wildfire state. However, it is unclear what is responsible for the observed heterogeneity between post-fire communities. Since historic research has shown that low nutrient soils largely limit the composition of undisturbed tundra, we hypothesize that the trajectory of post-wildfire community composition will be controlled by the availability of nitrogen and phosphorus in post-burn soils and that changes in composition will lead to changes in community productivity. To test this, we artificially added nitrogen and phosphorus fertilizers separately and in combination at a burned site and adjacent unburned site nine years after the 2007 Anaktuvuk river fire in Northern Alaska. After four years of fertilization, we observed a strong relationship between deciduous vegetation and community productivity where an increase in deciduousness led to a subsequent increase in productivity. Specifically, the individual addition of nitrogen and phosphorus did not result in a shift towards more deciduous vegetation. However, we observed a marked shift in community composition towards more deciduous vegetation as a result of combined nitrogen and phosphorus fertilization which, consequently, stimulated community productivity. With the area of tundra effected by wildfire projected to increase in the future, our results demonstrate the importance of nutrient availability in determining the composition and productivity of post-fire systems.