Predicting the effects of climate change on forests will require a thorough understanding of how species acquire and use limiting resources. However, certain controls over nitrogen (N) acquisition, a nutrient that commonly limits plant growth in northern forests, are not well understood. In particular, the factors influencing uptake of different N forms from the soil remain elusive, yet important, as climate change will likely alter the amount and types of N available to plants. Our research aims to improve our understanding of N uptake by examining the role of plant traits on acquisition of different N forms. Specifically, we asked whether trees common to northern temperate forests vary in their capacity to take up different N forms, and if N uptake is mediated by variation in root morphological traits. To examine these questions, we collected fine roots (< 2 mm diameter) of seven temperate tree species from the Cloquet Forestry Center in Northern Minnesota and measured their capacity to take up three N forms commonly available in the soil (glycine, ammonium, and nitrate) as well as four common root morphological traits.
Results/Conclusions
We found that species varied significantly in their ability to take up different N forms. Uptake rates were fastest for ammonium across all species, and paper birch (Betula papyrifera) had the fastest uptake rate overall. Tamarack (Larix laricina) and white pine (Pinus strobus) had the lowest N uptake rates of the species studied. Additionally, root length per mass (specific root length) was significantly and positively correlated with uptake rates for all species and N forms, though nitrate uptake was most tightly correlated with the trait. These results demonstrate the importance of controlling for root traits when measuring uptake rates, as variation in N uptake was driven more by root traits than by species. Additionally, our findings show that species like paper birch may be most benefited by increases in the availability of N as they are able to take up N more quickly. This improved understanding of N uptake will help scientists and forest managers make more accurate predictions of how forests may change in response to a warming climate.