Changing climate is expected to alter the future range of many species. While studies commonly focus on the direct impacts of climate, fewer evaluate how climate-mediated changes in N availability may contribute to shifts in plant species distributions. Understanding these impacts is particularly important in systems which may see changes in available N-forms, such as the temperate-boreal forest ecotone. Changes in N-form availability could impact species differently, as certain N-forms may not provide equivalent nutrition for all species. However, predicting which species may be affected by changes in N-forms remains challenging. This research examines whether traits associated with organic or inorganic nutrient economies influence plant responses to different N-forms. We hypothesized that species’ N-preferences will vary according to the type of mycorrhizal association, latitudinal range, and plant N demand. To test this hypothesis, we grew seedlings of 11 temperate and boreal tree species in pots with different N-forms (amino acids, ammonium, nitrate, or an equal mixture of all N-forms). After one growing season, we measured total biomass, total N, mycorrhizal colonization, and height. We defined the N-preferences of each species by comparing N-uptake and plant growth on each N-form to the N-uptake or growth on the mixture of all N-forms.
Results/Conclusions
The form of N supplied significantly affected plant growth across species, though not necessarily in relation to the traits examined. Tree seedlings did not exhibit different preferences for different N forms. Rather, NO3- was the N source preferred by all species regardless of latitudinal range or plant N demand (here represented by leaf Nmass). Mycorrhizal type influenced the degree of preference for NO3-, as ectomycorrhizal species showed greater increases in growth on NO3- than arbuscular mycorrhizal species. However, all species still had the greatest growth on NO3-. Overall, these results suggest that species varying across the traits examined do not differ substantially in their preferences for distinct N forms. Therefore, changes in the relative availability of different N forms, coupled with preference for specific N-forms, are not expected to drive changes in species growth or N uptake at the temperate-boreal forest ecotone. However, competition for the same preferred form of N could play a role in determining the future composition of forests in the boreal region. These results help inform our understanding and guide future research on the impacts of changes in N cycling on species composition.