Kronzucker, HJ, MY Siddiqi, ADM Glass. 1997. Conifer root discrimination against soil nitrate and the ecology of forest succession. Nature. 385: 59-61.
A common practice when restoring property is to plant trees which may be late-successional species. This has been noted to be problematic as many of the species do not survive. The problems associated with planting late-successional tree species are centered around the idea that there is an ecological order to succession, and when ecosystems skip stages, there soils may not yet have the composition to support later succession stages. Micro- and macronutrients, which are needed to support later succession stages, have not had the opportunity to accumulate in the soils since many of the organisms that fix nitrogen are associated with early successional stages, and the specific nitrogen compound changes from ammonium in late-successional forest soils to nitrate after disturbances. In this study, the researchers wanted to investigate the uptake of various forms of nitrogen by late-succession species, in this case white spruce.
The researchers use a radiotracer to quantify the amount of nitrate and ammonium taken up by white spruce seedlings. The trees had a much higher rate of uptake for ammonium than nitrate. This difference is though to be regulated by the protein channels in the plasma membrane. Because the protein channels limit the transport into the cell, it shows a preference for the ammonium. This is contrast to early-successional species which have an equal affinity for both nitrogen compounds. The researchers also noted that nitrate uptake was associated with nitrate exposure. Trees which had never been exposed to nitrates previously did not transport nitrates. After three days of nitrate exposure, the trees would reach their maximum nitrate uptake rate. Aspen, a pioneering tree species, does not require previous exposure to nitrates in order for it to be taken up by the tree. When the white spruce trees were exposed to both nitrates and ammonium, the trees would have an increase in ammonium uptake. This is to be expected as the tree already has structures in place to handle ammonium uptake.
This research presents an interesting look on the ecology of succession. The spruce trees are poor competitors in the early-successional stages. This may be a direct cause of the exact nitrogen compounds present in the soil while the early-successional species do not have a particular affinity for a given compound. The aspen are more opportunistic and will take advantage of any available nitrogen in the soil. It has been previously demonstrated that competition for light, water, and physical space are important for early-successional species but also access to other nutrients. Spruces become the climax species by growing slow and partitioning resources but they may not be as efficient as once thought.
Adam Warrix 