It is normally assumed that atmospheric nitrogen inputs to forests are entirely taken up directly by trees through the process of foliar absorption. However several studies suggest that uptake of nitrogen in forest canopies also occurs through other processes. Epiphytic lichens growing in the forest canopy can take up atmospheric nitrogen directly and have been shown to enhance nitrogen deposition to the canopy. Aphid infestation of the forest canopy has also been shown to affect nitrogen uptake as microbes which feed on the honeydew produced by aphids can directly utilise atmospheric nitrogen.
Competition therefore exists for nitrogen in the forest canopy between the tree, fungi and microbes.
The immediate availability of nitrogen for uptake by trees is reduced, although nitrogen may eventually become available from decomposition of fungal and microbial biomasses. It is expected that the influence of these processes on nitrogen uptake by the forest canopy will change seasonally and as the stand matures. Investigations of the fate of atmospheric nitrogen deposition in forests have been limited. Assessments of the different pathways of nitrogen uptake in the canopy and of their effects on the total nitrogen available to the tree are important in understanding the effects of nitrogen cycling on tree growth and carbon storage. The project aim is to determine the fate of nitrogen deposition in forest stands of different ages and to relate this to nitrogen availability for trees.