Air Quality Guidelines - World Health Organization Regional Office ...

effects of airborne nitrogen pollutants on vegetation
Table 36. (contd)
Ecosystem Critical load � Indication of exceedance
(kg N/ha
per year)
Trees and forest ecosystems
(contd)
Deciduous trees 15–20 # Nutrient imbalance; increased
shoot/root ratio
Acidic coniferous forests 7–20 ## Changes in ground flora and
mycorrhizas; increased leaching
Acidic deciduous forests 10–20 # Changes in ground flora
Calcareous forests 15–20 (#) Changes in ground flora
Acidic forests (unmanaged) 7–15 (#) Changes in ground flora and
leaching
Forests in humid climates 5–10 (#) Decline in lichens; increase in freeliving
algae
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a ## Reliable: a number of published papers on various types of study show comparable results.
# Fairly reliable: the results of some studies are comparable.
(#) Expert judgement: no data are available for this type of ecosystem; the critical load is based on knowledge
of ecosystems likely to be more or less comparable with this ecosystem.
b Use low end of the range for nitrogen-limited and high end for phosphorus-limited calcareous grasslands.
receptor groups according to the general relationships between abiotic factors
and critical loads for nitrogen (Table 37).
At this moment, the critical loads are set in values of total nitrogen inputs.
More information is needed in future on the relative effects of oxidized and
reduced nitrogen deposition. Critical loads for nitrogen are formulated as
reliably as possible. As most research has focused on acidification in forestry,
serious gaps in knowledge exist on the effects of enhanced nitrogen deposition
on natural and seminatural terrestrial and aquatic ecosystems. The
following gaps in knowledge are particularly important:
• more research is needed in Mediterranean, tropical and subtropical vegetation
zones;
• quantified effects of enhanced nitrogen deposition on fauna in all types
of vegetation reviewed are extremely scarce;
• the critical loads for nitrogen deposition to Arctic and alpine heathlands
and forests are largely speculative;