Nitrogen deposition :: Broadleaved, Mixed and Yew Woodland

Effects and implications:

Trees

• Increased growth, greatest where soil organic layer C:N ratio is high.
• Destabilisation; faster growth, reduced investments in roots leading to increased risk of drought stress (Anders et al 2002) and increased risk of uprooting. Damage to mature beech in the 1999 storm in Switzerland positively correlated with leaf N (Meyer et al. 2008)
• Nutrient imbalance, crown discoloration (chlorosis / yellowing) associated with base cation, Mg and K deficiency leading to reduced growth rates, reduced crown densities and abnormal branching patterns.
• Change in mycorrhizal flora and reduction in the numbers of large sporocarps, fruiting bodies, which appear particularly sensitive to NH₄⁺. Sensitive mycorrhizas are replaced by those preferring rich conditions, which tend to be those that are efficient at taking up P.
• Increased litter production
• N accumulation as NH₄⁺ or amino acids leading to increased sensitivity to abiotic and biotic stress - reduced frost hardiness, associated with effects on late growth cessation and early bud burst, as young tissue is highly frost sensitive.
• Winter desiccation; increased defoliation by leaf feeders; increased pathogen infection, though evidence is reported predominantly for beech.

Under-storey vegetation, ground dwellers and epiphytes

• Loss of species diversity (Haines-Young et al. 2003; Kirby et al. 2005).
• Loss of sensitive forbs and mosses and increases in nitrophilous plants especially grasses.
• Loss of ground dwelling Cladinas (Strengbom et al 2001).
• Loss of lichens with blue green algae (N fixing), particularly sensitive (Goransson 1990). Lichens with both green algae and cyanobacteria as their photobiont (photosynthetic partner) appeared to be more resilient (Dahlmann 2002).
• Pleurococcoid algae that grow epiphytically can be stimulated by even quite small amounts of N deposition (Bobbink et al 1996) particularly if P and K are available e.g. in rural areas.
• Epiphytes growing on Oak are highly sensitive, particularly to ammonia, probably mediated via increases in bark pH (Ter Braak).

Soil chemistry and soil fauna

• N deposition has the potential to decrease soil pH (e.g. acidification): NH₄⁺ may be retained by the system or nitrified to NO₃^-, releasing 2 moles H⁺. Nitrate can leach out, if not consumed by the vegetation or microbial population. Leaching has the potential to remove base cations, and reduce soil buffering capacity, (acid cations e.g. H⁺ and Al³⁺ can also be leached) through the mobile anion concept (Reuss and Johnson 1985). Thus in systems that cannot fully retain NH₄⁺ its deposition will cause acidification.  
• Gaseous ammonia (NH₃) deposition initially increases pH because in contact with water it ionises, consuming H⁺ to form NH₄⁺, however if this is then nitrified, acidification will occur as the reverse occurs and H⁺ ions are released. The degree of nitrification will depend on the initial soil pH, with rates being highest in less acid soils. 
• Nitrate leaching may not be a problem for the woodland as such, but it does present a threat to water systems, and is one of the more robust indicators that the system is N saturated.  
• N affects the composition of leaf litter through changes in species composition and changes in leaf litter chemistry. Cellulose activity may be stimulated. The level of lignins and phenol compounds which can restrict fungal activity and the activity of phenol oxidase often goes down, leading to increased rates of decomposition. Overall mineralisation tends to be increased by N deposition, potentially increasing nutrient availability.  

Soil fauna

• There have been few studies in this area, but most report negative effects, e.g .on fungal feeders (less mycorrhizal hyphae). 
• Changes in acidity will also have implications for soil fauna. 

Overview: evidence, processes and main impacts

N deposition is not believed to have a direct, major effect on tree growth in the UK.  BUT as illustrated above its indirect effects are many and varied: N can affect woodlands through eutrophication  and acidification and these changes are likely to predispose woodlands to these more highly deleterious indirect effects. Woodlands are complex ecosystems, comprising various compartments with different sensitivities to N. The key component are the trees, but in many woodlands there is an under storey of woody shrubs, forbs and grasses and below this, lower plants (cryptograms) carpeting the forest floor. Seasonally, sporocarps, fruiting bodies may appear and below ground there will be a diverse array of mycorrhizal fungi associated with plant roots which are especially sensitive to N deposition. In addition the trees may support epiphytic communities of bryophytes and algae. Thus woodlands, and the different vegetation types they comprise, provide a diverse habitat for wildlife, especially insects, birds and small mammals. N deposition can compromise this  biodiversity or conservation value through changes in cover (protection), food type, quantity and quality, changes in the overall environment for predators, and timing of food source availability via effects on phenology (bud burst, bud set, flowering).

Woodlands provide a rough surface and tend to intercept larger amounts of both dry deposited N and orographic deposition than less rough surfaces, e.g. grasslands. This is particularly the case for woodland edges, which experience the highest N deposition, especially where there is a local sourceof gaseous N, e.g. roads and / or intensive agricultural areas. Thus there is often a gradient of N deposition declining from the woodland edge. Generally the members of the lower plant compartment show the greatest sensitivity and  the  critical load will have been set lower to protect them where these make up a key component of the woodland type. In addition the critical load takes into account changes in soil chemistry associated with acidification and eutrophication which can lead to N leakage, either though leaching (nitrate) or emissions of the greenhouse gases NO or N₂O.

Effects of N deposition can be both direct and indirect, and some are not easily distinguished from issues concerned with management, especially where this involves changing light levels e.g. thinning. Inappropriate or insufficient management and wind throw can simulate N effects and may result in very similar outcomes to N eutrophication, e.g. enhancing grass growth.

Pollutant deposition type and risk areas

Woodlands surrounded by farmland and roads are most at risk from N eutrophication and invasion by ‘casual’ plants because of the greater availability of a seed source for such plants, compared to remote areas surrounded by more semi-natural habitats. Woodlands near intensive livestock units are particularly at risk from ammonia deposition, especially those growing on acid soils, which can be toxic to trees, ground flora and especially epiphytes (Krupa 2003).

Example evidence of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.

The majority of species specific responses have been undertaken by a Swiss group on beech woods.

Indicators of N enrichment

These depend on the extent of the deposition and form of the N but relatively robust examples include:

• Increased likelihood of algal presence.
• Increases in  soil KCl extractable NH₄ -N, soil C/N ratio,
• Increases in tree foliar and litter N concentrations and P/N ratio
• Organic layer C/N ratio, irrespective of soil and forest type, is significantly negatively related to dry NHx deposition (n = 110, r = 0.46, p<0.001) across Great Britain (Vanguelova and Pitman, 2009).

What factors modify N deposition impacts?

• The availability of other nutrients e.g. P and K, and any other growth influencing factors will determine whether N deposition increases growth or leads to the accumulation of N in the foliage.
• Climate: via frost and drought, these will interact with N to exacerbate the impact of N.