Nitrogen deposition :: Coniferous woodland

Effects and implications

Trees

• Increased yields greatest where soil organic layer C:N ratio high (Guerrieri et al 2011).
• Destabilisation; faster growth, reduced investment in roots leading to increased risk of drought stress (Anders et al 2002) and increased risk of uprooting.
• Decreased fine root biomass and numbers of root tips, with associated increased above ground biomass, indicative of micro nutrient deficiencies induced by excessive growth.
• Mechanical stability - broken stems associated with less non-structural carbohydrate and starch (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.
• Increased susceptibility to climate stress and fungal disease (de Vries et al. 1995).
  • Increased needle production and litterfall restricting light to ground flora.
  • Change in mycorrhizal flora and reduction in the numbers of large sporocarps, fruiting bodies, which appear particularly sensitive to NH₄⁺, possibly because this N form leads to the greatest diversion of C into amino acid production at the expense of C assimilate partitioning to support sporocarp formation.
  • Sensitive mycorrhizas are replaced by those preferring N rich conditions, which tend to be those that are efficient at taking up P.
  • Changes in recruitment and loss of biodiversity (Dobben et al. 1999). The increased N availability favoured Picea abies and Betula seedlings at the expense of Pinus sylvestris.

Soil chemistry and soil fauna

• Soil N retention can suppress the potential for N deposition to increase tree growth (Emmett et al 1998).
• Decreased organic matter decomposition, following initial stimulation leading to an increase in recalcitrant N.
• N deposition has the potential to change soil pH: 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, BC 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.  
• NH₄⁺-N deposition is largely retained by the soil on exchange sites maintaining low soil solution concentrations. When this occurs in acid humus layers it usually leads to base cation loss, especially where the NH₄⁺ is nitrified the more mobile nitrate moving down the profile will take the displaced base cation with it. Reductions in BC availability: in acid soils can lead to Mg deficiency and in acid soils P and K⁺ deficiency (Fluckiger and Braun 1999). This will lead to reduced rates of C assimilation and growth and lower water use efficiency.
• NH₄⁺ deposition, in excess of growth demands, leads to arginine accumulation which in turn feeds back to reduce NH₄⁺ uptake by trees, and consequently leading to greenhouse gas release as N₂O and NO. 
• N affects the composition of leaf litter through changes in species composition and changes in leaf litter chemistry. 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.  
• N deposition can affect the activity of methanotrophs, organisms that live in the aeroebic zone and convert methane (CH₄) to carbon dioxide, a less potent greenhouse gas (lower radiative forcing) than CH₄ per unit of C. Changes in emissions are due to changes in the vegetation and root exudates and the use of nitrate as an alternative electron source and the inhibitory effect of NH₄⁺ (Goulding et al 1998). N deposition tends to decrease methane oxidation (Butterbach-Bahl et al 2002). 

Overview: evidence, processes and main impacts

N deposition is not believed to have a large effect on conifer growth in the UK although where conifers occupy soils with low nutrient availability, low levels of N deposition would be expected to increase growth. The close planting and uniformity of species in most coniferous plantation forests, once they have closed canopy, restricts biodiversity interest: Several birds of prey e.g. Red Kites and Goshawks need the tall conifers to make their nests. The thick evergreen canopy, causing dark, shaded conditions, also prevents the growth of many higher plants, although mosses and fungi can flourish. This shelter suits mosquitoes and midges, a vital food source for several species of bats.  Mosses, fungi and epiphytic lichens are likely to be most sensitive to N deposition. There are however, indirect effects of N which do affect the trees: N effects through eutrophication and acidification can predispose woodlands to these indirect effects with potentially deleterious consequences.

Woodlands are complex ecosystems, comprising various component types with different sensitivities to N. Trees are the key component, but in many evergreen woodlands there is a field layer of lower plants (cryptograms) making up the forest floor. Seasonally, sporocarps, fruiting bodies, also belonging to saprophytes, may be present and within the litter layer and below ground there will be a diverse array of mycorrhizal fungi associated with tree roots. In addition the trees may support epiphytic communities of bryophytes and algae. Thus these woodlands, and the different vegetation types they comprise, provide a habitat for wildlife, especially insects, birds and small mammals. N deposition can compromise this biodiversity 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 dry deposited N than for example grasslands. This is particularly the case for woodland edges, which experience the highest N deposition, especially where there is a local source of gaseous N e.g. roads or rural agricultural areas (intensive livestock production, manure heaps, fertiliser application). 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 where these make up a key component of the woodland type the critical load will have been set lower to protect them. 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 or emissions of the greenhouse gases NO or N₂O. Effects of N deposition both direct and indirect are not always distinguishable from issues concerned with management, especially where this involves changing light levels e.g. thinning.

Woodlands surrounded by farmland, highways and wasteland 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. Although, depending of the age of the wood, opportunities for seedling establishment will be restricted by the ‘dark’ conditions.

Caledonian pine forests occur on a variety of low fertility soil types, but mostly in areas of low N deposition. However, there is evidence of N damage in Scots pine on low nutrient soils in Scotland, possibly associated with high needle N content and increased risk of pest and pathogen attack, leading to reduced needle retention (Kennedy (2003). These pine forests often contain other tree species including birch, willow, rowan and juniper with a ground flora dominated by ericoids. The main threats to these forests are overgrazing, fire and fragmentation.

Pollutant deposition type and risk areas

Indicators of N enrichment

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

• Forest floor CN ratio: < 25 
• Needle % N (> 1.4 %).
• Increased litter production and canopy
• Decreases in bryophyte and herb species richness
• Understory with increased Ellenberg scores
• Changes in under-storey species, with sensitive mosses and lichens likely to decline e.g.  increases in acid tolerant nitrophilic species (e.g. Deschampsia flexuosa) in non-fertilised Scots Pine stands over the last 20 to 40 years and decreases in Calluna vulgaris and Vaccinium vitis-idaea (Rodenkirchen 1992).

• Algae growing on the trees

• Reduction in faunal and floral biodiversity as dominant species take over.
• Reduction in large sporocarps and diversity of mycorrhizal fungi.
• Increased soil acidity
• Increase in N content of upper soil horizons, with subsequent effects on soil processes, e.g. mineralization.

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.