Acidity and nutrient nitrogen critical loads and exceedance for woodland habitats

The continuous evaluation and development of the critical loads methodology using current data is crucial for targeting implementation policies towards effective ecosystem recovery. For woodland, the approach must be appropriate to the breadth of environmental conditions and woodland types present in the UK, and also representative of both unmanaged woodland and current practices employed across the managed forest estate.

Woodland habitat mapping

Managed and unmanaged woodland have been mapped separately by combining three woodland cover maps from different sources. The managed woodland category has been further categorised into conifer and broadleaf woodland to enable critical load exceedances to defined receptors for each woodland type to be mapped.

Woodland cover maps (34K)

For acidity, the 'broadleaved, mixed and yew woodland' Broad Habitat is sub-divided into two categories :

  • Managed (productive) broadleaved woodland
  • Unmanaged (ancient & semi-natural) coniferous and broadleaved woodland.

For nutrient nitrogen, the unmanaged woodland is further divided into 'Atlantic oak woods' (for the effects of nitrogen on epiphytic lichens) and other 'unmanaged woodland' (for the effects of nitrogen on ground flora).

The coniferous woodland broad habitat only includes managed coniferous woodland. The unmanaged coniferous areas are included in the unmanaged (ancient & semi-natural) coniferous and broadleaved woodland category because the receptor (ground flora) and critical load are the same in both.

The unmanaged woodland category consists of ancient and semi-natural woodland, yew and Scots pine and is 'managed' for biodiversity or amenity, but not timber production. All other coniferous and broadleaved woodland is assumed to be primarily managed as productive forest where harvesting and removal of trees takes place. Both managed and unmanaged woodlands are included, since the long-term protection of the whole ecosystem function is important.

Acidity Critical loads for woodland

The methods used to calculate acidity critical loads for woodland habitats differ according to the soil type dominant in the 1km grid square in which the woodland occurs. For mineral and organo-mineral soils, critical loads are based on the Simple Mass Balance (SMB) equation and a critical molar ratio of calcium to aluminium (equal to 1.0) in soil solution. Critical loads of acidity for peat soils are based on a critical soil solution pH value of 4.4.

Acidity critical loads maps for woodland habitats (33K)

A detailed explanation of the latest changes made to the methods and criteria for the calculation and mapping of critical loads can be found in Hall et al., 2004 (The United Kingdom National Focal Centre - UK NFC).

Nutrient nitrogen critical loads for woodland

The critical loads for nutrient nitrogen for woodland, used for UK mapping, have been revised in the light of the conclusions of the Berne workshop  (PDF-673K) in November 2002. The Berne workshop, proposed a single nutrient nitrogen critical load value for forests of 10-20 kg/ha/yr, as quite reliable, and suggested that national experts use broad guidance on modifying factors to decide on national mapping values. However, an approach based on specific response variables was considered to be more appropriate for use in the UK. These responses, for defining critical loads in the UK are the effects on:

  • Nitrate leaching
  • Ground flora
  • Epiphytic lichens and algae.

The critical loads for managed conifer plantations are set to protect the soil on the basis that there is little ground vegetation present under most managed conifer plantations in the UK. Furthermore, it is likely that management practices will have more impact on the vegetation than pollution inputs. A summary of the nutrient nitrogen critical loads for the different woodland habitats is shown in Table 1.  Empirical and Steady state mass balance approaches are used for their calculation. Further details are given in Hall et al., 2003 (UK NFC).

Table 1. Summary of empirical critical loads for nitrogen (kg/ha/yr) for forests
Woodland habitat Critical load methodology Critical load Receptor
Unmanaged woodland Empirical


Ground flora
Managed conifer Steady state mass balance approach


Soil ecosystem
Managed broadleaf Steady state mass balance approach


Ground flora
Atlantic oakwoods Empirical


Epiphytic lichens

Critical loads exceedance

Critical load exceedance maps for nutrient nitrogen and acidity now account for forest management activities (thinning, harvesting and fertilisation) that can affect nitrogen and base cation removal or addition through growth uptake. This is achieved though the uptake term in the Simple Mass Balance equation based on growth estimates and nutrient concentration measurements made across the Level II network. These datasets have recently been updated.

Nutrient nitrogen exceedence maps

Managed woodlands (27K)

Unmanaged woodland (22K)

Acidity exceedence maps

Broadleaved, mixed & yew woodland habitat: unmanaged areas only (16K)

Broadleaved, mixed & yew woodland habitat: managed broadleaved woodland (18K)

Coniferous woodland broad habitat: managed areas only (16K)

The latest UK statistics on the percentage of habitat area exceeded for acidity and nitrogen is shown in Table 2.

Table 2. UK statistics on the percentage of habitat area exceeded for acidity and nitrogen based on the updated 2004 critical loads and the latest version of deposition data for 1999-2001 ( Hall et al. 2004)
Woodland habitat 2004 % habitat area exceeded
Acidity Nitrogen
Conifer woodland (managed) 70.5 92.8
Broadleaved woodland (managed) 69.1 97.8
Unmanaged woodland 58.6 95.8

Are our forests endangered by acid and nitrogen deposition as suggested by the critical load approach?

The latest critical load exceedance statistics indicate that between 50 and 70% of the UK’s woodland area is growing under excessive acidity deposition and between 93 and 97% for nitrogen (see Table 2). The critical load exceedance is based on calculated critical loads and current acid deposition.

The acidity critical loads are set to protect the soil while the nitrogen critical loads are set to protect the ground flora. Therefore tree growth and health are not the receptor defined in the UK approach to critical loads mapping. When steady-state critical loads are compared to deposition two outcomes are possible:

  • The deposition is below the critical loads, which implies that there is no critical load exceedance; there is no risk to the ecosystem and a reduction in deposition is not necessary
  • The deposition is above the critical load and thus critical load exceedance is apparent; there is a risk for damage to the ecosystem and reduction of deposition is necessary.

It is often assumed that a reduction in deposition will immediately remove the risk of harmful effects to the ecosystem. However, the response of soils, especially their solid phase, to changes in deposition may be delayed by decades or even centuries as a result of (finite) buffers, the most important being the cation exchange capacity. These finite buffers are not included in the critical load formulation, since they do not influence the steady state, but only the time to reach it. Calculations using exchangeable cations in the soil and the steady state critical load exceedance can be used to estimate the time-frame expected for detrimental effects on tree health to be observed.

For three Level II plots, this time was calculated (Kennedy et al., 2001) to be:

  • 1187 years for Alice Holt (Oak)
  • 66 years for Grizedale (Oak)
  • 13 years for Ladybower (Scots pine)

There is generally a time lag between a chemical change and a biological response. The same is true when an ecosystem recovery from excessive pollution loading is expected. Thus, the critical loads can be used as a measure of potential damage but not as a measure of damage that has already occurred. Dynamic models are now used to predict the time-frame over which damage to forest ecosystems is expected.

High variability in nitrogen deposition to forests is also evident. Critical load exceedance statistics are based on a 5 km grid. Nitrogen deposition can vary greatly over this area, potentially introducing an overestimation in the area exceeded. At the same time, the extent of exceedance at woodland edges will be underestimated. Another area of uncertainty is that where much of the N loading is derived from ammonium deposition, this nitrogen load is likely to be smaller to the ground flora as a result of uptake by the overstorey. Thus, nitrogen critical loads exceedances needs to be interpreted with caution.

Further research will improve the uncertainties associated with critical load calculations and exceedances.


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