Tree nutrition trends

Foliar samples being collected from the upper canopy at the Brechfa beech plot
Foliar samples being collected from the upper canopy at the Brechfa beech plot

Foliar chemistry provides an indication of the nutritional status of a tree. At ten of the Intensive Forest Monitoring plots, tree foliar chemistry has been assessed annually since 1995. Changes in tree nutrients can thus be related to shifts in local environmental conditions at these sites. For example, foliar nitrogen concentrations in Scots pine at the Thetford Intensive Forest Monitoring site have been in excess of 1.7% of foliar dry weight over the past ten years, rising to 2.2% in 2002 – a value indicating nutrient imbalance (Gundersen, 1999) and reflecting high levels of nitrogen loading at this site.

Graphs showing annual deposition of total nitrogen in throughfall, nitrate levels in soil solution and foliar nitrogen (% of dry weight) at Oak, Scots pine and Sitka spruce Intensive Forest Monitoring sites

It is important to note that high levels of nitrogen deposition and critical load exceedance do not necessarily result in reduced tree growth (Sheppard & Wallander, 2004). However, this situation may not hold in the long term and ‘super-optimal’ levels of foliar nitrogen remain a concern. At the onset of monitoring at the Lady Bower Intensive Forest Monitoring plot, foliar nitrogen concentrations were also above 1.7% of foliar dry weight, but have since declined. This corresponds with a deterioration in crown density, suggesting that excess nitrogen deposition is not a causal factor in the deterioration of crown condition.

There is a general downward trend in foliar sulphur levels at most sites, corresponding to a reduction in soil solution sulphate concentrations.

Foliar sulphur levels at one Scots pine Intensive Forest Monitoring site (Rannoch) are now classed as deficient according to Van den Burg (1985) and, if the downward trend continues, this is indicative of a real possibility that large areas of British woodland on sensitive soil types could become sulphur deficient in the foreseeable future.

Graphs showing annual deposition of total sulpher in throughfall, sulphate levels in soil solution and foliar sulpher (% of dry weight) at Oak, Scots pine and Sitka spruce Intensive Forest Monitoring sites

Two out of the three oak sites have foliar phosphorus (P) concentrations classed as deficient, according to Taylor (1991), reflecting extremely low available P in the soil. Other essential nutrients such as magnesium, calcium and potassium are at optimal levels, but in the future they may decline, especially in infertile soils, as a result of emission reductions. Base cation concentrations in rainfall have already fallen at most sites.

Future changes in climate, air quality and soil and water chemistry are likely to impact on tree nutrition. Monitoring at Intensive Forest Monitoring sites can give an early warning to forest managers of possible nutrient deficiency as well as inform policy makers of the timeframe and direction of forest responses to the implementation of pollutant emission control measures.

References

Chemical fluxes in time through forest ecosystems in the UK – Soil response to pollution recovery (2009) (PDF-701K)

Gunderson,P. (1999). Nitrogen status and impact of nitrogen in forests – indicators and their possible use in critical load assessment. Paper presented at Conference on Critical
Loads, Copenhagen, November, 1999.

Sheppard, L.J. & Wallendar, H. (2004).  Atmospheric nitrogen - pollutant or fertilizer? In 'Plant ecophysiology: Nitrogen Aquisition and Assimilation in Higher Plant'. Edited by S. Amâncio and I. Stulen. Springer Netherlands. pp. 65-98.

Taylor, C.M.A. (1991). Forest fertilisation in Britain. Forestry Commission Bulletin 95. HMSO, London.

Van den Burg (1985). Van Den BURG, J. (1985). Foliar analysis for determination of tree nutrient status – a compilation of literature data. Report No 414. Rijksinstituut voor Onderzoek in de Bos – en Landschapsbouw ‘de Dorschkamp’, Wageningen, The Netherlands.