Too much of a good thing?

In experiments in greenhouses and gardens woodland plants grow better with more nutrients. More nitrogen for example may help them survive better under dense shade, because the plants can produce larger leaves and capture more sunlight. In the field additional nutrients can be a disadvantage to some of the woodland flora. The plants may be lusher and more attractive to herbivores; so they are more likely to get eaten.  Additional nutrients favour tall, competitive species such as nettles that outgrow smaller, stress-tolerant woodland specialists such as primrose.

Nettles encouraged at a wood edge by inputs of nitrogen from the adjacent field; livestock farming and car exhausts are major contributors to nitrogen emissions.

Levels of nitrogen compounds in the atmosphere (various oxides and ammonium compounds) have been increasing from industry, transport and farming. However, in the late 1990s I was somewhat sceptical as to whether these were sufficient to really cause long-term changes in the internal vegetation of British woods. In the Bunce Survey, looking at changes in 100 broadleaved woods across the country between 1971 and 2001 there was no overall shift in species towards more fertile/eutrophic assemblages and no change in mean Ellenberg fertility score (Kirby et al., 2005). Increasing soil pH and high levels of intensive land surrounding the wood were associated with increases in Ellenberg fertility scores; and species increasing in cover were more likely to be associated with high (rather than low) nutrient status conditions.  Yet, in most of the woods I was looking at the changes being blamed on increased nitrogen seemed often to have a more likely cause such as deer grazing.

In 2007 it was suggested that increases in nitrogen were making this wood more grassy. Perhaps so, but the small cage trial indicated that deer grazing was more significant; in 2019 dog’s mercury still survives in the cages while grass dominates outside.

Since 2000 the effects of eutrophication on woodland vegetation have become more obvious and the evidence has built up accordingly. Sally Keith from Bournemouth University revisited woods in Dorset that had been recorded by Professor Ronald Good, a local botanist, in the 1930s when nitrogen pollution levels were lower. The mean number of species in each wood was much the same in 2008, but there were fewer differences between sites, with species of fertile soils more common in 2008 (Keith et al., 2009). The Countryside Survey 2007 found an increase in competitive species, which are favoured by high nitrogen, in woodland compared to the 1990s (Norton et al. 2012).

Changes in the vegetation at woodland edges are becoming more common. Cleavers and cow parsley are a more regular feature of the outer 10-20 m of woods where the wood-edge abuts farmland. Hedge garlic is another species that seems to be on the move. It is one of the food plants for the Orange-tip butterfly along with Lady’s Smock. In the early 1990s a study of the butterfly at Monks Wood in Cambridgeshire commented that Lady’s Smock was then the only food plant present in the wood (Dempster, 1997). By 2016 Hedge Garlic was common towards the edges of Monks Wood and it has similarly started to move into Wytham Woods.

Cow parsley, cleavers and hedge garlic are getting commoner in woodland edge zones. 

On the continent some studies have reported such edge effects and the expansion of high-nitrogen species such as rosebay willow-herb and raspberry (Brunet et al., 1998). However other studies looking at change over the last 20-40 years have not shown such clear results (Verheyen et al., 2012). In some of these studies this may be because the baseline for the comparison was after 1970, and the main changes in plant species caused by increased nitrogen might already have happened. Elsewhere the nitrogen levels may still be building up in the soil, but the effects on the vegetation have not yet come through because the plant growth is still more limited by light than nutrients.  A risk then is that the nitrogen time-bomb may be triggered when woods are opened-up by coppicing or thinning, that allows more light to reach the ground.

So, eutrophication of woodland has moved up my list of threats to woodland flora. While edge inputs might be reduced to some extent by creating buffer strips and hedges around woods, there is not an easy solution to the more general deposition of nitrogen compounds across the countryside, short of major lifestyle changes, such as all of us making fewer car journeys.

Some buffering is possible against edge sources; but are we willing to give up our cars?

BRUNET, J., DIEKMANN, M. & FALKENGREN-GRERUP, U. 1998. Effects of nitrogen deposition on field layer vegetation in south Swedish oak forests. Environmental Pollution, 102, 35-40.

DEMPSTER, J. P. 1997. The role of larval food resources and adult movement in the population dynamics of the orange-tip butterfly (Anthocharis cardamines). Oecologia, 111, 549-556.

KEITH, S. A., NEWTON, A. C., MORECROFT, M. D., BEALEY, C. E. & BULLOCK, J. M. 2009. Taxonomic homogenization of woodland plant communities over 70 years. Proceedings of the Royal Society B: Biological Sciences, 276, 3539-3544.

KIRBY, K. J., SMART, S. M., BLACK, H. J., BUNCE, R. G. H., CORNEY, P. M. & SMITHERS, R. J. 2005. Long-term ecological changes in British woodland (1971-2001). English Nature Research Report 653. Sheffield.

VERHEYEN, K., BAETEN, L., DE FRENNE, P., BERNHARD-ROMERMANN, M., BRUNET, J., CORNELIS, J., DECOQ, G., DIERSCHKE, H., ERIKSSON, O., HEDL, R., HEINKEN, T., HERMY, M., HOMMEL, P., KIRBY, K. J., NAAF, T., PETERKEN, G. F., PETRIK, P., PFADENHAUER, J., VAN CALSTER, H., WALTHER, G.-R., WULF, M. & VERSTRAETEN, G. 2012. Driving factors behind the eutrophication signal in understorey plant communities of deciduous temperate forests. Journal of Ecology, 100, 352-365.



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