Pollutant nitrogen and drought tolerance in heathland plants.
It has been suggested that pollutant nitrogen inputs adversely influence the response of
heathland plants to certain climatic and biotic stresses. One specific hypothesis being that
elevated nitrogen deposition may reduce drought tolerance in heathland vegetation.
However, there is little evidence from field studies that this is the case. The aims of this
research project have been firstly to establish the effects of increased nitrogen inputs upon
plant water relations in certain dwarf heathland shrubs under winter and summer drought
conditions. Secondly, to contribute to the understanding of how changes in plant water
relations caused by increased inputs of nitrogen affects the growth, physiological
perfonnance and consequent long tenn survival of heath land plant communities.
Experimental field work was carried out at an existing upland site in Clwyd, North Wales
near Ruabon from July 1995 to April 1999. Experimental work with potted plants was
carried out in the glasshouse at Crewe, Octoberl995 to June 1997. A lowland field site at
Budworth Common in Cheshire, consisting of twenty, 2 x 1 metre plots in a replicated
random block layout was established by the author during March 1996, remaining a site of
investigation until December 1998. At both the upland and lowland field sites the
vegetation was dominated by Calluna vulgaris (L.) Hull.
At Ruabon a twelve month study of shoot relative water contents in relation to soil
moisture deficits was carried out on a monthly basis. Visible frost injury was surveyed and
recorded each spring (April 1996 to April 1999). At Budworth Common (summer 1997),
following 12 months of nitrogen applications (0, 20, 60 and 120 kg N ha-1yr-1 (NH4N03» a
protracted period of drought was imposed on field plots by the use of drought shelters,
these covered the whole of each plot.Comparisons were drawn between watered and droughted plants in the field. This was
carried out by applying the equivalent summer rainfall for Budworth Common to one half
(1m2) of every treatment plot and droughting the other half (1m2). Nitrogen treatments
were continued at fortnightly intervals throughout. Budworth Common was used to study
the impacts of elevated nitrogen and drought for a five month period from May to
September 1997. Intensive experimental field work over the drought period recorded:
weekly measurements of soil moisture deficits, shoot water potentials and shoot extension
growth. Measurements of shoot relative water contents and infra red gas analysis were
taken, the flowering period was recorded. During the post drought year (summer 1998) a
spontaneous outbreak of Lochmaea suturalis (heather beetle) in the field plots showed the
beetles to have a preference for high N treated Calluna. An increase in the competitive
grass species Deschampsia jlexuosa was seen in droughted plots during summer 1998,
particularly the high N treatments.
As a result of experimental work carried out both in the field and laboratory this study has
shown that elevated nitrogen does have detrimental impacts on the drought tolerance of
certain heathland plants. Field work results compared positively with those obtained from
the pot experiment. A higher incidence of visible frost and drought damage was recorded
in high nitrogen plots. Lower soil moistures were recorded in high nitrogen plots.
Measurements of shoot water potentials revealed that high nitrogen treatments under
drought conditions exhibited more negative values than did low nitrogen plants. High
nitrogen increased the number of early opening flowers under conditions of full water but
in plants receiving high nitrogen and drought flowering was delayed. This study increases
the understanding of the impacts of elevated nitrogen inputs on the water relations of