The ecology of Lough Neagh woodlands

Abstract

Much of the woodland fringing Lough Neagh has developed on exposed ground as a result of lough lowerings carried out during the 19th and 20th centuries. Recent woodland dominated by Alnus glutinosa had colonised ground exposed since the last lough lowering in 1959, whereas older woodland, which colonised ground exposed before 1959, contained Alnus glutinosa, Fraxinus excelsior and Acer pseudoplatanus. These woodlands, which have a fragmented distribution resulting largely from agricultural land use, form part of the designated Lough Neagh Area of Special Scientific Interest.

The first part of this study is concerned with the tree population structure and vegetation of lakeside woods under conditions of changed drainage. This was examined by relating the ground flora and tree species variation of sample sites, to the position on the lakeshore cross-section and the edaphic, environment and management variables. The second part of the study investigates the ecology of the non-native species Impatiens glandulifera. An experimental approach was used to determine the response of the species to disturbance.

A stratified random sample of recent and old shore woodlands was carried out to determine their composition, structure and management. The most frequent trees were Alnus glutinosa, Fraxinus excelsior, Acer pseudoplatanus and Salix cinerea. The population of Alnus glutinosa consisted mainly of larger individuals, with few Alnus glutinosa seedlings being recorded. The Fraxinus excelsior population consisted of a high density of small-sized stems. Although there were small numbers of large Acer pseudoplatanus, it was frequently found as seedlings and saplings. The regeneration of the introduced Acer pseudoplatanus and the presence of non-native ground flora species suggests that change in both the composition and structure of the ground flora and canopy
will occur with time.

Classification of the tree canopy, based on basal area, resulted in two woodland types corresponding to woods on recent shores characterised by Alnus glutinosa and Salix spp., and woods on the older shores, characterised by Fraxinus excelsior and Crataegus monogyna. Classification of the ground flora also resulted in two groupings characterised by the species Hyacinthoides non-scriptus, found in well drained old shore quadrats and the species Phalaris arundinacea, Agrostis stolonifera, Filipendula ulmaria, Glyceria fluitans and Ranunculus repens found in wetter recent shore quadrats. Key ground flora species in both types of woodlands (> 70% frequency) were Urtica dioica and Poa trivialis, both of which are indicative of eutrophic soils.

Multivariate ordination showed that variation in the composition of the ground flora was related to woodland age and the environment gradients, soil drainage, soil depth and light
intensity and grazing management.

This study found no relationship between species richness and woodland area, shape or isolation. Although the presence of the non-native species Impatiens glandulifera and Heracleum mantegazzianum were not affected by woodland area, shape, perimeter or isolation, Acer pseudoplatanus was correlated with larger woods. The distribution of non-native species showed patterns of geographical variation on the lakeshore. Both Impatiens glandulifera and Heracleum mantegazzianum had a northern distribution, whereas Acer pseudoplatanus was associated with the eastern shore. Impatiens glandulifera seeds were found in drift deposits over a wide range of sites except in the south-east.

The response of woodland ground floras dominated by Impatiens glandulifera, to experimental treatments (soil disturbance, nutrient addition, soil disturbance+ nutrient addition, pruning), was monitored for one field season. Nutrient addition was associated with an increase in biomass index (stem number x height) and the percentage cover of Impatiens glandulifera, whereas soil disturbance was associated with a decrease. These effects indicate that the species is resistant to experimental soil disturbance and nutrient addition. The ground flora community responded to treatments in different ways depending on site and treatment. No changes to the ground flora community were detected as a result of the nutrient addition and pruning (removal of Impatiens glandulifera) treatment. Sites that had a ground flora more indicative of wetland habitats were most affected by both the nutrient addition and soil disturbance treatments. Species adversely affected by the treatments included Cardamine amara, Chrysosplenium oppositifolium, Filipendula ulmaria and Ranunculus ficaria, based on their ranked extreme scores on the primary axis of a Redundancy Analysis. Species associated with the soil disturbance treatments, based on ranked extreme scores on the primary axis of a Redundancy Analysis, included Galium aparine, Glechoma hederacea, Geranium robertianum and Aegopodium podagraria, all of which are ruderal species. The implications of disturbance events, such as grazing, on the ground flora of these wetland sites are discussed.

Date of Award	Oct 1996
Original language	English
Supervisor	Alan Cooper (Supervisor)