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Achieving effective Rhododendron control

PhD research project

PhD student: Eddie Daly
Supervisors: Nick McCarthy & Prof. John O'Halloran 


Project Description: 
One of the most serious invasive alien species posing in Ireland a threat to local biodiversity, particularly in woodlands and forests, is Rhododendron (Rhododendron ponticum L.). Rhododendron, despite being widespread in Ireland, is not a native Irish plant and was introduced here during the nineteenth century as an ornamental garden plant and has since become established.

Once established on a susceptible site, Rhododendron can kill local plant species in the ground vegetation layer and prevent the regeneration of native trees and shrubs, thereby also indirectly affecting the local fauna. 

Controlling the spread of invasive species in forest plantations traditionally involves either manual brush cutting or herbicide application. Effective results can be achieved using both methods, though they do have a number of disadvantages, such as public concern over water contamination and harmful effects to non-target flora and fauna. Consequently land managers and research biologists have been searching for alternatives methods of control that are effective and also environmentally safe.

This study investigated the potential for a recently discovered Irish fungal pathogen Chondrostereum purpureum (Pers.:Fr) to prevent regeneration of Rhododendron and Silver Birch (Betula Pendula).  During this 18 month trial it was shown that both Rhododendron and Birch are resistant to the isolate of C. purpureum used in this trial.

As a successful invasive species, Rhododendron is often found in areas where recent land management activities have taken place and disturbed substrate and the absence of predators provide optimum regeneration conditions. As most land management practices (particularly in a forestry situation) remove substrate and expose ground cover, the degree to which depth of substrate affects the germination of Rhododendron seeds was investigated. A negative relationship between litter depth and germination Rhododendron seeds was observed with deeper forest litter layers reducing the establishment of Rhododendron.

The exclusion of herbivores from young plantation forests using fencing is often used to prevent crop loss through grazing damage. This resultant exclusion of herbivores may also prevent seedling survival of native broadleaf scrub species. The role of grazing in the establishment and survival of Rhododendron was investigated and observations to date have demonstrated a negative impact of animal browsing on the seedling survival of the native broadleaf scrub species birch and holly. This has been established as a long-term trial to investigate whether herbivore fencing leads to an increase in native plant species and ultimately allow them to outcompete the invasive Rhododendron.

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Planning and Management Tools for Biodiversity in a Range of Irish Forests

PLANFORBIO Research Programme, Dept. of Zoology, Ecology & Plant Science, University College Cork, Distillery Fields,North Mall, Cork, Ireland