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BROAD-LEAVED TREE AND SHRUB INVASION OF CONIFER PLANTATIONS IN IRELAND
ABSTRACT
A survey of 2298 quadrats sited in 400 geographically dispersed conifer plantations in Ireland was conducted to ascertain the extent and diversity of the natural regeneration of woody plants within them. A number of environmental variables were also measured, including altitude, latitude, longitude, soil pH and light. The probability of occurrence of the 22 most commonly regenerating species were modelled against the measured environmental variables. A separate survey was carried out on the dispersal patterns of cache-dispersed species of trees within the conifer plantations.
Sixty-two species of regenerating woody plants were identified growing within conifer plantations. The most species-rich sites were found in older, thinned plantations of Scots pine growing on lowland, mineral soils.
The environmental resources of soil nutrient status, light and the amount of accumulated growing season heat energy were found to affect the form and size of the ligneous plant community that developed on a site. In highly resourced sites forest canopy species attained a large size while woodland shrubs established in the low light conditions under the developed canopy. In intermediately resourced sites small trees were able to colonise, requiring more light. In low resourced sites only small, moorland shrubs and conifers were capable of colonising, requiring high light conditions to do so.
The results suggest that accumulated growing season heat energy should be added to light and soil N as major limiting factors in mesic terrestrial habitats.
The interaction between these limiting factors did not produce a similar response in species distribution, drawing into question the assumption of a normal distribution curve for the environmental ranges of woodland species.
A forestry management system designed to produce high quality softwood sawlogs (longer rotations, thinning, high pruning and growing species other than sitka spruce), will also produce a more diverse woodland ecosystem than that obtained from a management system designed to maximise timber output, regardless of quality.
The grouping of the natural regeneration of the main hardwood timber species in Ireland (oak, ash, beech and sycamore) on sites that are invaded by ivy suggests that the presence of ivy indicates a proposed forestry site that will support a crop of broad-leaved trees.