VI
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.
VII
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.