Abstract
An understanding of underlying causes assists in the prediction of outcomes and management of plant successions. The purpose of this study was to investigate the causes of the succession from gorse to broad-leaved forest by experiments under controlled conditions and manipulative experiments in the field
In the controlled experiments, the effect of different light levels on germination, survival and growth was investigated for some of the major species in the succession (principally, Ulex europaeus (gorse), Kunzea ericoides, Coprosma robusta, Pittosporum eugenioides and Melicytus ramiflorus). Species performance was matched against the generalised behaviour of typical early- and late-successional
species.
While seed characteristics and germination responses were not consistent with those predicted on the basis of successional status, the study species had growth rates and biomass allocations consistent with species of similar successional status. Gorse had significantly higher growth rates than the native woody species at high light levels but not at lower light levels. Differences in growth rates with respect to light levels explained the early dominance of gorse and the later inclusion of native woody species in the canopy. There was little evidence of trade-offs between high potential growth rates and high growth rates and survival at low light levels.
Seed bank studies investigated seed availability in the field. Seasonal patterns of germination and the effect of canopy clearance and litter removal were examined over two years. Survival of emerging seedlings and the growth and survival of transplanted seedlings was monitored in clearings cut in the gorse, under a gorse canopy and under a native forest canopy.
Seed availability of native woody species was low in young gorse stands and higher in older sites closer to seed sources of native forest species. Gorse seed dominated the seed bank of all sites except those within native forest vegetation where gorse had not previously grown. Gorse seed germinated in every month of the study, with peaks in both summers and the first autumn of the study, whereas seeds of native woody species germinated only in spring. The germination of gorse seed tended to be promoted by canopy clearance whereas that of Kunzea ericoides was unchanged and that of Melicytus ramiflorus tended to be inhibited. Litter removal from under an intact canopy promoted the germination of all species. Survival of transplanted seedlings of all species was low under dense native canopy and in unfenced plots. Gorse had the highest growth rates of the study species in the clearing but there was little difference between growth rates under the gorse canopy. Artificial shading in the field had a similar effect on growth rates to the gorse canopy.
Differences in growth rates in the field were again sufficient to explain species replacements early in the succession, but differences in life span and stature were required to explain subsequent exclusion of gorse. Seed availability and germination would influence the starting point of the succession and could result in a longer phase of gorse dominance than that predicted from growth rates alone.
Findings were consistent with the hypothesis that differences in competitive ability along a changing gradient of light availability were one of the major causes of the succession from gorse to broad-leaved forest. However there was little evidence for the existence of the trade-offs that have been proposed as the cause of these changes in competitive ability.
This study allowed options for active intervention in the management of gorse-covered land that might promote the succession to broad-leaved forest to be proposed. These include addition of seed, removal of litter, planting of seedlings and limiting any disturbance of the gorse canopy to winter in order to minimise the effects of clearance on gorse germination.