Abstract
The net outcome of the interaction between woody and herbaceous plant species can be positive or negative depending on plant species and environmental conditions. Positive interactions (facilitation) are postulated to be more prevalent and negative interactions (competition) less prevalent, under high environmental severity. Positive interactions have been attributed to the so-called ‘fertile islands’ and ‘nurse plant’ effects. The ‘fertile islands’ and the ‘nurse plant’ effects are suggested mechanisms for plant interactions in arid and semi-arid environments worldwide. Some indigenous plants in dryland New Zealand are postulated to have grown under a woody canopy and may benefit from the restoration of the woody habitat. However, restoration of indigenous plants may be impeded by mammalian herbivory. In this study, the ‘fertile islands’ and the ‘nurse plant’ effects and their possible role in the ecological restoration of a predominantly indigenous dryland mixed woody and herbaceous vegetation, were investigated, in the presence and absence of rabbit herbivory, in the dry sub-humid Central Otago by field experiments and by glasshouse ecophysiological experiments.
The fertile islands effect was investigated by comparing soil properties under four shrub species to those of the adjacent grassland. Soils under Kunzea ericoides and Coprosma propinqua were more fertile compared to those of the grassland. Soils under two other species Discaria toumatou and Rosa rubiginosa were not significantly different in fertility from soils in the grassland.
The nurse plant effect was tested in the field by comparing the height growth and survival of five planted herbaceous plant species as well as the natural recruitment of herbaceous species under a shrub canopy to those of the adjacent grassland, with or without grazing. In general there was no significant difference in height growth or survival between plants under a shrub canopy and those in the grassland. Grazing had a negative effect on the height growth and survival of herbaceous plant species. Herbaceous plant species richness under a canopy differed with shrub species and season of measurement. Effects of a shrub canopy on herbaceous plant species richness ranged from mainly positive (Rosa rubiginosa) to mainly positive but inconsistent (Coprosma propinqua and Discaria toumatou) and mainly negative but scale-dependent (Kunzea ericoides). However, herbaceous plant species composition was not significantly different between areas under a shrub canopy and the adjacent grassland.
The nurse plant effect was also examined experimentally in the glasshouse by comparing the response of the same five herbaceous plant species to light and defoliation. All five herbaceous plant species showed some degree of shade tolerance with higher concentrations of leaf total chlorophyll under shade than in full light. Acaena buchananii, Luzula ulophylla and Carex breviculmis showed lower range of shade tolerance than Anthoxanthum odoratum and Acaena agnipila. Acaena buchananii and Carex breviculmis showed the highest degree of tolerance to defoliation in the glasshouse and may be good candidates for restoration.
The fertile islands effect has been demonstrated for Kunzea ericoides and Coprosma propinqua but not for Discaria toumatou and Rosa rubiginosa. However, the nurse plant effect has been demonstrated for Rosa rubiginosa and partly for Discaria toumatou and Coprosma propinqua and these three may the best woody species for ecological restoration. Moreover, because of their tolerance of shade and defoliation, Acaena buchananii and Carex breviculmis may also be the best understorey species for ecological restoration. Restoration of indigenous woody plants coupled with control of rabbit herbivory may be some of the best forms of management intervention for the ecological restoration of a predominantly indigenous dryland mixed woody and herbaceous vegetation in south-central New Zealand.