Many native New Zealand shrub species occur in montane short-tussock grasslands. Little quantitative information was available concerning effects of grazing by introduced mammal herbivores on growth, survival and recruitment of such shrubs. In 1959, two exclosures were constructed at 650 m altitude (primarily to study mountain beech Nothofagus solandri regeneration) on Cass Hill, Canterbury (South Island). This paper reports shrub community changes between 1960 and 1977.
The (post-and-wire fence) exclosures measured 20 x 20 m x 2.l m high. The lower 1.5 m was covered in 3-cm mesh chicken wire (the bottom buried into the soil). They excluded sheep, cattle, red deer Cervus elaphus and brown hares Lepus europaeus. One (‘boundary exclosure’) straddled the beech forest-tussock grassland interface. The other (‘grassland exclosure’) was in grassland 25 m from the forest edge. The main tussock grasses were fescue tussock Festuca novae-zelandiae and blue tussock Poa colensoi.
In December 1960 and 1977, each shrub and tree in an 8 x 8 m section of the grassland exclosure, a 16 x 16 m section of the boundary exclosure and adjacent unfenced control areas, were mapped and height measured.
Survival, establishment and height growth were determined for the six most common shrubs: matagouri Discaria tonmatou, golden bush Cassinia fulvida, mingimingi Coprosma propinqua,leafy coprosma C.parviflora, wire-netting bush Corokia cotoneaster and Hymenanthera alpina.
In 1977 there was little visual difference in vegetation appearance or height between the exclosures and their controls. For most shrub species populations were stable, with high survival, low height growth and some recruitment.
Survival: Overall, exclosure shrub populations showed no consistent differences to their controls. There was a tendency for higher survival in exclosures, but the only statistically significant difference was higher Cassinia survival in the grassland exclosure than its control.
Recruitment: Recruitment was lower for Hymenanthera, Coprosma, Corokia, and Cassinia in the boundary exclosure than its control (many new Cassinia in the latter), perhaps suggesting that some grazing disturbance facilitates initial shrub establishment (e.g. by providing open areas for germination). But the grazed grassland did not show this trend, with many new Discaria in the exclosure.
Height growth: Growth varied, some individuals declined and others increased regardless of site, but height increase was overall low. Shrubs in exclosures generally exhibited greater growth. Discaria had significantly greatergrowth in the exclosures. Coprosma in the boundary exclosure had significantly greatergrowth than in its control. No statistically significant differences were apparent for Cassinia between the grassland exclosure and control or for Hymenanthera between the boundary exclosure and control. Sample sizes were too small to allow further comparisons. Increases over the 17-year period varied from zero for Hymenanthera (boundary control), to 3.9 cm for Discaria (grassland exclosure), to 21 cm for Cassinia (grassland exclosure), to 52 cm for Coprosma (boundary exclosure).
Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.royalsociety.org.nz/Site/publish/Journals/nzjb/1978/55.aspx