Alluvial grasslands of Canterbury and Marlborough, eastern South Island, New Zealand: Vegetation patterns and long‐term change

Abstract

Grasslands of alluvial systems were studied in 35 valleys from the Mackenzie Basin in Canterbury to Marlborough in eastern South Island, New Zealand, to describe the relationship between environmental factors and vegetation types and gradients. Pastoral modification, represented by decreasing percentage of native species, was the primary determinant of the overall vegetation pattern. The least modified vegetation types, retaining native tussock canopies, were recorded on low pH, organic‐rich, phosphate‐poor soils, in higher‐rainfall reaches of valleys experiencing low summer temperatures. Secondary vegetation gradients, from Festuca matthewsii and red tussock grasslands to Festuca novae‐zelandiae grasslands, corresponded with trends from mesic intermontane to drier, more continental rainshadow climates, and from low‐lying floodplains to elevated terraces. The effects of grazing, and of the cessation of grazing, were examined at fenceline and exclosure sites. The abundance and abundance fraction of native species had decreased over time with grazing at all fenceline sites, and native species richness and native richness fraction had decreased at two sites. Grazing‐exclosed vegetation had higher native biomass than adjacent grazed vegetation at three sites, and higher percentages of native species richness at three sites. The presence or absence of native tussock grasses at the time of exclosure may be critical for post‐pastoral retention or recovery of native biomass. Trends along climate and modification gradients in the occurrence of three major introduced species were compared between valley groups. Agrostis capillaris appears to have reached its potential environmental range in most of the studied valleys. However, Festuca rubra and Hieracium pilosella may not yet occupy their potential range in the North and South Hurunui valleys in Canterbury, and in the Ahuriri, Hopkins, and Dobson valleys in the Mackenzie region.