Factors influencing natural regeneration of totara (Podocarpus totara D.Don) on grazed hill country grassland in Northland, New Zealand

David O Bergin,Mark O Kimberley

doi:10.1186/s40490-014-0013-8

Abstract

There is increasing interest in establishing indigenous forests on marginal pastoral hill country in New Zealand to improve soil conservation, water quality and indigenous biodiversity. Large-scale reforestation using native seedlings is extremely costly. However, given suitable seed sources, it is possible to encourage natural regeneration of grassland into native plants. A grazed, sloping pasture site in Northland where totara (Podocarpus totara D.Don) regeneration had been allowed to develop over a 10-year period was assessed using a grid of 2-metre diameter plots. Counts of regenerating woody species and cover of herbaceous species were obtained in each plot. Plots were classified on the basis of herbaceous species cover using the TWINSPAN classification procedure. Regression analyses were used to identify site factors and characteristics of the herbaceous vegetation associated with totara regeneration. More than 7,000 stems ha−1 of woody species, mostly totara and gorse (Ulex europaeus L.), had accumulated over the 10-year period despite the presence of cattle grazing. Regeneration of totara was associated with steeper slopes and sites with the greatest diversity of herbaceous species but with lower overall levels of herbaceous cover. The regeneration pattern of the other main woody species was similar although gorse colonised a wider range of sites. Regeneration of woody species was absent on flatter sites, which were dominated by a few high-fertility tolerant herbaceous species and had high levels of herbaceous cover. This study has shown that regeneration of totara on a pastoral hill-country site was most prevalent on steeper slopes of lower fertility with typically sparse vegetation cover comprising low fertility tolerant herbaceous species. Conversely, totara regeneration was absent on flatter areas of higher fertility. It seems likely that grazing assists totara regeneration by keeping pasture cover short, thus increasing light levels near ground level for newly germinated seedlings. The propensity for totara to regenerate on pastoral hill country in the presence of grazing and where there is a nearby seed source could provide a practical option for re-establishing indigenous woody vegetation on erosion-prone marginal land.

Highlights

There is increasing interest in establishing indigenous forests on marginal pastoral hill country in New Zealand to improve soil conservation, water quality and indigenous biodiversity
Porteus (1993) describes a system involving the removal of grass to allow natural regeneration of indigenous seedlings, followed by hand thinning of weed species, and continued protection of selected seedlings from grazing or browsing animals
Plots were classified into four groups on the basis of herbaceous species composition using TWINSPAN (Additional file 2: Table S2)

Summary

Introduction

There is increasing interest in establishing indigenous forests on marginal pastoral hill country in New Zealand to improve soil conservation, water quality and indigenous biodiversity. Less costly systems for establishing indigenous trees and shrubs on grasslands have been advocated such as direct seeding using manuka Porteus (1993) describes a system involving the removal of grass to allow natural regeneration of indigenous seedlings, followed by hand thinning of weed species, and continued protection of selected seedlings from grazing or browsing animals. This system would be too labour intensive to be applied on a large scale

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