The legacy effect of soil conditions on restoration of retired pasture

Abstract

Restoration of previously forested land back to forest is an increasingly common and important activity globally. Much of this restoration occurs on land previously used for agriculture, and this former activity can leave an imprint on the environmental conditions under which restoration is executed. My thesis research was to quantify the legacy effect of retired pasture on New Zealand ecological restoration near Auckland. I asked whether pasture soils used for ecological restoration are dissimilar from soils of reference old-growth forests, whether these soil characteristics influence growth of planted seedlings, and whether competition from residual exotic grasses also reduces seedling growth. I studied restoration plantings occurring at Long Bay, Shakespear, and Tawharanui Regional Parks located on the east coast, north of Auckland. Each of these regional parks are 'farm parks', with the balance between pasture and natural forests being gradually expanded to forests by retiring areas of pasture and planting with a mix of 4-8 pioneer species. I focused on plantings that were two to six years since establishment. I compared the soils of restoration areas with those of natural reference forests and nearby pasture. I also measured plant identity and size, and grass cover and height, including kikuyu (Cenchrus clandestinus), in randomly located 25m2 plots within the restoration areas. Soil parameters analysed were bulk density, soil moisture, pH, organic carbon, total N and Olsen P. My study revealed some clear differences in soil characteristics among pasture and restoration sites, and reference forests. Pasture and restoration sites had soils of higher fertility (higher P and N) and lower CN ratios than reference soils. As well, growth of the seedlings planted on the restoration sites was negatively correlated with slope, bulk density, kikuyu cover, and total N. Of the two most abundant species planted, kanuka (Kunzea ericoides) grew almost twice as fast as manuka (Leptospermum scoparium), but manuka was more successful in low fertility and harsh sites. My results show that an agricultural legacy does occur in the soils of these restoration sites, and does influence the success of restoration plantings in the critical early years. I recommend the investigation of management techniques to reduce this agricultural legacy, that future restoration plantings on these parks emphasise kānuka over mānuka, and that plantings at Shakespear, characterised by very low fertility soils, use models of natural low fertility ecosystems to inform the restoration there.