Eucalyptus Fastigata Site Productivity On Converted Farmland; Implications for Reforestation of Marginal Agricultural Land in New Zealand.

Up to 2.9 million hectares (24%) of New Zealand’s farmland can be defined as marginal; that is, land that yields a poor economic return or has a high erosion risk. A large percentage of marginal land is located in regions likely to experience adverse effects under climate change. There is increasing interest to establish plantation forests on marginal land, including Eucalyptus fastigata, and Sequoia sempervirens. However, little is known about productivity, or suitability, of marginal land for these species. This study investigated E. fastigata productivity on 15 sites throughout New Zealand that were converted from farmland to forest (ex-farm). Site productivity was defined as the height from the tallest 20% of trees at age 11. Soil analysis included total carbon (C) and nitrogen (N), various phosphorus (P) extractions, Bray extractable cations, and pH. Other environmental variables include air temperature, precipitation, elevation, latitude, and soil order. Mean annual air temperature, total soil N, and soil C:N had the highest individual correlations with productivity. However, multiple linear regression analysis found an optimal seven variable model that explained 78% of the variance in site productivity; air temperature, precipitation, soil water storage in the root zone, Bray-2 P first extraction, Bray-2 P total extraction, soil order, and sodium hydroxide extracted P. Also, P extracts contributed to a stronger overall model than total soil N and C:N. The results showed that the effects of soil on site productivity should not be analysed in isolation. Although temperature and rainfall strongly influenced site productivity, apparent soil fertility on ex-farm sites had an important impact. Soil nutrients must be considered in identifying optimal sites for growing E. fastigata and other species on marginal land.