Factors affecting eucalypt survival in South African plantation forestry

Abstract

A review was conducted to highlight the most important factors contributing to increased mortality and decreased growth in eucalypt plantations during re-establishment (0–12 months). Citations were ranked in terms of credibility, with the importance ratings (derived from the literature sources) applied to the different factors affecting survival and growth during eucalypt re-establishment. Of the various factors impacting early eucalypt mortality, water stress and planting stock quality were considered highly important. Other factors that negatively influence survival and early growth, albeit to a lesser degree than water stress and planting stock quality were the manner and quality of site preparation (soil and slash); planting practices (planting depth included); timing of planting (during dry, hot periods); various post-planting operations (incorrect fertiliser placement or herbicide drift) and insect pests and diseases. In South African forestry, soil water availability is a major contributor of re-establishment success and, as a water scarce country, research indicates the negative impacts this can have on survival and growth. Plant stock quality was also considered important, especially in terms of matching planting stock size (as influenced by age in container, species, season raised, etc.) to container type/size, as the planting of over- or under-prime planting stock will result in higher mortality. These factors cannot be considered in isolation due to the complex interactions that exist among them and determining the primary causes of mortality can be elusive, especially as their impacts tend to be additive by nature. Despite all the research implemented under controlled and field conditions, high mortality (and subsequently below optimal final yield) is still problematic. Most commercial forestry companies target a survival of at least 90% three months after planting, but it is not clear where this threshold level originates and it may be purely anecdotal. This raises key questions pertaining to the economic implications of various mitigation practices, including blanking, replanting and planting at higher densities that can ensure optimum stocking at rotation-end. The factors need to be looked at in greater detail to quantify the relative importance of each. The degree of interrelatedness among the factors should also be further researched.