Studies assessing the effects of biochar used as a soil amendment in agriculture and forestry have indicated variable results, from significant improvements in growth and health to no effect at all. Research into biochar use for trees within the urban landscape is extremely limited. This review is aimed at arboricultural practitioners and professionals involved in urban tree landscape management and provides a critical analysis of the use of biochar to support tree health and establishment. Biochar, specifically wood biomass-based biochar, has the potential to enhance tree establishment and survival. However, considerable variability in the physical and chemical properties of biochar currently limits universal application. Therefore, practitioners should aim to use biochar types suitable for the desired function, such as transplant establishment, remediation of declining mature trees, and pest/disease management. Biochar also represents a promising complementary amendment to more established soil management techniques such as mulching and fertilization, but further long-term studies in a range of conditions typical of urban environments are required to fully understand the effects of specific biochar types on urban trees.
High mortality rates result from transplanting bare-rooted plants into urban landscapes where unsuitable soil conditions, such as low fertility and poor structure, often exist. Coupled with little aftercare, these losses can cause high economic losses to the industry. Previous studies have shown lowered transplant stress and higher survival rates through the addition of soil amendments at the time of planting to improve soil conditions. The efficacy of three soil amendments applied singly and in combination—biochar, slow-release molasses, and an organic N:P:K fertilizer—were investigated for their potential to reduce transplant losses of Pyrus communis ‘Williams’ Bon Chrétien. Results of this investigation showed that use of these soil amendments in virtually all cases had significant positive effects on tree growth and vitality across two growing seasons. For example, all amendments reduced mortality of Pyrus communis ‘Williams’ Bon Chrétien’ by 20% compared to none in treated controls, while increases in fruit yield and crown canopy coverage per tree ranged from 19.3% to 46.7% and 14.4% to 31.1% over non-amended soils when averaged over two growing seasons. Amendments of biochar with an organic N:P:K fertilizer and an organic N:P:K fertilizer alone showed, on average, the highest improvements in vitality and growth. Results indicate use of biochar, slow-release molasses, and organic N:P:K fertilizer amendments offer potential for increasing bare-root transplant survival and establishment of Pyrus communis ‘Williams’ Bon Chrétien.
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