Forests are globally important ecosystems that provide goods and services to the planet. They have a crucial role in supporting biodiversity, storing carbon, and regulating the climate. The forests around the world, including temperate European forests, are under threat by the global increase in temperature and unpredictable climate conditions caused by climate change. These climatic changes are also causing the spread of new pests and diseases northwards as the planet warms. In recent years, some devastating diseases have affected trees and forests in Europe and the UK. To understand the role of soil, both its biotic and abiotic components, in tree health and disease development, I analyse the effects of soil treatment on diseased and healthy trees. Using the study system of Oaks affected by Acute Oak Decline and Horse Chestnuts affected by Bleeding Canker of Horse Chestnut, I investigate if the treatments successfully improve tree health and prevent disease by analysing phenotypic measurements of the tree, the soil and tree microbiome and the tree metabolome.