ABSTRACT The general view of trees is that they threaten heritage structures, their roots disrupt archaeological features, or that they create microclimates conducive to biodeterioration. The conservator's experience in assessing the impact of trees on various cultural assets highlights the positive role they can play in modifying the outdoor environment to reduce stress mechanisms. Trees intercept sunlight and rain, absorb ground water and stabilise soils. This is particularly beneficial at rock art sites where infrastructure must be kept to a minimum to maintain the spiritual and aesthetic ambience. While focussing on studies demonstrating the protective role of trees, it is critical to make balanced assessments that recognise both positive and negative implications; a tree can both shade an object and increase the risk of mechanical damage from roots and falling branches. Objective assessments consider all implications rather than through intellectual and operational bias. Laboratory studies demonstrate hydrothermal stress to be greater than heating or wetting alone, and freeze–thaw that has been traditionally considered one of the most destructive mechanisms disrupting outdoor stone. Interventive approaches for reducing hydrothermal stresses in stone rely upon chemical treatments to repel water, and consolidation to better resist such stresses. A well-designed tree canopy can substantially reduce thermal expansion and almost completely remove rain from the object through interception, funnelling to the trunk and uptake of groundwater.