Tree-ring carbon (C) and oxygen (O) stable isotope (SI) chronologies spanning the period 1983–2012 were analysed at three Pinus nigra Arn. sites located in the upper Orcia Valley (Tuscany, Italy) in a badlands landscape. The goal of the study was to determine the extent to which tree-ring stable isotopes (SI) can serve as a proxy for mass wasting processes. To this end, we applied an established dual-isotope model to detect physiological changes between trees growing at three sites, one along the upper border of a well-studied shallow landslide, one on the landslide, and one in a stable area, as control. We further analysed whether trees at the three sites showed different δ¹⁸O responses to climate and to precipitation δ¹⁸O. Tree-ring δ¹³C and δ¹⁸O variations and trends revealed impairments of the photosynthetic process at the landslide site. We found that trees growing on the landslide show signs of reduced photosynthetic capacity since the onset of the landslide in 1993, whereas since 2000, while producing compression wood during periods of landslide activity, the trees show trends of higher average maximum net photosynthesis. The correlation analysis performed between the SI chronologies and the climatic variables revealed that the climatic signals at the site located on the landslide are masked by growth stress induced by the mass wasting processes. The most distinct differences in climate responses between sites were found in tree-ring δ¹³C in response to mean temperature and to mean temperature range, and in tree-ring δ¹⁸O in response to precipitation δ¹⁸O. Our research confirms that it is possible to reconstruct mass-wasting processes on forested slopes and to date geomorphological events by considering the trees' physiological conditions as recorded by stable C and O isotopes in tree rings, and by comparing affected with unaffected sites.