This study reports ethnographic and experimental analyses of inter-generational changes in native Itza' Maya and immigrant Ladino populations of Guatemala's Petén rainforest concerning understanding of ecological relationships between plants, animals, and humans, and the perceived role of forest spirits in sustaining these relationships. We find dramatic changes in understanding ecological relationships and the perceived role of forest spirits. Itza' Maya conceptions of forest spirits (arux) are now more often confounded with Ladino spirits (duendes), with Itza' spirits no longer reliably serving as forest guardians. These changes correlate with a shift in personal values regarding the forest, away from concern with ecologically central trees and towards monetary incentives. More generally, we describe how economic, demographic, and social changes relate to the loss of a system of beliefs and behaviours that once promoted sustainable agro-forestry practices. These changes coincide with open access to common pool resources.