The limestone cliffs of the Jura mountains in northwestern Switzerland harbour a diverse flora with several relic populations of arctic-alpine and Mediterranean plant species. Overgrowing forests increasingly threaten rare, relic plant species with high light demands on cliffs, because traditional forms of forest use, including the collection of firewood and grazing by goats, were abandoned. We examined the temporal and spatial patterns of overgrowing forest at six cliffs in the northwestern Swiss Jura mountains by analysing tree cover on time-series air photographs made between 1951 and 2000. Overall tree cover increased from 60% to 85% at the six cliffs examined between 1951 and 1964 and then levelled off. The increase in tree cover showed distinct spatial patterns. The cover increased significantly in the talus and on the cliff face, but not on the plateau (at the top of the cliffs). Our results confirm the assumption that the cliff forests became denser during the last decades. Forestry practices such as selective thinning or controlled grazing by goats are suggested as a method of promoting the threatened relic plant species.