Ecological constraints to 'old-growth' lichen indicators: Niche specialism or dispersal limitation?

European landscape conservation includes the recognition of inter-related 'ancient' and 'old-growth' woodland. Ancient woodland is defined by its temporal continuity, which can be measured through its consistent occurrence on historic maps over a period of time, typically several centuries. Old-growth woodland has attributes of both temporal continuity and structural complexity; European old-growth woodland is now extremely rare and a valuable conservation resource. Indicator species provide recognition of old-growth woodland, through traits that are sensitive to its defining features: (i) dispersal limitation demanding temporal continuity of suitable habitat prior to colonisation (as is associated with ancient woodland), and/or (ii) specialist niches associated with old and senescent trees (which may or may not be found in ancient woodland, depending on its past management). To test the response of indicators to each of these features, niche models were developed for lichen epiphytes in an ancient and structurally diverse woodland stand, thus corresponding to 'old-growth' condition. Models were projected for the ancient and an adjacent regenerated stand. There was less suitable habitat in the regenerated stand, and a lower proportion of suitable habitat was occupied. Nevertheless, indicators had colonised from the ancient to the regenerated stand within 50 years. Viewed against the background of previous work, we conclude that landscape context - the spatial relationship between ancient and regenerated woodland - is critical to the interpretation of indicators, which are perhaps better conceptualised as markers of threat and conservation value than independent measures of woodland history.