Moss on Rocks: Evaluating biodeterioration and bioprotection of bryophytic growth on stone masonry
Keywords: masonry
cultural heritage
bioprotection
bryophyte
biodeterioration
limestone
moss

Lower plants and organisms growing on cultural heritage sites have been identified as key sources of biodeterioration and degradation of the underlying substrates. Within this field, bryophytes (mosses) have been relatively under-researched and the effects of their growth on heritage sites is comparatively unknown.
The main aim of this thesis is to characterize the impacts of moss growth on historic limestone masonry, and to determine if their presence on built structures can be categorized as having either an overall bioprotective or biodeteriorative effect on built structures. This thesis addresses the objectives and three primary research questions related to the perceptions of moss growth, visualizing the moss-lithobiontic interface, and moss impacts on moisture regimes using a mixed- methods approach involving field and lab analyses. To address the first objective, it provides a qualitative exploratory survey of built heritage experts and visitors about their perceptions of moss growth on cultural heritage. To address the second objective, the thesis develops a workflow to visualize the impacts of moss on limestone. To address the third objective the thesis uses a range of field and lab-based analyses to investigate the impacts of moss on water regimes on limestone and mortar surfaces. It uses both weathered laboratory samples of limestone and in-situ limestone walls and cementitious mortar to evaluate moss impacts under a range of conditions.
The project is based on research at several built heritage sites in Oxfordshire, England, including the UNESCO World Heritage site Blenheim Palace, Kelmscott Manor, and Oxford Botanic Gardens. It investigates the of impact one of the world’s most common mosses, Brachythecium rutabulum on a variety of wall structures at these built heritage sites, and on some experimental limestone blocks with known colonisation histories.
Key findings from this project uncover both aesthetic and tangible impacts of moss colonisation on built heritage. Findings from the survey showed that both visitors and built heritage experts like the aesthetic qualities of mosses on historic stonework, and feel that removal of moss should only occur if clear biodeteriorative impacts are found. The survey also highlighted the need for further research on how mosses affect the underlying materials. The thesis also explored a possible workflow method for utilizing weathered samples from cultural heritage sites, and found visual evidence of mosses acting as secondary colonisers within lichen perithecial pits. Key scientific findings from this project are that mosses play a role in increasing both surface and subsurface water absorption and uptake, and that removing mosses from weathered surfaces may be more damaging than leaving them in-situ. Additional findings are that mosses may play a role in ’shielding’ underlying substrates from light rainfall events. The evaluated findings show initial evidence that in most cases, moss growth is not an indicator of serious damage, and in some cases is neutral or even protective.
The research project has contributed to advancing the knowledge of moss impacts on cultural heritage, and in creating a set of methods and protocols for investigating the moss-rock interface. The findings also have implications for conservation decisions regarding the cleaning and maintenance of mosses on cultural heritage.