The management of plants and their impact on monuments in historic gardens: Current threats and solutions

The conservation of historic gardens is crucial for safeguarding monumental, aesthetic, historical, ecological and economic values in many countries of the World, as well as associated services, such as carbon stock, microclimate and water regulation, biodiversity conservation, pollution removal, and recreation. In historic gardens, architectural and sculpture elements coexist with an abundant plant component, which is currently often precarious due to senescence processes occurring nowadays. Unhealthy plants and reduced structural stability of trees represent a threat for both garden artistic structures and buildings, as well for the visitors’ safety. Awareness in garden managers about the most relevant and current threats is necessary for garden conservation. This review, through a global survey of the literature since 1990, addresses two main questions (1) which are the most relevant threats on historic gardens vegetation as affected by environmental, biological and anthropogenic causes, and how do they impact on monuments? (2) Which are related strategies to counteract these threats? Regarding the whole analysed period, the impact of the biotic component on monuments was the most discussed threat; in recent years a growing concern on the effects of climate change and pathogens and pests on historic garden plants also emerged. Strategies to address current and future challenges of historic gardens are hereby identified from experiences reported in worldwide literature and discussed. Best practices are collected in tables to provide managers of historic gardens with a valuable tool and guide to conserve and enhance their value. Due to the heterogeneity of the threats to be addressed, a multidisciplinary approach to ensure the conservation of historic gardens is recommended.