Remote sensing assessment of biological activities on built historical structures in a tropical region:  Case of the Osun-Osogbo Sacred Grove UNESCO World Heritage Site, South-Western Nigeria.

Abstract

The relationship between biological communities and the deterioration of cultural heritage in temperate and tropical countries remains a subject of debate. In humid tropical climates, plant growth plays a crucial role in both natural and built environments by contributing to increased biodiversity, stormwater and micro-climate regulation, soil protection, food and medicine provision, and global warming mitigation through carbon sequestration. Despite these significant contributions, there is a lack of understanding regarding the role of vegetation on historic cultural heritage sites in tropical countries and how the distribution of 'greenery' impacts the thermal and physical conditions of heritage buildings and monuments. While laboratory and field studies in temperate regions have shown the complex relationship between greenery (algae, moss, ivy) and stone/rock heritage structures, including bio-erosion, protection, and conservation, rather than solely biodeterioration, similar research is lacking in Nigeria. One such example is the Osun-Osogbo sacred grove, a UNESCO World Heritage site surrounded by dense sacred trees and heritage structures colonized by algae and moss. The influence of these greenery and weathering processes on the physical conditions of walls and monuments has not previously been assessed. This research addresses this gap by using a mixed methods approach, including satellite imageries, air and land surface temperature data, rock surface temperature and humidity measurements and millimetre-scale surface change data to examine the effects of greenery on heritage structures within the Osun-Osogbo Sacred Grove UNESCO World Heritage Site (WHS) to improve historical documentation of the Geoheritage site conservation and degradation processes. The heritage features of interest are the forests, walls and sculptures at the heritage site that are in deteriorating conditions aggravated by the devastating floods in 2019 and climate change. The Landsat (30 m) time-series imageries will be used to investigate the spatial and temporal distribution of forest cover and reveal air and surface temperature trends and wet/dry periods within the sacred grove site. To map and develop a weathering/erosion database, crowd-sourced historical photographs, repeat photographs subjected to Structure-from-Motion (SfM) photogrammetry and LiDAR data (2019 - 2023) will be used and integrated into a geographic information system (GIS) to complement micro-climatic records. Surface change (soiling, cracks) and erosion patterns of heritage structures influenced by biology and other weathering processes will be isolated and assessed using three-dimensional (3D) point clouds, digital elevation models (DEMs) and orthophotographs. Findings from this study have the potential to serve as a guiding framework for identifying the extent and nature of monument degradation to inform sustainable remedial practices and improve heritage site conservation in Nigeria and Africa.