Outstanding remnants of nature in compact cities: patterns and preservation of heritage trees in Guangzhou city (China)

Heritage trees (HTs) play pertinent ecological-landscaping roles in cities, yet the few known studies are mainly descriptive or resource inventories. This paper explores the intertwined natural and cultural spatial associations between trees (species, dimensions and age) and city (landscape impact, urban fabric, district history and landuse), assessing the integrated city-tree complex at the macro-scale. The study area includes eight main built-up districts (covering 116 km2) of Guangzhou, a compact and major city in South China. A survey of historical and inventory records identified 348 HTs which encompass the city's officially recognized HTs. Principal component analysis (PCA), Jaccard Index of Similarity (JIS), and other statistical tests helped to unravel spatial associations. The 25 HT species, compared to 254 species in the city's entire urban forest, are dominated by four common species. The number of HTs increases with district age of 25-2500 years. The more stressful and cramped roadside habitat accommodates more HTs, followed by spacious park, and intra-lot interstitial spaces in education, government-institution, and religious sites. The HTs are dominated by the 100-200-year age bracket, with few older trees, indicating a generation gap and the inability of old districts to preserve older trees. Park and religious sites accommodate more of the oldest and largest trees. Roadside contains more wide-crown trees mainly due to effective protection from motor vehicles by the inner bicycle lane. District is indicated by JIS as the major determinant of species variations, with landuse playing a secondary role. Large trees polarise into normal and degraded tree forms due to physical site constraints. Tall-wide and short-wide tree forms with high landscape impact contribution dominate landscape impacts. PCA extracted three factors that explained 71.9% of the variations, namely age-dimension, potential growth, and district history. Increasing compactness and the present management regime may fail to sustain the HT stock and deprive the city of HT precursors. The concept of ecological brinkmanship and transgenerational arboriculture are proposed to enhance HT conservation.