Urban CO2 imprints on carbon isotope and growth of Chinese pine in the Beijing metropolitan region

Abstract

Rapid urbanization has occurred globally and resulted in increasing CO₂ emissions from urban areas. Compared to natural forests, urban forests are subject to higher atmospheric CO₂ concentrations in view of strong urban-periurban-rural gradients of CO₂ emissions. However, relevant insights in the CO₂-associated urban imprints on the physiology and growth of regional forests remain lacking. By sampling foliage and tree rings of Chinese pine (Pinus tabuliformis) in the Beijing metropolitan region, China, we explored whether and how urban CO₂ emissions affect stable carbon isotope ratios (δ¹³C) and tree growth spatially and/or temporally. The results indicate a significant decrease in foliar δ¹³C values towards the urban center and this pattern was mainly explained by the urban-periurban-rural gradients of CO₂ emissions as surrogated by trunk road density. Tree-ring δ¹³C values showed a significant decrease over last four decades and this trend was mainly explained by rising levels of CO₂ and secondarily mediated by the variations of aridity index during growing season. Moreover, annual basal area increment of Chinese pine was significantly accelerated during last two decades, being mainly driven by increasing CO₂ emissions and secondarily mediated by climate variations. These findings reveal significant CO₂-associated imprints of urbanization on plant growth and provide empirical evidences of significant CO₂-induced alteration of carbon cycles in urban forests.

Keywords: Basal area increment; CO(2) enrichment; Stable carbon isotope; Tree ring; Urban forest.