Species richness, stand structure and carbon storage under an age chronosequence in Tectona grandis plantation at agricultural landscape of Indian Eastern Himalayan Foothill

Understanding the variations in plant diversity, soil physico-chemical characteristics, and biomass storage under an age chronosequence of farm forests is crucial to advance our knowledge of their potential role in biodiversity conservation, soil management and climate change mitigation. The present work was conducted from September 2018 to February 2020 in randomly selected young unmanaged farm forests categorized into ≤ 5, 5–10 and 10–15 years old age classes with standard procedures of estimating diversity indices, biomass, carbon and soil physico-chemical parameters. Overall species richness of the plantations was 28, of which 11 were trees, 9 herbs, 3 shrubs, 3 climbers and 2 ferns. The quantitative vegetation parameters and diversity indices estimated indicate that the farm forests’ plant assemblages constitute mainly native species. Based on IVI values, herbs were the most critical species, followed by shrubs and trees. With the increasing farm age, the species richness, the vegetative parameters and diversity indices decreased. The soil of the plantations was inconsistant nature and unstable characteristics as they did not exhibit any discrete trend with plantation age. Total biomass and organic carbon storage increased with increasing farm age. The overall mean total plant biomass, soil organic carbon, and ecosystem carbon storage in the farm forests were 87.94, 39.62 and 80.96 Mg ha − 1 , respectively. Every subsequent increase of farm age by five years increased the ecosystem carbon stock by 33.22% and 4.09%, respectively while; the total increment was 38.67% from lowest to highest age class. The quantum of carbon storage in these farm forests was less than those reported for the native forests. Still, it can convert an unutilized agricultural landscape to a net carbon sink or even earn carbon credits. Moreover, the vegetation heterogeneity developed in these farm forests within a short span of 15 years can develop into semi-natural secondary forests with multiple social and ecological benefits in the long run.