Species richness and aboveground carbon stocks in the homegardens of central Kerala, India

Tropical homegardens with high agrobiodiversity have high potential for carbon (C) sequestration, especially under changing environments. Floristic diversity, richness, and aboveground C stocks and how size of homegardens influence agrobiodiversity and C sequestration potential were assessed in the Kerala homegardens. A total of 839 homegardens in 28 panchayaths (lowest unit of local self-government) of Thrissur, Palakkad, and Malappuram districts were surveyed through a stratified random process. Information was gathered on holding size, floristic composition, plant height, and girth at breast height (GBH) of all trees and shrubs (>20 cm GBH). Aboveground C stocks of trees were computed using allometric relationships, assuming C as 50% of biomass. The homegardens were also classified into small (<0.4 ha), medium (0.4–1.2 ha), and large (>1.2 ha). In total, 473 species were recorded, of which 208 were trees (>20 cm GBH), 86 shrubs, and 179 herbs. Simpson's floristic diversity index (0.64, 0.41, and 0.46 for a subset of small, medium, and large homegardens, respectively), species richness, and tree density (per hectare) were highest for small-sized holdings. Large-sized homegardens, however, had more stems per garden. Average aboveground standing stocks of C ranged from 16 to 36 Mg ha−1, with small homegardens having higher C stocks on unit area basis than large- and medium-sized ones. Implicit in this is the potential for C sequestration and agrobiodiversity conservation, especially by small homegardens.

Research highlights▶ Species richness increased as garden size increased, but number of species per unit area declined. ▶ Simpson's floristic diversity index (0.64, 0.41, and 0.46 for small, medium, and large homegardens respectively), species richness, and tree density (per hectare) were highest for the small-sized homegardens. ▶ Large homegardens had more multipurpose trees and shrubs per garden. ▶ Average aboveground standing stocks of C ranged from 16 to 36 Mg ha−1, with small homegardens having higher C stocks on unit area basis than large- and medium-sized ones.