Estimation of carbon stock in urban parks: Biophysical parameters, thresholds, reliability, and sampling load by plant type

At present scarce information is available on what biophysical parameters of plants are important to what kind of vegetation carbon stock, and what thresholds should be adopted in sampling different types of plants, and what species/plants store carbon most efficiently. The answer to these questions is significant as it dictates the reliability of the carbon stock estimate and estimation cost, and can guide the design of efficient urban green infrastructure. This study aims to assess the carbon storage capacities of plants, their biophysical parameters important to the carbon stock by plant type, and the influence of parameter thresholds on carbon estimate and sampling load. In total, 1,681 woody plants were surveyed in 20 plots in Auckland parks and reserves. Trees (51%), shrubs (22%), and ferns (27%) co-existed spatially, and stored 115.30, 2.05, and 5.94 Mg C, respectively, at a ratio of 19.4:0.3:1. On average, a tree, shrub, and fern/palm stored 138.09 ± 333.7, 5.59 ± 6.0, and 13.36 ± 15.5 kg C per plant. The top five carbon-contributing tree species were exotic pines (29.1%), kauri (9.8%), rimu (3.8%), kahikatea (2.9%), and matai (2.1%). Matai should be the choice in revegetation projects to maximise carbon stock given its unparalleled carbon contribution index (17.2). Stem length was totally ineffective in indicating plant carbon, followed by canopy area. Diameter at breast height (DBH) was a better indicator of tree and shrub carbon . Total vegetation carbon would be underestimated by 3.7% if a DBH threshold of ≥10 cm were adopted for trees, and ≥14 cm for shrubs and ferns, all causing the carbon stock of the respective plant type to be underestimated by about 1.60 Mg C. However, these thresholds would reduce the overall sampling load by 47.2%. The established relationship between DBH and cumulative carbon allows calibration of the estimated carbon in accordance with the DBH threshold adopted in sampling different plants. Whether the amount of calibration still holds true in other types of urban plants requires further study.