Evaluation of Morphological Properties of Avenue Tree Species and Concentrations of Pollutants Under and Outside Their Canopies in Port Harcourt, Nigeria

Abstract

We evaluated the morphological attributes of three major avenue tree species (Delonix regia, Gmelina arborea and Terminalia mantaly) and the concentrations of pollutants under and outside their canopies at four locations reflecting different levels of anthropogenic activities that contribute to pollution, in Port Harcourt, Rivers State, Nigeria. The sites are: Shell Petroleum Development Company (SPDC) Agbada (II) Flow Station, Aluu; LNG Seaport (LNG), Trans-Amadi; Isaac Boro Park (IBP), Mile 1; and Rivers State University (RSU), Nkporlu. Three trees belonging to each of the three species were randomly selected at each site and measurements of their total height (THT), diameter at breast height (DBH), crown diameter (CD), and basal area (BA), were taken. Concentrations of carbon monoxide (CO), nitrogen dioxide (NO₂), ammonia (NH₃), methane (CH₄), sulphur dioxide (SO₂), and particulate matter (PM) under and 50 m outside their canopies, were also measured. One-way Analysis of Variance (ANOVA) was used to test for significant difference (p ≤ 0.05) in tree attributes and the concentrations of pollutants under canopies of each tree species among sites, while t-test was used to test for significant difference (p ≤ 0.05) in the concentrations of the pollutants under and 50 m outside tree canopies at each site. THT, DBH, CD and BA of the three tree species varied significantly at the four sites with highest and lowest mean values recorded for RSU and LNG, respectively, except for D. regia where DBH and BA were highest and lowest at IBP and LNG, respectively. The concentration of pollutants under tree canopies at different sites did not vary significantly (p > 0.05) in most cases (except PM) despite the significant differences in tree growth/morphological attributes. There was no significant difference between the concentrations of pollutants under and 50 m outside tree canopies in all the sites. Higher concentrations of CO and PM were recorded under trees at LNG Seaport while NO₂, SO₂ and CH₄ were higher under trees at SPDC, indicating that the level of pollutants at each site reflected the nature of the activities associated with the sites and not differences in tree morphological attributes. However, concentrations of the pollutants, except CH₄, were below recommended exposure limits under and outside tree canopies, at all the locations.