Distribution of invasive alien plant species in Akure forest reserve,Ondo State
The study investigated the composition and spatial distribution of the invasive aliens and native species and determined the composition of native and alien invasive species in soil seed banks of the various vegetation physiognomies in Akure Forest Reserve and their contributions to above ground vegetation. This was with a view to assessing the extent to which invasive species have affected the biodiversity of the Reserve. The study was carried out in five study sites named (A,B,C,D,E) representing all the physiognomies present in the Akure Forest Reserve.Site A is a Natural forest, B is a Teak plantation, C is a Taungya system, D is Taungya + Teak + Gmelina while site E is Teak + Gmelina + Pinus Plantation. In each of dry and rainy seasons, ten study plots each measuring 25m x 25m indicating two plots from each of the five physiognomies were used. Within each plot,forall woody species that were one(1m) and above,complete enumeration, separation of enumerated stems into aliens and natives, measurements of density, basal area, height,girth at breast height were determined . The distance of each of the study plots from the nearest built up area and cluster of aliens were recorded. The geographic location of each study plot was also determined using a Global Positioning System (GPS).The dispersal mechanism of each encountered species was noted.In each plot, five replicate soil samples were randomly collected in the dry and rainy seasons to a depth of 0-15 cm and were monitored for seedling emergence tests for six (6) months in each season to determine which of the emerged seedlings were native or alien and their percentage contribution to the seed bank. Shannon wiener diversity index and also Sorensen’s index of similarity were used to measure the degree of similarity among the sites in their species diversity and floristic composition and also the number of woody species in the seed bank and standing vegetation.Differences in speciesrichness,Shannon- wiener index and evenness between the most invaded and the least invaded physiognomies were used to measure the effects of invasion on the biodiversity.Student’s t-test was used to test for significant differences in basal area, stem density and size distributions between aliens and natives. Analysis of variance was used to compare seedling emergence in all the sites.Correlation analysis was used to establish the relationships between alien density and distance from built up areas and cluster of alien. Results showed that there were ninety three (93) woody species consisting of five(5) alien species and eighty eight (88) natives in all the study sites though the aliens had high densities. Alien species found in the study area were Tectonagrandis, Gmelina arborea, Pinuscaribaea, Gliricidia sepium and Carica papaya. The most invaded site which is a disturbed site being a source of revenue to the Government is the Teak plantation while the Natural forest which is relatively undisturbed is the least invaded with no alien species. There was no significant difference in the stem densities of the alien and native species. Size distribution of all stems fall into the lower size classes.The least invaded site had the highest basal area, species diversity, species evenness, species richness while the most invaded site had the lowest in each case except the basal area. Other sites had intermediate values. The most invaded sites were closer to the built- up areas. Distance from built up area was negatively correlated with alien density(r = -0.611,p= 0.01) and also nearness to cluster of aliens (r = -0.626,p=0.01). The lowest similarity index of 5.55% was observed between the least invaded and most invaded.Seedling emergence was higher during the dry season than the rainy season with the least invaded site having the lowest mean seed bank density. Seed bank was dominated by herbaceous stems. Similarity between the seed bank and the standing vegetation was low and also the proportion of alien to native species. There was no significant difference in seed bank density of both the aliens and indigenous species in sites A,B and C (P>0.05) while in sites D and E, there was a significant difference (P<0.05). It is concluded that invasive alien woody species are well distributed in Akure Forest Reserve though the Natural forest is still intact and not vulnerable to invasion by alien species. Natural areas resist plant invasion while disturbed areas are prone to invasions. Other factors that make an area invasible include nearness to built-up areas,cluster of aliens, water bodies,dispersal mechanism and species characteristics of the aliens. Invasion by alien plants have significant impacts and effects on the biodiversity of the Forest reserve by reducing species richness, diversity and evenness. There is no similarity between the standing vegetation and the seed bank and also aliens in the standing vegetation were absent in the seed bank, therefore the seed bank may not produce an effective natural regeneration for alien species.