Ecology and Phenotypic Variations of Vitellaria Paradoxa Subspecies Nilotica (Kotschy), A.N. Henry et. al. in Ethiopia

The ecology and the phenotypic variations of Vitellaria paradoxa (Gaertner f.), a multipurpose tree, highly valued for its oil was studied in Gambella region: Ethiopia. The objective of the study was to examine the ecology and the phenotypic variations between and within the natural population of Vitellaria paradoxa. The sections under ecological studies were done through inventory of the woodland based on random sampling method, in green house and in laboratories while the phenotypic variations were analyzed based on morphological traits. Results from population survey of 46 ha of woodland consisting of 230 quadrats showed that Vitellaria paradoxa accounted for 9.9% of the total trees and its basal area was estimated at 1.68 m2 ha -1, representing an 30.15% of the total cover. There were significant differences, at P<0.05, in terms of mean diameter at breast height, height and basal area between the populations. The total stocking rate was estimated only at 18 trees ha-1 and estimation of the diameter class of the species showed that 48.54% of the trees had diameters lying above the mean. The recruitments were estimated at 22 seedlings ha-1 and the species demography was also not encouraging as the ratio between the matured trees and the recruitments were approximately equal (1:1). There were also no viable seeds of Vitellaria paradoxa found in the soil seed bank. The soil seed bank was dominated by grasses (43%) and sedge (22%) and there was no correspondence between the seed bank regenerations and the natural populations particularly for trees and shrubs. The regeneration trend showed that the population of Vitellaria was threatened. The soil analysis result showed that the species requires medium textured soil and pH <6 and the foliar analysis result depicted that the species was not deficient in nutrients. The indicator value of the species was 53.8% and it occurred in more than half of the quadrats and its pattern of distribution was much influenced by elevation gradient. There were variations in the main mean tree characteristics such as crown diameter, basal area, diameter at breast height and height and the major mean leaf characteristics such as petiole length, total leaf length, lamina width and petiole ratio along the altitudinal gradient and the variation in crown diameter and lamina width was highly significant at P<0.05. Small sizes of leaves and petiole lengths were found in Elia and larger sizes of leaves and petiole lengths were recorded in Abbot and Phugnido. Medium size leaves and petiole lengths were registered in Agnota. Similarly, short trees with smaller diameters were found in Elia, tall trees with spreading crown and bigger diameter were found in Abbot and Phugnido and taller trees with prominent crowns were concentrated in Agnota. Cluster analysis of the six qualitative traits also showed the close relatedness of the four populations where Abbot and Phugnido formed one cluster and Agnota and Elia formed clusters of their own. The allometric relationships between mean tree parts and the mean leaf characteristics and the mean leaf characteristics with each other were also significant at P<0.05. For instance, diameter at breast height (dbh) was strongly related to the basal area (ba), (dbh = 0.988ba +1.128, r2= 0.976), basal area (ba) was strongly related to the crown diameter (cd), (ba= 0.542cd +0.116, r2= 0.288). Among tree characteristics, only crown diameter was related to lamina width (lw), and petiole ratio (pr), (cd= 0.396lw + 0.436pr + 1.605, r2= 0.150 (lw), r2=0.176 (pr). All the major leaf characteristics were also strongly and positively related with each other. For example, petiole length (pl) was strongly related with total leaf length (tll) (pl=0.818tll +0.310, r2=0.669), lamina length (ll) was strongly related with total leaf length (tll), (ll = 0.954ll+ 0.690, r2=0.909) and lamina width (lw) was strongly related to leaf ratio (lr) (lw= 0.711lr +11.992, r2=0.502). The cluster membership result showed a random distribution of the trees from one eco-geographical region to the other. For example, groups 81, 82, and 84 were found in all the four populations, group 87 was found in two populations (Abbot & Elia), groups 83 and 88 were found in three populations (Abbot, Elia and Phugnido) while group 85 was found only in Elia. The variations in the traits, that formed the cluster groups, were explained by four axes that accounted for 77% of the variance in the sampled trees by quantitative traits matrix of the sampled plots. In general, the phenotypic variation result showed that the in between population variation was higher than within population variation for the quantitative traits and this phenotypic variation due to in between populations was 86.52%. Similarly, the in between populations variation was higher than the within population variation in terms of the qualitative traits and the percentage of the phenotypic variance due to within population accounted for 92.61%. Thus, based on the total trend in the demography of the species and the location of the variations, two complimentary strategies are suggested, firstly, in-situ conservation approach involving as many populations as possible should be considered since there are high inter population variation, secondly, ex-situ conservation involving as many selected individuals within a group from all cluster groups should be undertaken concurrently as there are high inter population variations to reserve as well as to initiate improvement program for Vitellaria paradoxa ssp. nilotica.

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