Ecological Investigations On The Potential Of Three Woodland Tree Species For Agrofrestry Practices

Three indigenous tree species namely Acacia tortilis (Forssk.) Hayne, Acacia senegalrn(L.) Willd. and Balanites aegyptiaca (L.) Del. were studied to explore their potentialrnfor agroforestry practices. Studies on the ecology and growth characteristics of the threerntree species and their influence on environment were conducted at three different sites;rnAdamitulu, Awash and Abajata Shalla National Parks in the rift valley. A total of 15 rnreleves (50 x 50 m), five releves in each of the sites were established. Inrneach of the releves presence and abscence and frequency of all tree species wasrnrecorded. From each site a total of 15 trees, five for each study tree, were randomlyrnselected for detailed investigation. Tree characteristics which included tree height,rndiameter at breast height (DBH), canopy depth, canopy radius and canopy diameter ofrneach of the study trees were measured. Cover of undercanopy vegetation was estimatedrnfor each of the study trees to assess the influence of trees on undercanopy speciesrndiversity. To investigate the influence of trees on undercanopy soil, samples wererncollected from 50, 250, and 450 cm distances at 02.5, 5.0 - 8.0, and 20 - 23 cmrndepths along the transect radiating from tree boles. Texture, pH, conductivity, organicrncarbon, total nitrogen, available phosphorus and exchangeable potassium of the soilrnsamples were analyzed. For comparison purposes cover of vegetation outside treerncanopies and soil physical and chemical properties beyond the canopies of trees werernalso investigated. Of the three sites, Abijata Shalla area was characterized by high tree rndensity (61/ 2500 m2 ), followed by Awash National Park (36/ 2500 m2 ) and Adamitulu rnarea (17/ 2500 m2 ). Acacia torti/is had the highest DBH (0.42 ± 0.12 m), canopy diameter rn(13.64 ± 2.25 m) and canopy radius (6.91 ± 0.79 m) whereas the highest tree height (5.18 ±rn0.77 m) and canopy depth (2.14 ± 0.73 m) were recorded for B. aegyptiaca.rnUndercanopy vegetation diversity of A. tortilis was highest (2.13) foIlowed by A.rnsenegal (1.92) and B. aegyptiaca (1.78). In all sites undercanopy species diversity wasrnsignificantly higher than vegetation diversity outside of tree canopies.rnTexture and pH did not vary between under and outside canopy soils implying that treerncanopies did not affect these soil properties. Surface soil conductivity, organic carbon,rntotal nitrogen, available phosphorus and exchangeable potassium were significantlyrnhigher under tree canopies for all the study trees compared with outside canopy soils.rnComparison of the study trees top soil nutrient contents indicated that under canopyrnsoils of A. tonilis had higher values foIlowed by A. senegal and B. aegyptiaca,rnrespectively. Acacia tortilis and A. senegal had comparable seedling growth rates whereas the formerrnhad a higher seedling emergence vigor. The results showed that the study trees significantlyrn enriched soil under their canopies and created a suitable environment for the growth of undercanopyrn vegetation. Over all A.tortilis had better growth characteristics and improVed most undercanopy soil. The significance of the findings for agroforestry are discussed.

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