Canopy effect of Prosopis juliflora on selected grass species by using soil

Introduction

Prosopis juliflora is native to Central and South America and then started to spread from southern Mexico to Panama and from the Caribbean Islands to Venezuela and northern Peru (Bokrezion, 2008). Nowadays, it can be found in various semi-arid and arid climate zones, including Ethiopia. It was first introduced to the Awash area of the Afar Regional State some 30 years ago (Zaraye, 2008). Prosopis juliflora competes with grasses and indigenous trees and has invaded farmland and rangelands in the area. The farmland causes great damage to biodiversity, livestock and food production. Prosopis has a wide adaptability of soil ranging from sand dunes to clay soils, from saline to alkaline soils, from areas below 200 to more than 1500 m above sea level and mean annual rain fall ranges between 50 to 1500 mm. It can also withstand and survive temperatures from as high as 50°C (air temperature) and 70°C (soil temperature). It is one of the most common trees in semi-arid and arid parts of the sub-tropical and tropical zones (Pasieczik et al., 2004).

In Ethiopia, Prosopis juliflora is considered a major threat because of its invasive nature. It has an aggressive invasive character, invading pastureland, irrigated cultivated lands and irrigation canals, causing an irreversible displacement of natural pasture grasses as well as native tree species. In terms of Prosopis juliflora coverage, the area's most adversely affected nationally include the Afar and Somali Regions in the east and southeast of the country and the area around Dire Dawa City. There are also moderately affected areas in Amhara, Oromia, Southern Nations Nationalities and Peoples (SNNP) and Tigray Regions, that is, in the mainly dry lands of Central, East and North Ethiopia (Steele et al., 2009). 

Prosopis juliflora is one of the invasive alien plant species that are threatening the native plant species. It grows in very hot, dry climates, with a temperature up to 48°c and annual precipitation from 150 to 750 mm. Prosopis root reaches to great depth in the soil searching for the required water. It can grow in a variety of soil types, including saline and alkaline areas in sandy and rocky soils. 

The tissue of Prosopis juliflora is photo synthetically active throughout the year, presenting a wide spread root system through which the tree fully exploits the available water resources. Its low nutritional requirements and resistance to water deficit give Prosopis juliflora a great plasticity of response, which allows its wide distribution in arid and semiarid zones in the tropics (Pasiezik et al., 2001). However, the Prosopis decline in growth of the annual grasses like A. adscensionis during invasion was most likely due to competition for light (shading) from invading Prosopis juliflora, the increases in the annual grasses C. virgata and S. verticillata, the perennial grass C. dactylon and the non-succulent shrubs A. lindleyi and S. tuberculata were most probably in response to the establishment of favorable environmental conditions in the sub canopies and neighborhoods of the trees. Aristida adscensionis performs poorly in shade while C. virgata, C. dactylon and the alien non-succulent shrub A. lindleyi tolerate shade and are hardy colonizers of bare or disturbed soil (Nidhlovu et al., 2016). 

Allelopathy is one of the interesting properties found in some plants as Prosopis species with a depressive effect on the associated flora attributed to the release of chemicals that affect other plants. Moreover, the negative impact of the plant could be through light deprivation, competition for water and nutrients, or leaching of Allelopathic compounds. Some of the plant secondary metabolites are called allelochemicals or their products that are released into the environment and may affect neighboring organisms by affecting their physiological activities. These species are causing enormous damage to biodiversity as well as agricultural systems (Bais et al., 2003). Therefore, this research was initiated to study the canopy effect of Prosopis juliflora on selected grass species through soil aqueous extract.