Elaeagnus Angustifolia Colonization and Understory Floristic Successional Patterns at Mid Valley, North Patagonia, Argentina

From circa 1970, Elaeagnus angustifolia (olivillo, Russian olive) has been a notorious invader in the valley livestock fields at Mid Valley of Rio Negro, in North Patagonia, Argentina. The species colonized riparian and plain areas forming different stands. Once introduced in this semiarid region, the seasonal and yearly climatic variations, as well as the flow regimen of the river, influenced the colonization speed. Depending on the landscape, E. angustifolia stands may develop as monospecific groups or in associations with other trees, leaving some uninvaded areas in-between. The different shapes and the main species forming the canopy of the stands, affect the floristic composition, and the cattle forage quality, of the US (Understory) community, if compared with the uninvaded zones. Other herbaceous community can be distinguished at the stands’ border. The description of the different stages on E. angustifolia colonization in Mid Valley for 25 years, shows that the colonization was hastened by rainy years and wet soils. Once established, and while the new specimens reached the reproductive age, the enhancement of the population was the result of vegetative sprouting and diminished US diversity. This period was immediately followed by drought years, but the amelioration of soil quality of the already nitrogen fixating roots and the attenuation of incident sunlight, benefited herbaceous strata under the canopy of all the stand with Russian olive. Posterior changes in trees composition and management of the cattle fields, influenced US diversity. Today, the herbaceous composition studies show that, under dense monospecific population of E. angustifolia, the pasture reduces forage quality because of the dominance of non-palatable species. Stands’ border areas combine an enhanced nitrogen content in soils, and enough light to develop a herbaceous community of annual and perennial grasses and forbs. An increment on biodiversity and spontaneous forage biomass is noticed after the Russian olive plants are removed.