Abstract:
This study was conducted in Nech Sar National Park , Gama Gofa zone, Southern Ethiopia
with the objective of determining the major encroaching woody plant species in the park,
their seed distribution pattern, their impact on grass biodiversity as well as the possible
causes of encroachment in the area. Systematic sampling method was used to collect
vegetation data. A total of 80 quadrats each with 400 m
2
(20 m X 20 m) were established
along four line transects for woody species data collection. Grass species data was collected
from 1 m
2
sub plots at the 4 corners and center of the 20 main plots. To know the pattern of
seed distribution for dominant species soil samples were taken from the center and four
corners of 15 selected plots. Data on species cover abundance, presence of a species, height,
'
altitude and geographical coordinates were recorded from each quadrat. Density, frequency,
dominance and abundance were used to describe vegetation structure. Alpha, Gamma and
Beta diversity and also Shannon-Wiener diversity and evenness index were used to assess the
levels of diversity. Correlation analysis was conducted to investigate the impact of woody
species on grass diversity. A total of 24 woody species belonging to 15 genera and 9 families
were recorded in the park. Species such as Cyathula orthacantha, Abutilon figarianum,
Dichrostachys cinerea, Barleria acanthoides, Solanum incanum, Acacia oerfota and A.
mellifera were found to be the major encroaching species. The plant community analysis
revealed the existence of four community types. Density of woody species in the park found to
be 4402 ha-
1.
Overgrazing, fire suppression, climate change in terms of erratic rainfall and
drought are considered to be among the main factors that caused woody plant encroachment.
Expansion of bush cover is confirmed to have a negative impact on grass diversity. Since,
diversity of grass species negatively correlated with woody density (r=-0:943, p<0.10). it is
. believed that bush encroachment has already started to affect sustainability of the park
particularly in the form of habitat change. Grazing animalsare assumed to be the primary
victims of the current dynamics. Therefore, implementation of holistic approach that involves
the whole community and experts to control expansion of bush encroachment and decrease
the contribution of over grazing and settlement to bush encroachment are recommended as possible solutions.
