Effect of Disturbances on Non-tree Species Richness, Diversity, Distribution, and Abundance in Seasonally Dry Riverine of Engareolmotonyi, Arusha, Northern Highlands of Tanzania

Abstract

The nontree species including shrubs, herbs, sedge, grasses, and ferns are vital for the sustainability of the forest ecosystem. The study was conducted at the Engareolmotonyi seasonally dry riverine forest in northern Tanzania to determine the nontree richness (S), diversity, distribution, and abundance. A total number of 20 plots of 20 x 20 m2 (m2) were purposively established, within which nested plots of 1 x 1 m2were set to determine herbs, sedge, grasses, and ferns; whereas the 2 x 5 m2 subplots were established to identify the woody nontrees for their botanical names, counted for their number of stems, and disturbances were recorded. Shannon index (H’) of diversity was used to determine species diversity. Relative frequency (RF) was used to determine the distribution percentage and abundance. A total of 50 non-trees species were identified. Of all those, 30 were non-woody while from while 20 species were nontree woody plants. Non-woody plants had higher H’ and RF than woody nontrees. The nontree woody plants had less density than no-n woody plants. The most abundant species had an RD of 26.121 ± 15.30.3, while the medium had the RF of 10.290 ± 4.222, and the rest were the least abundant with an RD of ≤ 4.222. The recorded disturbances were; cutting sticks, poles (for snaring), very small trees, collection of livestock fodder plants. The relative frequency of disturbances ranged from 37.50% ± 3.13%. The most dominant disturbance was as a footpath(s) with a relative frequency (RF) of 37.50%), followed by cutting (21.88.14%), livestock fodder collection (12.50%), the intact plots and firewood collection (19.38%), blue monkey and baboon foot tracks (6.25), and snares had an RF of 3.13%. The effect of disturbances on nontrees has been noticed in Engareolmontonyi forest even though the plant diversity remains relatively high. This implies that moderate disturbance has no significant damage to plant richness, diversity, distribution and abundance. The widely distributed plants with a larger number of stems have more advantage of survival than the least distributed in terms of any damage occurring to a part of the seasonally dry riverine forest. Conclusively, the tropical vegetation can tolerate moderate or controlled activities, while excessive disturbances will always lead to a decline in richness, diversity, distribution, abundance, and even extinction of the least distributed and even the most abundant plants depending on the disturbance severity. Restoration of gaps is needed, cutting of trees and snaring should be discouraged, livestock fodder collection should be controlled.