Effects of woody species thinning on herbaceous layer Charasteristics of a wooded grassland community in Kibwezi, Makueni district Kenya

Abstract

In Kibwezi Dryland Field Station, Makueni district, the vegetation has changed physiognomically from grassed shrubland to dense wooded shrubland making the area unsuitable for cattle grazing. In October 1992 a study was initiated to document overstorey-understorey relationships following tree and shrub thinning. Four treatments of varying tree and shrub density each replicated four times were randomly imposed in plots measuring 20m by 15m. They consisted of thinning trees and shrubs to the following canopy covers, 0% (complete clearing), 35.5% (500 shrubs and 167 treesjha), 55.3% (1167 shrubs and 167 treesjha) and an average of 67.6%- (unthinned treatment with approximately 1800 shrubs and 400 treesjha). A clipping method was used to determine the biomass of herbaceous species while their basal cover and frequency was obtained by the point frame method. Average biomass production and basal cover of herbage components (grasses and forbs) for the study period in relation to woody canopy cover was analysed using F-test statistic. To quantify these relationships, simple linear and quadratic regression analyses were conducted. XII Canopy cover reduction had significant effects (P<O.05) on increases in biomass production and basal cover of grasses, forbs, and total herbage. There was a strong relationship between biomass production, basal cover and frequency of herbaceous species with the over storey canopy cover. The relationship between woody canopy cover and herbage production was described by a 2nd degree polynomial equation, i.e. y-=3493.4-9.806X-O.171X2 (p=O.OOOl, r2=o.946, n=16). The equation y-=12.07-0.013X-O.0005x2 (p=O.OOOl, r2=o.966, n=16) described the relationship between woody canopy cover and basal cover of total herbage. When linear regression equations were fitted for the two vegetal classes (forbs and grasses) at varying woody canopy cover, the overall rate of biomass and basal cover increase for grasses was slightly higher than forbs. Forbs did better at the lightly thinned treatments. Competition between woody and herbaceous species both in the tree and shrub canopy zone and in their root zone reduced performance of herbaceous spec i.es in the lightly thinned treatments. A degree of protection from competition will give herbaceous species advantage over woody species; this can be achieved through thinning as shown by results of this study. It was concluded from the data that in order to maximize herbage production, tree and shrub density should be reduced to achieve a canopy cover of not more than 35.5%. After achieving the desired production potential, a balance between herbaceous plants and woody species should be maintained through cattle grazing and fire.