Ecology and Development Series No. 10, 2003 - ZEF

Impacts of human use on the forest vegetationabundant than the other plant species. The abundance of coffee was omitted in order tocompare the contribution of non-coffee plant species to the vegetation structure.Because, inclusion of coffee in the abundance data contributes only slightly to the totalabundance of the NATFOR and SECFOR. On the other hand, in the managed forestcategories coffee overwhelmingly dominates (as high as 50%) due the low abundance ofother species. In the structural analysis, plants were grouped into four major heightclasses (HC): HC 1 (< 1.5 m tall), HC 2 (1.5-12.5 m), HC 3 (12.5-24.5 m), and HC 4(>24.5 m). This height class with different ranges was used to capture the existingconditions of the forest vegetation structure with respect to coffee. The first class, HC1,represents seedlings and young saplings that are too small for diameter measurement,and does not contribute to the basal area. The stems or individuals in the HC2 includeshrubs, saplings and the majority of small trees. Individuals in the HC2 occupy a similarheight class with mature coffee trees and hence are the ones that compete with coffeefor space. Height classes HC3 and HC4 constitute shade trees in lower and uppercanopy layers.5.2.2 Analysis of differences in species compositionThe difference in the mean number of species per plot among the studied forestcategories was tested using one-way analysis of variance (ANOVA). One-way ANOVAis similar to the t-test except that it allows comparison of more than two groups ofdatasets. In order to test how one forest category differs from another, the Mann-Whitney-U-test was used. The U-test is the nonparametric equivalent of the independentsamples t-test and is recommended for counting data such as number of species andabundances of this kind.The Shannon index was calculated using the software EstimateS 6.01b(Colwell 2000). The Shannon index represents the information statistic indices of themeasures of heterogeneity. It takes into account the evenness of the abundances ofspecies. Additionally, Shannon evenness (E) was calculated as the ratio of the Shannondiversity index (H) to the maximum diversity (H max ). The value of H max is the naturallogarithm of the number of species (Magurran 1988).The relationship of the number of species and abundance can be used to detectthe prevalence of human intervention in a community, using species abundance models.71