Evaluating the stick-gap method in a Mexican tropical dryland: Effect of stick length and sample size

Abstract

The degradation of drylands by human activities is a serious problem in many developing countries. Monitoring in these countries therefore is basic to prevent or mitigate land degradation. The stick-gap method is one graphical approach that has been used to monitor rangelands in eastern Africa. This method collects data on vegetation cover and structure with a 1-m-long stick at 20 locations per site. These data are then used to calculate land health indicators. Because the stick-gap method is easy to implement and to understand, it is suitable for monitoring drylands in other developing countries. However, the effect of stick length and sample size on the estimation of indicators such as basal gaps should be first evaluated. In this study, we measured basal gaps with 1-m- and 0.5-m-long sticks at 20 and 100 locations per site in two mesquite shrublands with different vegetation structure within a Mexican tropical dryland area. We correlated the basal gaps estimated using different stick lengths and sample sizes with the basal gaps estimated using one standard tape method. The correlations between these estimates were not significant considering 20 locations per site, independent of whether 1-m- or 0.5-m-long sticks were used. However, the correlations between these estimates were significant considering 100 locations per site. Also, the correlations between these methods were slightly higher when 0.5-m-long sticks were used in comparison to 1-m-long sticks. These results were similar in mesquite shrublands with either open or closed canopies. We conclude that increasing sample size to 100 locations per site and decreasing stick length to 0.5m would be sufficient for estimating the proportion of basal gaps in both open- and closed-canopy mesquite shrublands.