Land degradation in a Mexican tropical dryland: An assessment using soil, vegetation, and human disturbance indicators

Background/Question/Methods

Land degradation is a serious problem in drylands that comprehends biophysical (soil erosion, vegetation loss) and socioeconomic (firewood collection, livestock raising) issues. It has been suggested that understanding this problem requires the analysis of these issues to determine the causing factors and the spatial and temporal scales at which they act. The Zapotitlan Valley is a tropical dryland in central Mexico with land degradation problems because land-use change and habitat fragmentation. The goal of this work was to conduct an assessment of land degradation using soil, vegetation, and human disturbance indicators. By using these indicators we tried to consider the biophysical and socioeconomic issues. The work was conducted in fluvial terraces of the valley because these sites have the most severe degradation problems. Terraces were classified according to soil texture (fine, intermediate, coarse) and vegetation cover (closed and open mesquite shrublands, abandoned field crops) in eight environmental units. At each unit, we conducted 3-5 transects to obtain data on soil properties, plant cover, and human activities. Quantitative indicators such as soil stability, percent plant cover, and number of firewood piles among others were calculated from data. Indicators were compared among units with parametric and non-parametric tests.

 Results/Conclusions

All indicators varied among environmental units depending on their vegetation cover. This pattern was similar across soil texture. Indicator analysis suggested that units with open mesquite shrublands and abandoned field crops tended to have worst biophysical and socioeconomic conditions than units with closed mesquite shrublands. Specifically, soil indicators showed that units with open shrublands and abandoned fields have soils with low infiltration rates (4-16 cm/h), aggregate stability (18-31%), and porosity (44-54%). Soils also had low organic matter (1.1-1.9%) and nitrogen (0.05-0.33%) contents. Vegetation indicators showed that plant cover was low (10-47%), canopy gaps were large (5-10 m gaps: 9-35%; >10 m gaps: 18-72%), and litter cover was low (12-44%). Bare ground cover was high (11-77%). Human impact indicators such as number of goat droppings, human trails, and firewood piles showed that local people conduct several activities at these units. Firewood collection however was the most frequent activity because the number of recent stem cuts (0.06-0.1 cuts/m²) and firewood piles (0.13-0.15 piles/m²) were high. These results suggest that environmental units varied in land degradation. Open shrublands and abandoned fields had higher land degradation than closed shrublands. These results also suggest that land degradation is seemingly independent of soil texture.