Leaf Area and Root Density Measurements for Use in Cover Performance Evaluations on Semi-Arid Reclaimed Mine Lands

Abstract

Evapotranspiration (store and release) soil covers have been proposed as a means to limit acid mine drainage. Soil water balance models, like UNSAT-H are commonly used to assess the effectiveness of store and release covers. Plant-related attributes are required as inputs to these models. In particular, UNSAT-H requires leaf area index (LAI) and root length density (RLD) inputs. Published LAI and RLD data are generally lacking for semi-arid plant communities. To resolve this data gap, we collected leaf area and root density measurements in native and reclaimed shrub-grassland communities in southwestern New Mexico. Leaf area indices were determined using digital image analysis of harvested leaves at the end of the growing season. These data were used to estimate peak LAI and develop an annual LAI distribution. The average LAI ranged from 0.29 in reclaimed plant communities to 0.42 in native shrub-grasslands. LAI values for the reclaimed site did not correspond to soil cover thickness, which ranged from 23 to 62 cm. However, higher LAI values were typically associated with plots with higher amounts of shrub cover. Preceding drought and heavy grazing probably affected the LAI data in both native and reclaimed areas. Root density was measured in soil excavations using a grid-count method. Root density measurements indicated that nearly two-thirds of the roots occurred in the upper 20 cm of the soil in both the reclaimed and native areas. Very few roots occurred below 1.0 m. RLD was described by the quartile function 69-20-7-4 in the upper meter of soil. Preliminary water balance simulations using a 100-year climate record indicate that average drainage was less than 1 percent of mean annual precipitation when the measured LAI and RLD functions were applied to a 60 cm thick cover.Additional Key Words: leaf area index, root length density, soil cover, soil water balance model, UNSAT-H

Upward Migration of Constituents in Soil Covers at Semi-Arid Mine Sites

“…Vegetation canopy cover is sufficient to control erosion on the tailing repositories. Mean maximum leaf area index (LAI) measured on the tailing repository was 0.29, which provided adequate transpiration demand based on unsaturated flow water balance modeling (Romig et al, 2006). Thus, soil covers similar to those evaluated here support sufficient levels of vegetation to maintain cover functions.…”

“…The surrounding vegetation is characteristic of the transition zone between desert grassland and the mixed evergreen woodland. In 2004, vegetation cover on the repository averaged 38 percent and was dominated by warm season grasses, forbs, and shrubs (Romig et al, 2006).…”

“…The chemical conditions in the basal contact zone are generally deemed non-limiting for plant growth, especially considering that they occur below the primary depth of rooting for vegetation in this region (Peace et al, 2004;Romig et al, 2006). Specifically, 66 percent of the roots occur in the upper 20 cm (8 inches) and almost 90 percent occur in the upper 50 cm (20 inches) of the soils based on measurements made in both native and reclaimed sites in the project area (Romig et al, 2006). Salinity levels in the basal contact zone are typically in the 2 to 4 dS/m range, which even sensitive plants can tolerate and the highest salinity level (8.5 dS/m) is within the acceptable range for more tolerant plants (Rhoades et al, 1992).…”