Roots of Chaparral Shrubs Still Fail to Penetrate a GeoSynthetic Landfill Liner after 16 Years

In the past, legislation in the United States (USEPA 1989) and most of Europe (Forster 1993) restricted the planting of woody shrubs and trees on landfills, largely because regulators feared that roots of woody plants would penetrate the landfill liner (Dobson and Moffat, 1995). This limitation precluded revegetating landfills with predisturbance ecosystems, such as chaparral, in which woody species are an important part of the vegetation. More recently, a wider range of species, including shrub and tree species, have been permitted for revegetation on landfills, but root morphology and depth remain major criteria for species selection (Calrecycle 1999; USEPA 2006).

Most previous studies across several temperate ecosystems suggest that tree roots do not penetrate landfill liners (Gillman 1989, Dobson and Moffat 1995, Robinson and Handel 1995, Handel et al. 1997, Hutchings et al. 2001). Moffat et al. (2008), however, found that tree roots in Great Britain occasionally penetrated weaker areas of a mineral landfill liner, particularly when the soil layer over the cap was < 1 m. In an earlier paper, we reported that the roots of 11 species of California coastal chaparral shrubs did not penetrate a geosynthetic landfill liner, but shrubs and trees were only 3–5 years old at the time and growth is notoriously slow in the sandy infertile soils and low rainfall conditions (Holl 2002). Here we provide an update on this study after 16 years in order to provide more conclusive guidance on whether woody chaparral species should be used in landfill revegetation.

In 1997, we established the study at the former Fort Ord Army base in the city of Marina, Monterey County, California. Maritime chaparral in this region is dominated by shrubs, including several species of manzanita (Arctostaphylos) and ceanothus (Ceanothus), and a high diversity of annual herbs (Griffin 1978). The soils are mediumgrained sands (92–96 percent sand) that are well-drained, and have low organic matter content and fertility (Holl 2002). Mean rainfall is 475 mm per year with high interannual variability (275 to 957 mm; National Climate Data Center, Asheville, NC).

We planted eight shrub and one tree species separated by 1.5 m on an experimental area with a 40-mil polyethylene geomembrane liner (Poly-Flex Construction, Inc., Grand Prairie, Texas) covered with approximately 65 cm of soil in order to match regulations at the time. For more experimental design details see Holl (2002). In 2013, 16 years after planting, we used shovels and trowels to excavate 2–3 of the largest individuals of four planted species: chamise (Adenostema fasciculatum), sandmat manzanita (Arctostaphylos pumila), Monterey ceanothus (Ceanothus rigida), and coast live oak (Quercus agrifolia). We also excavated three naturally colonizing individuals of Torrey pine (Pinus torreyana) (> 4 m in height and > 10 cm diameter at breast height), as well as one particularly large (2 m height, ~10 m² aerial) cover coyote brush (Baccharis pilularis) on the landfill liner. All originally-planted coyote brush had senesced. We followed all major roots of each plant until they became 2–3 mm diameter. We also excavated three Torrey pine and one coast live oak tree in maritime chaparral adjacent to the experimental area to compare taproot morphology off the landfill liner.

No roots of any of the six species penetrated the landfill liner, although roots of most species excavated often reached the liner, consistent with our earlier results (Holl 2002). Roots of four of the species, chamise, sandmat manzanita, coyote brush, and Monterey ceanothus usually had a taproot that split into 2–5 medium sized (> 2 cm diameter) roots that primarily spread laterally in the top 30 cm of soil. Occasionally, the roots would grow deeper and run along the liner for up to 4 m. The lateral extent of the excavated roots (> 2–3 mm diameter) was generally only 1–2 m longer than earlier excavations (Table 1, Holl 2002).

Both Torrey pine and coast live oak have thick taproots; for example in Torrey pine the main taproot of all individuals was 8–10 cm diameter immediately below the soil surface. On the liner, the taproots of both...