Soil Amendment Increases Tree Seedling Growth but Reduces Seedling Survival at a Retired Gravel Mine

Restoring forest vegetation within denuded settings requires identifying the abiotic factors that limit plant establishment (Bradshaw 1997, Whisenant 1999). In heavily disturbed sites such as quarries and gravel pits that have been denuded of native soils, degraded soil processes may limit planted tree seedling survival or growth (Williamson et al. 2011). To improve vegetation establishment on denuded sites, soil amendments are commonly used to manipulate soil fertility (Biederman and Whisenant 2011a, Hough-Snee et al. 2011a), introduce organic matter, soil microorganisms, or propagules (Sinnett et al. 2008, Hough-Snee et al. 2012), or to create heterogeneous microhabitats (Biederman and Whisenant 2011b, Hough-Snee et al. 2011b) that improve plant survival. In forest restoration, amendments that facilitate soil development can lead to increased plant survival and growth (Ortiz et al. 2011). Specifically, amendments that increase soil carbon and nitrogen have been shown to improve planted tree seedling growth (Wilson-Kokes et al. 2013). Soil amendments for denuded sites are typically designed to increase planted tree growth so that seedlings may outcompete early seral vegetation and survive to maturity (Bradshaw 1997). In this study we examined how soil amendments change soil properties at a highly disturbed site and how these amendments impact the growth and survival of three early successional tree species. We tested two sets of hypotheses:

Table 1.

Mean soil carbon and C to N ratio differed between amended and unamended plots at both 0–15 cm and 15–30 cm depths. Soil moisture and nitrogen were higher at shallow depths in the amended plots, while bulk density did not differ between amended and unamended plots. Letters in 2002 and 2005 data indicate significant differences (ANOVA; F_1,36 > 4.12, p < 0.05 for all tests) between amendment treatments within a given soil depth and year. Due to limited sampling, no statistics were performed on pre-restoration soil monitoring.

1. 1. Soil amendment will increase soil carbon and nitrogen, C to N ratio, soil moisture, and decrease soil bulk density.

2. 2. The amendment-driven increase in soil fertility will increase the survival and growth of planted black cottonwood (Populus balsamifera), red alder (Alnus rubra) and Douglas fir (Pseudotsuga menziesii) seedlings relative to unamended seedlings.

The restoration site was a 1.7-ha retired gravel mine located on an alluvial terrace near Goodell Creek, a tributary to the Skagit River (Washington State, USA, elevation: 162 m). Gravel operations ceased in 1990 when the mine and surrounding area were incorporated into North Cascades National Park. The surrounding matrix consists of mature Douglas fir- and western hemlock- (Tsuga heterophylla) dominated conifer forest in uplands and black cottonwood-, red alder-, and western red cedar- (Thuja plicata) dominated riparian forest. Soil parameters were sampled within both forest types and the gravel mine to identify soil conditions prior to restoration (Table 1).

The primary restoration objective was to use amendments to establish soil properties that facilitate early-successional, coniferous-deciduous forest stand development. Temperate forests of Washington’s western Cascades are generally low in available nitrogen, so amendments were designed to raise soil organic matter content and moisture retention capacity without increasing N mineralization that would favor competitive ruderal weed establishment. Prior to amendment application, the entire site was graded to a ~7% grade and stockpiled sandy loam aggregate was evenly spread to a depth of 15 cm. Soil amendment consisted of a secondarily digested paper pulp sludge stabilized with fly ash (Smukler 2003). The amendment had high initial nitrogen content, so the carbon to nitrogen (C to N) ratio was increased prior to application by adding partially decomposed alder sawdust (36% C, 0.29% N, C to N ratio = 125; Smukler 2003). In the summer of 2001, the amendment was spread across two 0.45 ha blocks and tilled into the sandy loam topsoil (15 cm deep) over the mine’s semi-compacted subsoil. Two 0.45-ha blocks remained unamended and were not tilled. Thirty-six 20 m² circular experimental plots were created, nine per block. We measured soil percent...