The effect of organic matter and fertilizer additions in growth and survival of Jeffrey pine Pinus jeffreyi in a reforestation experiment on mine spoil in the Sierra Nevada, Alpine County, California, USA

  • Published source details Walker R. F. (2003) Comparison of organic and chemical soil amendments used in the reforestation of a harsh Sierra Nevada site. Restoration Ecology, 11, 466-474


Addition of organic matter to soil is being examined for its potential to enhance reforestation attempts, especially on sites with harsh substrate conditions such as produced by surface mining, which frequently have exposed spoil materials severely lacking in organic matter content. Sawdust, wood chips, tree bark, raw agricultural residues and municipal biosolids have all been trialed for this use, but such trials have largely been conducted in humid regions. This study looked at the effect of organic matter, fertilizer addition coupled with Pisolithus tinctorius inoculation of tree seedling roots as techniques to enhance Jeffrey pine Pinus jeffreyi gowth and survival on mining spoil at a site in the southwest USA.

Study site: The study was conducted on open-pit mine spoil located on the east slope of the Sierra Nevada (2,200 m altitude), Alpine County, California, USA. Excavation ceased in 1962, and the abandoned mine and dumped spoil (mostly andesites) covered approximately 100 ha. The mine soil is predominantly porous silica with small amounts of clay. Annual precipitation averages 50 cm, falling mostly as snow. The adjacent forest composition indicated that Jeffrey pine Pinus jeffreyi, California white fir Abies concolor lowiana and Sierra lodgepole pine Pinus contorta murrayana were the main constituents of the vegetation before mining commenced, with prevailing environmental conditions suggest that Jeffrey pine was dominant. Earlier attempts to revegetate the mine with herbaceous species were unsuccessful, with vegetation exceedingly sparse and consisting primarily of scattered Jeffrey pine around the site periphery. Largely in symbiotic association with Jeffrey pine, was the mychorizal fungi Pisolithus tinctorius introduced via wind-borne spores.

Soil analysis: At the start of the study five soil samples were collected from the planting trial area and pooled for analysis. This revealed the following properties: 68% sand, 18% silt, and 14% clay; organic matter, 0.2%; and pH 4.7. Elemental concentrations were (in μg/g): total N, 688; P (Bray 1), 32; K, 282; Ca, 3,841; Mg, 471; S, 301; Fe, 313; Mn, 84; Zn, 3.6; Cu, 26.7; B, 0.7; Mo, 1.8; and Al, 134. Poor water-holding capacity due to the coarse soil texture and near absence of organic matter, coupled with low precipitation, indicated that seedling moisture stress over a significant portion of the growing season is inevitable. The soil was low in total N, reflecting the lack of organic matter, and was evident from severe foliar chlorosis in the Jeffrey pines that had colonized due to N deficiency.

Seedling inoculation: The planting stock consisted of bareroot Jeffrey pine seedlings (grown over two growing seasons in a nursery bed using locally collected seed). All seedlings were graded for height (≥12 cm), stem diameter (≥4 mm), and shoot and root quality. Before outplanting the seedlings were acclimatized. Prior to planting the root systems of half of the seedlings were coated with Terra Sorb Root Dip Gel (Plant Health Care, Inc., Pittsburgh, PA, U.S.A.) that contained P.tinctorius basidiospores originating from the study site. Root systems of uninoculated seedlings were coated with Terra Sorb with no spores. All seedlings were rolled in moist burlap to prevent desiccation.

Seedling planting: In early May 1997, 400 seedlings were planted in 20 plots in a randomized split-plot experimental design with each plot with 20 seedlings in two rows of 10, divided into two subplots (40 in total) consisting of one row of 10. Spacing was 1 m within and 2 m between rows (typical for plantings on mine sites as tight spacings helps reduce erosion). One of four main treatments was applied to each of five randomly chosen plots (five replications of each main treatment):

1) organic matter (shredded and composted plant materials derived from municipal landscape maintenance operations) applied at a single rate of 2.0 L per seedling;

2) Gromax 21-6-2 + Minors Forestry Dry Site Starter Pak controlled-release fertilizer (Salinas Gromax, Inc., Salinas, CA, U.S.A.) applied at a low rate of one packet (8 g);

3) Gromax 21-6-2 + applied at a high rate of two packets (16 g) per seedling;

4) an unfertilized control.

Gromax has a release period of approximately 15 months at 21ºC, but on at this site the duration of nutrient release was at least three growing seasons (i.e the duration of the entire study period). The organic matter and fertilizer were applied to augered planting holes (13 × 38 cm). Organic matter was added so that it was uniformly distributed around the roots. For the fertilizer treatment, a Gromax packet was placed at mid-depth for the low application rate and one each at mid-depth and at the base of the planting hole for the high rate. Subtreatments consisted of inoculated or uninoculated seedlings.

Field measurements: Seedling height and stem diameter were measured when planted, in May 1997, and at the end of the first, second and third growing seasons (October 1997, 1998 and 1999) along with an assessment of survival. The height and diameter measurements were used to calculate an estimate of shoot volume.

An evaluation of the inoculation effects on ectomycorrhizal formation was conducted in October 1998 by harvesting the roots of a randomly chosen seedling from every subplot within the unfertilized treatment (i.e. five of inoculate and five of uninoculated, seedlings). Mycorrhizal development was quantified by determining the proportion of short roots bearing ectomycorrhizae.

Estimation of the organic matter decompositon was assessed by excavating the soil surrounding the roots of six seedlings that had received organic matter (three each from within the inoculated and uninoculated treatments) at the end of the final growing season.

Pine needle analysis: Pine needle samples were collected from each surviving seedling and pooled into one sample per subplot in the fourth week of July 1997, and again in the fourth week of July 1999. Needles were about 80% elongated when collected and were analyzed for total N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B, Mo and Al.

Mycorrhizal development: The only mycobiont on the roots of the inoculated or uninoculated seedlings (assessed at the end of the second growing season) was P.tinctorius. Differences in colonization between these two treatments were marginal, 44% of the short roots in inoculated seedlings (range 28–59%) and 41% in uninoculated seedlings (range 23–52%).

Organic matter mineralization: By the end of the third season only a small proportion of the organic matter had decomposed and that remaining differed only marginally, with 82% remaining surrounding the roots of inoculated seedlings (rang, 77–88%), and 84% around uninoculated seedlings (range 79–91%).

Survival & growth: The organic matter treatment induced substantial seedling mortality, as did inoculation but to a lesser degree. Most mortality occurred during the first season, survival patterns remaining virtually unchanged in the subsequent seasons.
Seedling volume was unaffected by treatment after the first season. After two seasons, the fertilizer treatments became significant, with the largest seedlings being those receiving 16 g of Gromax, those with organic matter and unfertilized seedlings were overall smallest. After three seasons 16 g Gromax again produced largest seedlings and had accelerated growth rate compared with the previous year. Seedlings fertilized with 8 g of Gromax and unfertilized seedlings were about the same size. The 16 g Gromax treatment produced a 74% increase in seedling volume. In comparison with unfertilized seedlings, organic addition resulted in seedlings 28% in volume less and a deceleration of growth rate. Neither inoculation nor the fertility × inoculation treatments proved significant in any year.

Nutrition: Organic and Gromax treatments produced increased N concentrations, and the former also increased P and K (but largely apparent in uninoculated seedlings only). Foliar Mg was higher overall in the 8-g Gromax treatment, but differences were generally small between treatments. Inoculation resulted in higher concentrations of N and Ca.

For Fe and Mn lower concentrations were found in the organic matter treatment, whilst Fe was in higher in the 8 g Gromax treatment, and these responses most pronounced in uninoculated seedlings. Differences among treatments in Cu concentration were small, but 16-g Gromax treatment had less overall. Foliar Mn was also influenced by inoculation with higher concentrations generally in inoculated seedlings, most apparent within the organic matter and unfertilized treatments. Concentrations of Zn, B and Mo were unaffected by treatment during the first season but Al had lowest concentrations found in the organic matter treatment and the highest in the 8 g Gromax treatment, again most apparent in uninoculated seedlings.

During the third growing season the 16 g Gromax treatment had the highest N concentration and unfertilized seedlings the lowest. Similarly, P and K were also highest in the 16 g Gromax treatment and lowest in unfertilized seedlings, but although the 8 g Gromax treatment exhibited intermediate values for these two elements, concentrations in the organic matter treatment exceeded those in unfertilized seedlings only marginally. Ca and S responses were different to the other macronutrients in that fertilizer produced substantially lower concentrations overall, most apparently with the high application rate.

Fertilizer effects on micronutrient concentrations were significant for Fe, Mn and B during the final season, for each the lowest concentrations were in the 16 g Gromax treatment. The fertility × inoculation interaction was also significant for Cu (primarily high concentration in uninoculated seedlings of the unfertilized treatment). Nevertheless, inoculation alone did not influence any micronutrient concentration in the third season, and as was true during the first season, Zn and Mo concentrations were unaffected by fertility treatment also. The 16 g Gromax treatment again produced the lowest Al concentrations, whereas inoculated seedlings generally had lower concentrations than uninoculated seedlings, most apparent within the 8 g Gromax treatment.

Conclusions: The inoculation procedure failed to induce substantially greater P. tinctorius colonization in inoculated seedlings which may have also impaired water relations. Overall, these findings indicate that organic addition or mycorrhizal inoculation methods used in this study failed to enhance restoration efforts on this dry, mine spoil site.

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