Journal of the NACAA
ISSN 2158-9429
Volume 3, Issue 1 - July, 2010


Quantifying the Effect of Tree Species on Nutrient Pools in Lincoln County Oregon

Angima, S., Lincoln County Extension Agent, Oregon State University
Downing, T., Tillamook County Extension Agent, Oregon State University
Reeb, J., Lincoln County Extension Agent, Oregon State University


Soils under seven different forest stands (15-30 years old) were analyzed for pH, exchangeable hydrogen ions, organic matter (OM), nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na) to establish how tree species affect nutrient content in different soils in Lincoln County Oregon. Soil sampling was divided into two different sections; the upper soil profile (0-16 inches) and lower soil profile (16-32 inches). The forest stands were Western hemlock, Douglas-fir, red alder, big leaf maple, shore pine, green ash, and Sitka spruce. We established that tree stands were growing on Alfisols, Spodosols, Inceptisols, Andisols and Entisols. Green ash stands favored higher pH of soils and had significantly higher nutrient levels of N, P, Mg, and Ca than all other stands for both upper and lower soil profiles. However, this was only true for a specific soil order (Inceptisols) and not found in other soil orders. Other than the Inceptisols, there were no obvious differences in nutrient concentration in soils under all other forest stands. The combined results from this study can be used by forestry researchers and educators to develop research protocols to address forest nutrition under different types of soil orders. Knowing how a specific tree species will affect future nutrient pools in the soils may help forest landowners develop a comprehensive forest plan that may include reduced inputs depending on selected species and type of soil.
   The influence of tree species on soil properties has been of interest to scientists for decades (Grayston and Prescott 2005). Soils differ dramatically under different types of vegetation and within forest vegetation under different species of trees (Binkley 1995), while composition and productivity of forests differ strongly among sites that differ in soil properties (Binkley and Giardina 1998). Tree species influence the distribution and size of nutrient pools across soils horizons and may affect soils through mycorrhizal activity, soil solution uptake, and organic matter exudation (Spears et al. 2001).
   Forests differ fundamentally from other vegetation types because they develop surface O horizons that greatly modifies the microclimate at the soil surface as well as the physical, chemical and biological features of the soil. Individual trees can affect soils at a scale of 33 ft or more compared to 3-16 ft for shrubs (Binkley and Giardina 1998). The spatial variation around individual trees follows a pattern of stem flow input of water and chemicals. It is generally accepted that differences in nutrient uptake, litter quality, coupled with diversity in soil microorganisms contribute to changes in soil carbon and nitrogen dynamics within a span of decades (Binkley 1995). For example, sugar maple (Acer saccharum) and eastern hemlock (Tsuga Canadensis) in Michigan influenced nitrogen (N) availability while in Canada, western hemlock (Tsuga heterophylla) or western red cedar (Thuja plicata) influenced N and P availability patterns favoring these trees to become dominant species (Spears et al. 2001).
   Trees influence soils by providing quantities of organic matter of varying chemical composition which in turn may contribute to different levels of organic matter degradation under different species of trees (Grayston and Prescott 2005). For example, red alder (Alnus rubra) promotes productivity and long term sustainability through its higher litter nutrient concentration and accelerated nutrient cycling, while vine maple leaves (Acer circinatum) increase N, P, K, Ca, Mg, and Zinc (Zn) and decompose much quicker than conifer litter (Hibbs and Bower 2001). Soil communities do adapt to types of litter produced within stands; litter in stands of origin usually decompose faster than if the same litter is introduced to different species.
   Lincoln County, Oregon is about 90% forested and the forest industry is a great economic engine to the economy of the county. Our objective was to determine existing nutrient pools (pH, organic matter (OM), exchangeable hydrogen (H+), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), Magnesium (Mg), and sodium (Na) under different forest stands 15-30 years of age on different sites throughout the county. This data can be used as a tool for forestry researchers and educators to help landowners and policy makers make informed decisions especially in compiling comprehensive forest management plans for watersheds and land under forests.
Material and Methods
Site Description
   The study site was Lincoln County Oregon (Figure 1), located in western Oregon bordering the Pacific Ocean. Lincoln County occupies 1,194 square miles with over 60 miles of coastline. The population in the county is about 46,000 people with population density estimated at 45 people per square mile (US Census Bureau 2010). The climate in Lincoln County is cool and wet during fall, winter, and spring while the summer months are dry and warm. Average temperatures range from approximately 40 °F in the winter months to approximately 80 °F in the summer months (NOAA 2008). Precipitation ranges from approximately 7 inches per month in the winter months to less than 1 inch per month in the summer months June to September.

Figure 1. Lincoln County Oregon (see white boundary) showing locations within the County where soil samples were taken under different tree species.
Soil Sampling and Analysis
   Sixteen high school students from three different school districts were trained July 2008 on how to take soil samples and how to estimate tree age by visual observation and height measurements for trees 15-30 years old. Site selection criteria included being away from roads or other disturbances, and having trees as the dominant plant species. For each site, global positioning system (GPS) location was recorded and used later to identify the soil order from the online web soil survey (NRCS 2010). The soil samples were taken in two sections 0-16 and 16-32 inches. The soils were air dried and sent to Kinsey Agricultural Services laboratory in Charleston Missouri for nutrient analysis. Nutrients analyzed were N, P, K, Mg, Ca, Na, OM, pH, and H+The web soil survey provided soil series which were used to determine soil orders under which these forest stands were growing. Where applicable, data were subjected to analysis of variance using SAS (SAS 1997). A Duncan multiple range test procedure was used for mean separations at P<0.05. Proc GLM under SAS was used for multivariate analysis of variance since the number of soil samples taken under each tree species were not the same.
Results and Discussion
   Thirty locations were sampled and ranged in tree species from big leaf maple (Acer macrophyllum), red alder, Sitka spruce (Picea sitchensis), western hemlock (Tsuga heterophylla), Douglas-fir (Pseudotsuga menziesii), green ash (Fraxinus pennsylvanica), and shore pine (Pinus contorta). From the web soil survey, we determined that these trees were growing in the following soil orders: Spodosols, Entisols, Inceptisols, Andisols, Ultisols, and Alfisols. To asses and compare nutrient pools between forest stands, we used soil order as a basis for comparison.
   Only two species of trees were found on Alfisols (high fertility soils); big leaf maple and Douglas-fir. There were no distinct differences in mineral content between upper and lower soil layers except one site that had relatively higher levels of P, K, Mg, Ca, and Na. This site may need further investigation as to prior management practices. Even though the soil orders are similar, the soil series are different and may contribute towards the observed differences. On Andisols (volcanic soils), Sitka spruce and Douglas-fir stands were the dominant species. Only one site dominated by Sitka spruce had very high organic matter that was coupled with relatively high levels of N, P, K, Mg, Ca, and Na both in the upper and lower soil depths. However, most of the areas sampled were covered with Inceptisols (young soils on steep hills) as forests in this area are on steep hills where normal agriculture is not possible. Soil properties under different tree species in this soil order were fairly uniform with pH ranging from 4.7-5.8. Soils under green ash stands had the highest pH of 5.8 but the lowest OM of 2.8 %. However, P, K, Mg, and Ca for both upper and lower soil depths were higher than other stands. We expected to find higher levels of N for red alder soils (through N fixation) but the data did not support this. It is important to note that since red alder trees are usually not planted but regenerate from seed through wind dispersal, they were found to be dominant in this soil order (soils on steep slopes) compared to other soil orders. The predominant trees species in Spodosols (light colored soils) were the shore pines and western hemlock. All these sites were in close proximity (within 5 miles) to the Pacific Ocean. The pH was higher than any other soil order ranging from a low of 4.9 to a high of 7.1. Red alder trees growing in these soils had the lowest nitrogen levels indicating that they might not do well in these soils. Red alders were dominant in Inceptisols. 
   High variation in mineral soil properties makes it very difficult to assess the effect of species on pools of nutrients. Research, however, suggests that species effects can be substantial. For example, Binkley and Valentine (1991) found that soils under green ash and white pine (Pinus strobus) had significantly more (about double) K, Ca, and Mg in mineral soil than under Norway spruce (Picea abies). In contrast no significant (or even apparent) differences were detected in extractable cations in four conifer species in Ontario Canada (France et al. 1989). In our study we compared soil properties among two soil orders that were sampled the most; Inceptisols and Spodosols. For Inceptisols, nutrient pools under green ash were significantly higher than big leaf maple, Douglas-fir, and red alder on pH, H+, OM, N, P, and Ca on both upper and lower soil profiles (Table 1). For Spodosols, however, observed differences were not significant.
Table 1. Mineral soil properties at two soil depths for different trees species growing under Inceptisols soils in Lincoln County Oregon
Soil Property
Depth (inches)
EXCH H (%)
OM (%)
N (lb/ac)
P (lb/ac)
K (lb/ac)
Ca (lb/ac)
Mg (lb/ac)
Na (lb/ac)
Means followed by the same letter across rows do not differ at P < 0.05 using Duncan multiple range mean separation test.
   Studies have shown that soil organic matter and N content are higher in stands with N-fixing species, typically by 10-40% (Binkley and Sollins 1990, Homann et al. 1992). In this study there were no significant differences between Red alder (N-fixing tree) and other tree species in terms of OM and N pools for both upper and lower soil layers of up to 32 inches on Inceptisols soils (Table 1). These results are similar to those of Homan et al. (1992) who found no substantial differences in exchangeable soil cations under a 50-year-old alder stand compared to an adjacent 50-year-old Douglas-fir stand in Oregon.
   This study provides information on mineral nutrition status for different soils and within different tree species. The data provides foundation information about in-situ nutrient status that can be used for future research and education in forest productivity. Some distinct observations do stand out. The Spodosols support western hemlock and shore pines and to some extent Sitka spruce. These soils have the highest pH of all the soils studied. Inceptisols that are usually younger soils support red alder, western hemlock, green ash, and Douglas fir species. They seem to be moderately acidic and contain higher levels of N, P, K, Mg, and Ca compared to other soil orders. Andisols and Alfisols support Douglas fir, maple, and Sitka spruce and are relatively rich in N, P, K, Ca, and Mg. Other than Spodosols, all other soils are very acidic and may be fixing plant available nutrients. Green ash stands seem to increase nutrient pools and generally have a higher pH compared to similar stands on Inceptisols. The lack of significant differences in nutrient pools in other forest stands within the same soil order may suggest that landowners can develop nearly identical comprehensive management plans for their forest stands.
  We wish to thanks the Lincoln County Natural Resource Crew headed by Mr. Parker Ogburn for organizing high school students to conduct soil sampling. We are grateful to Starker Forest Inc. for helping with soil analysis.
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