Willamette Valley Pine Soil Correlation Study

Kristin Kirschner

Editor's Note: Kristin Kirschner, Environmental Science Major, Willamette University. was commissioned by the Association to review the relationships between Willamette Valley Soils and current Valley ponderosa pine stand fragments. This review highlights selected portions of her report. Complete copies of the 28 page report are available through the WVPPCA.

Ponderosa pine has been growing throughout the Willamette valley for several thousand years according to pollen records and other historical evidence. Sediment cores collected from the central Willamette Valley indicate that ponderosa pine pollen concentrations rose approximately 7,000 years ago. At around the same time, other tree species found in today’s Willamette Valley also became common. Surveyors’ records from the early to mid 1800’s indicate that an oak-pine savanna dominated many sections of the Willamette Valley. Ponderosa pine was said to be found in the foothills, on the floodplain and in riparian forests.

Despite the fact that ponderosa pine has thrived in the Willamette Valley for thousands of years, today’s pine population has been reduced to a few scattered pockets. Since 1850, much of the Valley pine has been lost to harvesting, land conversion for urban and agricultural development, and forest succession. As Hibbs et al. recognized, “the exclusion of fire from the Valley has allowed other species to thrive and prevent the regeneration of ponderosa pine”. If current ponderosa pine distributions are compared to historical distributions, it can be seen that some pine stands are present in most of the locations, except for the west side of the Valley on the Coast Range foothills.

The project was designed to locate soil types where ponderosa pine was currently present in order to be able to estimate where it might be possible to establish it in the future. To create a GIS program, which would compare the current locations of the ponderosa pine and soil type, data first needed to be collected and downloaded. Luckily, the majority of the information needed for this project was available online in a digital format. Other necessary data was easily added to the existing information using Microsoft Excel and Arc View GIS.

Members of the Willamette Valley Ponderosa Pine Conservation Association had previously plotted locations of Valley pine stands (Figure 2). The Native Stand Mapping Project was completed in the fall of 2002. As native stand data was collected, it became clear that there were far more ponderosa pine in the Willamette Valley than originally believed, so the mapping project was amended to include stand density. Stand density was estimated through field observation and recorded along with a GPS point for the stand’s location. The mapping project succeeded in pinpointing over 950 sites, ranging from Beaverton to Cottage Grove, giving a general indication of stand location throughout the Willamette Valley. However, the project was not comprehensive in that it is not known if all stands were located.

Valley pine stands that were known or appeared to be planted (i.e. in rows) were not included in the stand mapping project. All of the information collected through the Native Stand Mapping Project was available online through the Oregon Department of Forestry.

Soil and Ponderosa Pine

Ponderosa pine stands are distributed throughout the Willamette Valley. However, the locations in which they are found show distinct trends. After reviewing the number of stands found in each soil type, it was determined that the stands are primarily found on ten soil series. These ten soil series represent 53% of the stands, while all other soil series present in the valley make up the remaining 47%. Each of the most common soils contains over 20 pine stands or 2% of the total number of stands in the valley, while all other soil series in the valley contained less than 1% each.
Discussion of Results

The locations in which Willamette Valley ponderosa pine is currently found share many characteristics. They are most likely comprised of one of the following soil series: Dixonville-Philomath-Hazelair complex, Aloha, Philomath, Veneta, Bellpine, Witzel, Nekia, Linslaw, and Clackamas. The sites have soil textures that are silty clay loam, silt loam, or loam, and are made up of particles that are fine, fine-silty, clayey or very-fine. They have moderately deep to deep soils and can range from being somewhat poorly drained to well-drained. They are in soil families with the properties of mixed mineralogy and a Mesic temperature regime and are often in the Haploxerolls great group.

By considering all of these trends together, it is possible to identify specific soil phases that would match all of the criteria and may therefore be appropriate sites for the growth of Valley pine. An Aloha silt loam (somewhat poorly drained, deep soil with fine-silty particles), 0-3% would be one example. These results are slightly different than expected because Valley pine is not commonly found on the dry, sandy, and gravelly soils typical of other ponderosa pine. The trends seen for soil drainage however do seem to agree with previous beliefs that Valley pine would be found on the ends of the soil moisture spectrum. To graphically represent this data, a collection of maps was created. These maps depict the Willamette Valley through a color gradient representing the number of Valley pine stands found within each soil series, and are available through the Willamette Valley Ponderosa Pine Conservation Association.

The results of the correlation between pine stands and soil type in the Willamette Valley present a general overview of the types of soils that Valley pine currently is growing in. The data presents a picture of where the stands are currently located. If pine does not grow in a particular soil that does not mean that, they are not able to grow there. It just shows that the current distribution does not include that particular soil series or type. When interpreting these results, it should be recognized that the soils discussed are not necessarily the “best” soils for Valley pine. It appears that current pine distributions may in fact be a mixture of stands occurring on very productive soils where the pine does extremely well and those soils where it can grow yet not at its most productive levels. For example, current stand distribution shows an equal number of stands in both the Hazelair soil series and the Dixionville-Philomath-Hazelair soil complex. When this data is combined with the limited information available from the Willamette Valley Ponderosa Pine growth study, it is clear that despite having the same number of stands, the growth rates of the stands varies significantly. The growth study reports 50-year site indexes at 92 feet for Hazelair silty clay loam, yet only 63 feet for the Dixonville-Philomath-Hazelair complex. Since only 12 soil types were used in the growth study, additional research is needed in this area, with future growth studies being completed to find out if the other most common soil types presented here are in fact the “best” soils for the Valley pine.

After identifying trends for the Valley pine stands, they were compared with general trends seen in the study area. This was done to determine if trends seen were in fact trends related to the Valley pine or they were merely reflections of soil trends within the Willamette Valley. If trends for the pine vary from the trends of the valley, it can be said that the Valley pine is preferentially found on those sites. In terms of soil series, the pine was found both on sites that reflect the general valley trends as well as sites that do not. However, pine stand trends for soil texture and slope mirror the general trends of the valley. Additional research will be required in order to determine if there is an existing relationship between the Valley pine and these factors.


Because other factors such as urbanization, logging and cultivation of land for agriculture have greatly affected the distribution of Willamette Valley pine, only the current trends are shown. If both presence and absence data for the pine stands were available, it would be possible to compare the soil types in which the pine was removed with the soil types in which pine is currently found. However, historical pine distribution is limited to general descriptions of areas within the valley and not at the scale and scope needed.

Soil Trends

Depth: Moderately deep to very deep

Drainage: Well-drained or some what poorly drained

Texture: Silty clay loam, silt loam or loam

Slope: Less than 30%

Mineralogy: Mixed

Additionally, this comparison addressed only the issue of stand location and soil type. The stands vary in size and most likely in vigor and health as well. This comparison therefore does not address these variables and seeks only to establish general baseline trends, which can be further investigated in the future. The variable of stand density was ignored in this study because of the fragmented nature of current distribution. This distribution pattern reflects not only the preferences of the pine, but also the influences of human activities on the distribution and relating stand density directly to soil type would be not considering these influences. Trends in stand density related to soil type may be investigated in the future, but any results must be acknowledged from within this framework. Finally, vigor and health of the stands were not included due to the lack of available data and a lack of time to visit and study stands in a variety of soil types.


Last Updated 02/24/08