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Photo plots


contributed by Karen Colson and Jason Karl

Description and Uses

Photo plots are close-up photographs taken to provide a qualitative record of condition from year to year within a defined small area (plot). Photographs are taken from the same location and same specified height each time, providing both a permanent visual record of the past and a means to evaluate changes over time. Photoplots typically involve placing a standard-sized frame on the ground.

Photo plots are regularly used in combination with other monitoring methods. They can be used to document and evaluate disturbances and changes such as:

  • Exotic Species Changes
  • Successional Changes
  • Soil Disturbace
  • Trampling

Advantages and Limitations

Photo plots are a versatile means of photo documenting disturbances and changes over time. However, in shrub ecosystems it can sometimes be difficult to see small changes over time. In many instances this method may require taking the photographs at the same time each year to avoid seasonal fluctuations when comparing photos. In addition, poor retakes of photos in some years (e.g., poor quality of photograph or failing to retake the photo in the correct position) can cause a loss of data for that year. As a qualitative technique, comparing photographs requires personal interpretation which can lead to observer bias or inconsistencies.

This method can also be used quantitatively:

  • Density (if you can identify individual plants within the plot in the photos). This approach is useful when field time is limited, since plants can be counted in the off-season, however it is still a lengthy process once you get back to the office and begin counting plants in the photographs. In addition, it is often more difficult to identify individual plants in a photo than while sampling on the ground. It is also likely that density will be underestimated due to individuals not able to be seen under taller plants. For these reasons, this method should be tested before use.
  • Cover. There are two ways you can use photo plots as permanent sampling units for cover: 1. lay a grid over the photo with a specified number of intersections and count the number of “hits” you have on a target species, 2. define canopy polygons on the photo and planimeter the area encompassed by the polygons.

Each have their drawbacks however. When counting “hits”, species with low cover can be missed completely and it is sometimes difficult to identify small individual plants. When defining canopy polygons it is easy to overestimate plant canopy (e.g., for plants with lacy canopies) and boundaries are difficult to define for some plant species. Drawing these boundaries can vary depending on the individual observer, causing inconsistencies from year to year.

Manuals

Technical and Application References

  • Booth, T.D. Cox, S.E., Fifield, C., Phillips, M., and N. Williamson. 2005. Image analysis compared with other methods for measuring ground cover. Arid Land Research and Management 19(2):91-100.
  • Booth, T.D., and S.E. Cox. 2008. Image-based monitoring to measure ecological change in rangelands. Frontiers in Ecology and the Environment 6(4):185-190.
  • Booth, T.D. Cox, S.E., Louaichi, M., and D.E. Johnson. 2004. Technical note: lightweight camera stand for close-to-earth remote sensing. Journal of Range Management 57:675-678.
  • Seefeldt, S.S. and T.D. Booth. 2006. Measuring plant cover in sagebrush steppe rangelands: a comparison of methods. Environmental Management 37(5):703-711.
  • Booth D.T., S.E. Cox & R.D. Berryman. Point sampling digital imagery with ‘SamplePoint’. Environmental Monitoring and Assessment 123: 97-108. Download SamplePoint from http://www.ars.usda.gov/services/software/download.htm?softwareid=246.
  • Johnson, D.E., Vulfson, M., and M. Louaichi. 2003. VegMeasure 1.6 user's manual. http://vegmeasure.us/index.htm

Similar Approaches

Photo points: Photo points are also photographs that are retaken from the same spot and filling the same frame each time the photograph is repeated, however they are landscape or feature photographs, unlike photo plots which are taken at the plot scale.

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field_methods/photo_plots_-_visual_record.txt · Last modified: 2012/03/08 15:14 by jgillan