Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

Veg Sampling
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Estimates of Utilization

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Understanding and measuring Utilization

The Concept of Utilization

A really basic principle of biology is that solar energy can be captured by plants and converted to chemical energy in the form of sugars, carbohydrates, and fats. From the plant's perspective, this nearly magical act requires leaves to capture sunlight and house the enzymes and cell structures of photosynthesis. 

No surprise; plants need leaves to photosynthesize.  The problem is that ecosystems are full of animals and processes that remove leaves on a regular basis; like herbivorous insects, large grazing animals, fires, wind, mowers, etc., etc.  If too many leaves are removed, the plant will not be able to create the sugars it needs to grow, reproduce, and recover from disturbances.

In this biological simplicity is an important management principle. The managers and stewards of land need to pay attention to how much green, photosynthetic material is removed from the plant from grazing or other forces to make sure the plants stay healthy.  The concept of how much biomass is removed from the plant is called utilization.

Rangeland and forest plants have evolved with grazing and fire so they therefore have ways to replace plant material that are removed. Plants vary in how much utilization that they can handle without survival or reproduction being negatively affected. In fact, some plants are healthier if they are grazed or pruned than if they are not.  The key for the manager to understand reasonable levels of utilization and be able to estimate utilization amounts that the plant is experiencing.

The idea is simple - the manager want to make sure that excessive and unsustainable levels of utilization are not occurring.  The application of this idea is difficult - how do you measure the proportion of the plant that has been removed.  There are whole books written on the topic. A good synopsis was written by range scientists from the University of Arizona called "Principles of Obtaining and Interpreting Utilization Data."  This publication is recommended as a place to start to understand the challenges of measuring and interpreting utilization.

Assessing Biomass to Estimate Utilization

There a variety of direct and indirect methods to estimate utilization. A good description of major methods for rangelands are outlined in:

Methods can be applied to grasses and shrubs and utilization can be measured or estimated.  The amount of a plant community removed by grazing can be estimated if you have an area that is grazed and compare it to a similar are that is not grazed.  Most often this is accomplished by measuring a small area that is excluded from grazing and comparing it to the surrounding or adjacent grazed are.

  • An ungrazed area can be created by a "utilization cage" made of net wire to create an area that will not receive grazing.

  • An important activity if using utilization cages is to move the cages on a  yearly basis.

  • Several cages will be needed across a landscape. The more variable the vegetation, the greater number of cages needed.

  • These utilization cages are not designed to reveal long-term changes in vegetation composition or structure. They should just be used to examine how much biomass is removed during a grazing season.

Photo from BLM in Bishop California, www.blm.gov/ca/st/en/fo/bishop/range0/r_monitoring.html

Making the Comparison to Estimate Utilization

  • Find or create a set of grazed and ungrazed plots.
  • Clipped and weighed the grazed and ungraed plots.
  • Make the comparison as follows:

  • If you can measure or estimate biomass in a grazed and ungrazed area you can estimate utilization.
  • Make sure the grazed and ungrazed area are similar in vegetation composition.

Summary Questions

  1. Why are land managers interested in the proportion of a plant or plant community that is removed?

  2. Utilization can be estimated directly with estimates of biomass or indirectly with other plant attributes. What other aspects of a plant could be measured to get an indirect estimate of utilization?

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