9.1 Vegetation Height and Structure Overview

Video Presentation

Learning Guide

Introduction to Vegetation Structure

When we describe vegetation, we often use terms like shortgrass or tallgrass prairie, open woodland, savanna, closed canopy forest, or shrub steppe, just to name a few.    These terms evoke images of what the vegetation looks like, or the spatial arrangement of the plants (Figure 1).    We refer to the three-dimensional arrangement of plants as vegetation structure.

Figure 1.  Three plant communities with distinctly different vegetation structure.

Vegetation structure is an important attribute because it provides essential information about different qualities of wildlife habitat.  To evaluate habitat suitability, we need to determine the ability of a site to satisfy the food and cover requirements for the species being considered.  The ability of a site to provide cover for shelter (e.g., for nesting, perching, bedding, or thermal regulation), concealment (e.g., for hiding or hunting), or escape are affected by the spatial distribution of vegetation on both horizontal and vertical planes.   Plant spacing, height, and the arrangement of canopy layers influence many aspects of animal behavior.  As such, vegetation structure is also an important consideration for livestock management.

We live in a three-dimensional (3D) world, and gaining a 3D perspective can greatly assist our understanding of landscapes at many scales.  Think back to when you first started to learn about the structure of glucose molecules, or the structure of the solar system, and how much easier it was to understand these topics by studying a 3D model compared to a two-dimensional diagram (Figure 2).

Figure 2.  Complex systems at many scales can be better understood when the three-dimensional relationships are clearly depicted.

Unfortunately, building 3D models won’t help us describe the spatial arrangement of the vegetation on a site.  In order to describe the three-dimensional aspects of vegetation, we need information about the vertical distribution of plant biomass in canopy layers, such as tree, shrub, and herbaceous layers.  We also need information about the distribution of plant biomass on a horizontal plane, which includes plant spacing and the horizontal spread of plant canopies. The 3D nature of vegetation cannot be described with a single measurement, but we can use a combination of measurements to provide this information.   The primary measurements used to describe vegetation structure are canopy cover, gap intercept, vegetation height, and visual obstruction.  This lesson focuses on methods used to measure gap intercept, vegetation height, and visual obstruction.

Advantages of Vegetation Structure Measurements

• Most measurement methods are relatively easy and can be done fairly rapidly, so relatively large areas can be surveyed in a short amount of time
• Measurements from different communities are readily comparable
• Can be used to describe the spatial distribution of plant biomass
• With sufficient training, measurements from most methods are objective and repeatable

Limitations of Vegetation Structure Measurements

• Most visual obstruction techniques require two people to efficiently collect data
• Without information about horizontal structure, vertical structure measurements do not provide sufficient information to describe trend
• Lack of standardization among visual obstruction methods

Methods to Assess Vegetation Structure

There are three main types of measurements that are used to describe structure:

Gap Intercept

Gaps are spaces between plant canopies or plant bases that are not occupied by rooted vegetation when viewed from above.  It is a vertical projection from the canopy or plant base edge to the ground (Figure 3).

Figure 3.  A canopy gap is here measured as the 36 cm space between plant canopies that does not include any other canopy or rooted vegetation.  Image originally published in Herrick et al. 2009a.

The decision to focus on canopy gaps or basal gaps depends on the site characteristics and your research or monitoring objectives.  In the discussion of using the gap intercept method to provide information about vegetation structure, we are going to focus on canopy gaps.

Canopy gaps are measured using a line-intercept method similar to the continuous line intercept method for determining cover, except that the measurements are of gap length. This method works best with vegetation that can be observed from a standing position (most herbaceous plants and short to mid-height woody plants), but the method can be adapted to measure gaps in taller vegetation as well.  It is important to decide whether to include or exclude annual plants, and to be well-trained on the protocol before implementing the method.

The length of a gap is measured along a transect tape by recording the “start” and “end” measurement of the gap.  There are two critical rules in the protocol for this method: minimum gap size to begin a measurement and the amount of canopy interception required to end a gap.  The minimum gap size is 20 cm, and gaps less than 20 cm are not recorded.   The gap is broken when at least 50% cover of any 3 cm segment of the transect tape edge is intercepted by canopy or rooted vegetation (Figure 4).

Figure 4.  Graphic illustration of canopy gap intercept measurements along a transect tape.

Gap size (difference between the “start” and “end” measurements) is calculated for each gap, and each gap is classified into one of four gap-size classes:  25-50 cm, 51-100 cm, 101-200 cm, and > 200 cm.  The data are reported and interpreted based on the percentage of the line that is included in each of the 4 gap size classes.  For example, transects can have the same total amount of gap, but very different distributions of gap sizes (Figure 5).

Figure 5.  Graphical illustration of 3 transects that each have 50% total gap, but differ in the number of gaps and the distribution of gap sizes.

The distribution of gaps has important implications for risk of invasion by weed species and susceptibility to wind erosion (Herrick et al. 2009a).  The gap intercept method can also be used to measure basal gaps, the gaps between plant bases.  Basal gaps provide valuable information about susceptibility to water erosion.  See Herrick et al. (2009a) for a more detailed explanation of this method and interpretation of gap data.

Vegetation Height

By measuring vegetation height we can gather critical information about the vertical structure of vegetation. Vegetation height provides valuable information about wildlife habitat as well as the potential for wind-driven soil erosion on a site.  Vegetation height measurements are usually taken along transects, often in conjunction with line-point intercept (LPI) measurements, although height is usually measured at fewer points on each transect compared to LPI.

Height measurements are usually done using a stick or rod that is marked in centimeter increments.  Measurements are taken at designated points along the transect.  Essentially, all of the plant material that occurs within an “imaginary” 30 cm diameter cylinder is examined, and the height and species of the tallest part of a woody plant and the tallest part of an herbaceous plant within that cylinder are recorded.  Observers may hold a ruler next to the stick (Figure 6), or lower a 30 cm diameter disc (perforated with a hole at the center) to maintain a radius of 15 cm around the stick.

Figure 6.  Vegetation height measurement method.  a) 15 cm radius around the height stick is established using a 30 cm ruler centered on the stick; b) the tallest part of the woody plant within the 30 cm diameter cylinder around the height stick is recorded as woody plant height.

Observers should record only the tallest part of the plant that is included within the 30 cm diameter cylinder, even if the plant has taller stems, leaves, or inflorescences that are outside the cylinder.

Figure 7.  Graphical illustration of vegetation height measurements on the tallest part of woody and herbaceous plants within a 15 cm radius of the height stick.  Note that the height recorded for both plants is the tallest part within the measurement area defined by the 15 cm radius, not the maximum height of the plant.

Height measurements are made to the nearest centimeter for measurements < 2 m, and to the nearest 30 cm (1 ft) for plant measurements that exceed 2 m.  Vegetation height data are calculated in three ways: as the average height of all plant measurements, the average woody plant height, and average herbaceous plant height for each transect.

Visual Obstruction

Visual obstruction is a measure of the degree of cover or concealment provided by vegetation. Essentially, it measures vertical cover but is dependent on the distance and height from which measurements are taken (Figure 8).

Figure 8. Illustration of the common components for visual obstruction methods using a cover pole.  The measurement is taken from a specified viewing height and viewing distance from the banded measurement pole.   Viewing height, viewing distance, and the precision of measurement height vary with different measurement protocols.

A variety of methods have been developed to measure visual obstruction.  The most common methods use cover poles, cover boards (also called profile boards), and the amount of the pole or board that is obscured from view is recorded.  An examination of the literature in which visual obstruction measurements have been reported reveals a lack of standardization in measurement methods:  the dimensions of different types of equipment or apparatus are highly variable, as are the measurement protocols.  Therefore, if you plan to measure visual obstruction, take time to determine the protocol that is best suited for your management and sampling objectives, and clearly document the protocol followed.

Visual obstruction methods typically involve one person holding a marked board or pole while another observer determines the percent of the board or pole that is obscured from view by vegetation. Sometimes observers record the height at which the measurement object is obscured. Measurements from different positions are often taken for a single placement of the measurement object.  Some poles, or boards, may be modified to be self-supporting, enabling measurements to be taken by a single observer.  Here we briefly review the most common types of equipment, and will follow with a description of a measurement protocol using a cover pole.

1. Cover boards: Cover boards are generally constructed from light-weight plywood, and are usually narrow in width compared to their height.  Common dimensions are 30 cm wide by 2.0 m or 2.5 m tall. The board may be painted in alternating bands of contrasting colors, such as black and white, or red and white. Numerous variations on this theme exist: the boards may be wider, the width of painted bands may vary, and in some cases the board may be painted in a checkerboard pattern.  Silhouettes of wildlife have sometimes been painted on the boards (Figure 9).

Figure 9.  Multiple sizes and configurations of poles and boards exist for measuring visual obstruction.  A few configurations include the Robel pole (Robel 1970), cover pole (Herrick et al 2009b),  profile/cover board (Nudds 1977; Griffith and Youtie 1988), and an alternative cover board (Adams et al).

2. Cover poles: Cover poles are narrow poles that are marked with alternating bands or segments of contrasting colors. Cover poles are sometimes called “Robel poles” in reference to R.J. Robel’s technique of using a cover pole to measure visual obstruction and standing crop (Robel et al. 1970). This method works well in grassland and small shrub communities with vegetation less than 4 feet tall. Cover poles are usually marked in 10 cm bands of alternating colors, but in some applications the bands may be as narrow as 2.5 cm.  Cover poles can easily be constructed to be self-supporting (Figure 10).

Figure 10.  Visual obstruction measurements being taken by one (a.) and two (b.) observers. Note that the observers in the image on the right are using a tether between the cover pole and viewing pole to ensure that observations are made from the appropriate distance.

The most common approach to taking measurements with cover poles is to identify the highest band or segment of the pole that is completely obscured by vegetation, or to record the lowest band that is partially revealed.  Protocols may be adapted to accommodate multiple measurements when alternating portions of the pole that are obscured or revealed (Figure 11). Again, these measurement methods have been adapted for many different applications.

Figure 11.  An example of how a pole might be read if we were using the Toledo et al 2010 method.  Bands that are less than 25% visually covered are considered non-obscured, while bands that have 25% or more visual cover are considered obscured.

Cover Pole Measurements and Calculations

The following example describes one approach to taking measurements with a cover pole.  An excellent video tutorial by the USDA-ARS Jornada Experimental Range supports the method described here and in the Herrick et al 2009a manual.  The cover pole used in this example is self-supporting, 2 m long, and divided into four, 50 cm segments. Each segment is subdivided into five, 10 cm bands, which alternate in color (20 bands total).

Measurements are taken at pre-determined points along a transect.  Two measurements from opposite directions are taken for each placement of the cover pole; these measurements are taken at a 5 m viewing distance from the cover pole.  The observer views the cover pole from a 1 m viewing height, and examines each of the 20 bands to determine whether at least 25% of the band is obscured by vegetation. Data are recorded for each band, scored as a “1” if at least 25% of the band is obscured, and a “0” if it is not.  The data are summarized as the percentage of observations within each segment in which the bands were visually obscured.   For example, if the cover pole was placed 5 times for 10 total observations (2 observations per placement), that would result in 10 observations on each band, or 50 observations for each segment.  If we recorded an obscured band 30 times for the lowest segment of the pole, we would report 60% visual obstruction for that segment.

A Note of Caution:

Visual obstruction measurements may be difficult to interpret because they do not consistently reflect variation in vegetation structure. For example, Figure 12 (from Toledo et al. 2010) demonstrates how three different configurations of vegetation structure can produce identical visual obstruction measurements.

Figure 12.  Graphical illustration of three situations that produce identical visual obstruction measurements without reflecting variation in the arrangement and spacing of plants.  Image originally published in Toledo et al. 2010.

This can occur because obstruction is influenced by the vegetation in the immediate vicinity of the cover pole, and by the vegetation in the line of sight between the cover pole and the viewing pole.  Research has shown that a combination of line-point intercept, canopy gap intercept, and vegetation height are more consistent indicators of vegetation structure.