SLATS Star Transect Protocol

This protocol pages provides a brief summary of the collection process for FPC field data, which is largely taken from the ABARES ground cover monitoring field manual (Muir et al. 2011).

 The metric of overstorey vegetation cover adopted in many Australian vegetation classification frameworks is Foliage Projective Cover (FPC). FPC is defined as the vertically projected percentage cover of photosynthetic foliage of all strata, or equivalently, the fraction of the vertical view that is occluded by foliage. Overstorey FPC is defined as the vertically projected percentage cover of photosynthetic foliage from tree and shrub life forms greater than 2m height and was the definition of woody vegetation cover adopted by SLATS. Overstorey FPC is one minus the gap probability at a zenith angle of zero and therefore it has a logarithmic relationship with effective leaf area index. Since Australian plant communities are dominated by trees and shrubs with sparse foliage and irregular crown shapes, overstorey FPC is a more suitable indicator of a plant community’s radiation interception and transpiration than crown cover.

Ground cover is the non-woody vegetation (forbs, grasses and herbs), litter, cryptogamic crusts and rock in contact with the soil surface. Ground cover changes in response to climate variables, vegetation dynamics and land management. Factors such as grazing pressure, tillage and stubble practices, drought and fire all affect ground cover. The quantity of ground cover affects water infiltration, runoff, erosion and carbon sequestration. It is a key indicator of land condition such as soil degradation, pasture production and biodiversity. Estimates of ground cover and changes in the quantity and spatial arrangement of ground cover over time provide land managers, policy-makers and scientists with valuable information for use in planning, monitoring and modelling applications.  

FPC and Ground cover can be monitored using remote sensing. From a remote sensing perspective, FPC is the woody green cover in the overstorey while ground cover is the fractional cover of the non-woody vegetation and litter near the soil surface. The field measurement protocol described here is used to derive three categories of cover from satellite imagery— photosynthetic vegetation (PV), non-photosynthetic vegetation (NPV) and bare soil (BS).

Plot Selection

Each 1 ha plot is a small homogenous area representative of a particular landscape.  As many sites as possible are selected within the super site to ensure sampling of the major of landscapes present (determined by landform, vegetation, land surface, soil and other land features).  Capturing the variability at the site should also be a priority and the overall sampling strategy should include a range of soil colours and textures and vegetation compositions. Site selection can be informed by viewing satellite imagery or aerial photography.

 The following points need to considered when locating sites:

  • Plot areas should be homogeneous, minimising within site variation;

  • The edge of a site should be at least 100 metres from roads, power lines or other features not characteristic of the plot;

  • Sites should be located away from water run-on areas as moisture can affect reflectance characteristics; and

  • Sites should be located on level or near level ground. If a sloped site is necessary, avoid western and southern slopes as these can be affected by shadow.

Plot Layout

Each plot should consist of three transects laid out in a star shape, with 100 observations along each (300 observations in total).  The first transect runs from north to south, the second from 60º to 240º and the third from 120º to 300º.

Star Transect Plot Layout

Data Collection Process

Basic Data Required

The following data must be recorded for each plot, for data management purposes.

  • Geographic coordinates (easting, northing,zone) at plot centre

  • Operators: who collected the data

  • Date: consistent format (dd/mm/yyyy)

  • Time: consistent format (hh:mm)

  • Plot name: name of specific plot

 Field Equipment Checklist

  • GPS (differential preferred)

  • 3 x 100 m measuring tapes

  • Stake or star picket

  • Hammer

  • Compass

  • Clinometer - A clinometer measures inclination or slope.

  • Munsell charts and Water bottle (for wetting up samples)

  • Optical wedge prisms (factors 1.0 and 2.0)

  • Digital camera

  • Field sheets and clipboard

  • Electronic site forms and notebook computer (optional)

  • Vegetation identification books

  • Densitometer and telescopic pole for mounting densitometer

  • Laser pointer and tape for attaching to pole

  • Biomass images for estimation

Process

  1. Lay out transect

  • Hammer stake into ground at centre of site.

  • Use compass to find 0° bearing and run a tape out at this bearing for 50 metres. The tapes should be placed on the ground underneath vegetation and be straight and follow a constant bearing. The operator should sight from the centre of the tapes along the bearing, and choose a landmark like a tree, to walk towards. Avoid trampling along the transects where observations will be made.

  • Return to centre and twist tape once around the stake to secure in place. Run remaining 50 metres of tape out at 180°.

  • Repeat for the second and third tapes, at 60°-240° and 120°-300° respectively.

  • Average GPS readings to obtain site centre coordinates.

2. Take Transect Measurements

Transect measurements are taken using a specialised measuring device consisting of: a densitometer for measuring woody vegetation; a laser pointer for measuring the ground cover and low woody vegetation; and a telescopic pole to which the laser pointer and densitometer are attached. The densitometer is attached to the top of the telescopic pole, while the laser pointer is attached to the same pole near the bottom, pointing downward.

Specialised FPC/Ground Cover Transect measuring device.

Transect measurements are recorded in three vegetation categories: non-woody and ground cover; woody <2 m; and woody >2 m. An observation, using the measuring device, is made every metre starting at the 1 metre point of each transect. Observations are completed north to south on the first transect, 60 to 240 degrees on the second transect and 120 to 300 degrees on the third transect.  A measurement is always recorded for the ground cover category. Measurements for the other two categories are only recorded if they are visible.

The vegetation categories measured.

Measuring ground cover

This category includes non-woody vegetation (such as grasses, forbs and herbs), litter, cryptogams, soil and rock. There is no height restriction for the non-woody vegetation.

  • Position the pole with the laser pointer attached vertically next to the metre mark on the measuring tape. Use level in densitometer to ensure pole is vertical.

  • Press the power button of the laser pointer.

  • Record the first intercept of the laser beam when looking downwards in the appropriate category from the list below:

    • Crust—the hard surface layer of soil.

    • Disturbed soil—cracks in a soil crust, ant nests or other disturbances in the natural surface e.g. by animal hoof prints. In ploughed agricultural sites most soil recordings will be disturbed.

    • Rock/lag—rock includes all stones and rock material greater than 2 cm. Lag includes all single grains that can be differentiated by the naked eye, approximately 2 mm to 2 cm.

    • Cryptogam—a biological crust composed of lichen, moss and algae.

    • Green leaf—a leaf with green pigmentation (one that is actively photosynthesising) attached to the plant. Sometimes the leaf in this state may appear more yellow than green. In this case a judgement call must be made as to whether it is placed in the green or dry category.

    • Dry leaf—a leaf with non-green pigmentation (one that is not actively photosynthesising). This can include senescent (alive) vegetation as well as dead vegetation. It must be attached to the ground or plant. 

    • Litter—dead plant material that is not attached to the ground. Includes branches, leaves or fallen tree trunks.

 Measuring woody vegetation less than 2 metres in height

All vegetation with a woody component and a height of less than 2 metres. These are generally shrubs and small trees.

  • Maintain the pole in the same position as where the ground cover measurement was taken.

  • Determine if there is an intercept of woody vegetation <2 m with the pole directly above the point recorded for the ground cover.

  • Record the intercept in the appropriate category—green leaf, dry leaf, branch.

    • Green leaf—a green leaf attached to a plant.

    • Dry leaf—a dead or dry leaf attached to a plant.

    • Branch—woody component of the plant (branch or trunk).

  Measuring woody vegetation taller than 2 metres

All woody vegetation with a height of 2 metres or more—trees and tall shrubs.

  • Maintain the pole and densitometer in the same position as used for the ground cover and woody vegetation <2 m measurements.

  • The observer checks visually to determine if the pole is positioned below a live tree crown or a dead/defoliated tree, or neither. If it is difficult to determine if the tree is completely dead, assume it is live.

  • If within a live tree crown the densitometer is positioned vertically using the spirit levels and the observer peers through the mirror sight to determine the first intercept directly above the viewpoint.

    • when in canopy gap (no intercept), record as "In Crown" only

    • when there is an interception with a canopy element record as "In Crown" AND one of "green leaf" OR "dry leaf" OR "branch”

  • If the tree is completely dead or defoliated:

    • record as "dry leaf" OR "branch" only (i.e. no recording is made for "in crown")

  • If the pole is not positioned below a live tree crown or dead tree then no recording is made for any category in the woody vegetation >2 m.

Illustration of the various crown measurement categories.

Site Description

To adequately describe the site a description of topography, vegetation structure, erosion characteristics, deposited materials, soil and rock colour, and tree basal area is recorded. Most of the data required is assumed knowledge or sufficiently described within the field data forms and data entry templates.  Further information for some of data elements is described below.

 Hillslope

To measure hillslope an observer faces in the direction of the slope, and levels the sight of the clinometer at a point in the distance which is approximately the same height as their own height The slope is recorded in per cent (0 – 100%).

Soil and Rock Colour

The Munsell Soil Colour Charts are used to record soil, rock and lag colour. Three readings are taken: hue, value and chroma. To obtain the reading a small amount of soil is held under the colour chart, to find the closest match. Both wet and dry recordings for soil crust (hard compacted surface soil) and disturbed soil (loose soil) are taken. A bottle is used to carry water to the site, to dampen the soil for the wet recordings.

Basal Area

To calculate the site tree basal area, 7 readings (one at plot centre, 6 at the halfway point of each arm of star transect). using the optical wedge prisms are taken and averaged. To use the optical wedge prism it is held at arms’ length and the observer looks through the prism at the tree being counted. If the tree trunk appears to overlap the tree viewed without the wedge prism the tree is counted (Figure 5a). If the trunk does not overlap the tree is not counted (Figure 5b). In cases where the trunk just touches, the tree is given a 1/2 count (Figure 5c). With the prism kept at a fixed point, the observer rotates 360 degrees around the prism and counts all trees that are ‘in’. The basal area is the number of ‘in trees’ multiplied by the basal area factor of the wedge prism or gauge. In Australia only optical wedge prisms with a basal area factor of 1.0 or greater are available from suppliers. Count all trees when using the wedge prism at sites with average tree diameter less than 0.3 metres.

Using an optical wedge prism to count tree basal area. a) an ‘in tree’—with overlap; b) an ‘out tree’—space between tree and offset image of trunk; c) a borderline tree—slight overlap between tree and offset image of trunk (Source: https://en.wikipedia.org/wiki/Wedge_prism )

Data Recording and Storage

Data in the field will either be directly entered into specially prepared ODK forms, or into field sheets and then transcribed later.