Vegetation structure

Physical measurement of vegetation structure characteristics is possible with varying levels of guidance available depending on the habitat. However remote-sensing approaches such as LiDAR show promise for simplifying and standardising measurement of vegetation structure across habitats in the future.

Forest:
Collection of Vegetation structure data can be carried out alongside data collection for Vegetation biomass, Tree diversity, Seedling regeneration, and Tree Age..
The UK National Forest Inventory (NFI) provides a standardised methodology to establish fixed area survey plots and record tree growth categories (Forestry Commission 2020). Young trees (seedlings and saplings), diameter at breast height (DBH) < 4 cm and mature trees, DBH > 4 cm, are recorded.

• Forest is defined as having >20% canopy cover
• 0.01 ha plots (5.64m radius) are established, the number of plots is determined by the size of the forest area
• The physical layering of the vegetation is recorded (tree canopy, shrub layer, field layer, ground layer)
• Forest storeys (distinct forest layers) and their upper- and mid-crown heights are recorded
• Within each plot DBH is recorded for all mature trees (DBH > 4 cm) + crown width and timber height for the dominant tree in each storey
• Saplings (height > 50 cm, DBH < 4 cm) are recorded in a 2.52 m radius plot at the centre of each 0.01 ha plot
• Seedlings (height < 50 cm, DBH < 4 cm) are recorded in a 1.78 m radius plot at the centre of each 0.01 ha plot
• For saplings and seedlings browse class and origin (planted, regeneration, sucker) are also recorded

The NFI Survey Manual provides the methodology:

• How to allocate plots – Chapter 12 Plot Assessments
• Recording vegetation layers – Chapter 9 Sub-Component Data
• Recording forest storeys – Chapter 8 Components
• Recording mature trees – Chapter 13 Tree Assessment Procedures
• Recording young trees – Chapter 15 Young Tree Assessments

Deriving metrics:

• Vertical structural diversity: a simple metric is the number of forest layers, more complex is standard deviation of tree height (Mura et al. 2015)
• Horizontal structural diversity: standard deviation of DBH (Mura et al. 2015)

Shrub-dominated ecosystems (heathland, scrub):

No standardised method available in the UK, but modification of protocols used elsewhere may be possible.
E.g. A method of vegetation monitoring in shrubland is given in Wood et al. 2012, based on the BBird protocol developed by the Montana Cooperative Wildlife Research Unit (Martin et al. 1997)

• Measurement should be taken at the peak of the growing season
• Plots 50 m radius are dispersed throughout the habitat and should be surrounded by 100 m of the same habitat and 300 m from other sampling points
• Within each 50 m radius plot are four 5 m radius subplots, one central, three at 0, 120 and 240 degree angles at random distances between 20 and 80 m from the centre
• In each subplot, at the centre and 5 m N, S, E, W of the centre, a Wiens pole 12 m tall subdivided into 30 cm subsections is placed upright in the vegetation
• The number of vegetation intersections in each subsection are recorded
• At each 50 m sampling point 20 foliage-height tallies are generated
• At each sampling point foliage height diversity can be calculated based on the number of hits in each subsection, this can be averaged across the 20 samples taken
• Horizontal diversity can be calculated as the standard deviation of canopy height across all 16 sample points

Herbaceous-dominated ecosystems (grassland, peatland, saltmarsh, wetland):
No standardised method available in the UK, but modification of protocols used elsewhere may be possible.
E.g. A method of vegetation monitoring in grassland is given in Wood et al. 2012, based on the BBird protocol developed by the Montana Cooperative Wildlife Research Unit (Martin et al. 1997) and Wien’s 1969 method of assessing grassland vegetation structure.

• Measurement should be taken at the peak of the growing season
• Plots 50 m radius are dispersed throughout the habitat and should be surrounded by 100 m of the same habitat and 300 m from other sampling points
• Within each 50 m radius plot are four 5 m radius subplots, one central, three at 0, 120 and 240 degree angles at random distances between 20 and 80 m
• In each subplot, at the centre and 5 m N, S, E, W of the centre, a Wiens pole subdivided into 10 cm subsections is placed upright in the vegetation
• The number of vegetation intersections in each subsection are recorded
• At each 50 m sampling point 20 foliage-height tallies are generated
• At each sampling point foliage height diversity can be calculated based on the number of hits in each subsection, this can be averaged across the 16 samples taken
• Horizontal diversity can be calculated as the standard deviation of canopy height across all 20 sample points