Global stem shape models for single-stem tree species

Introduction

The Cape Verde islands provide a wide spectrum of growth conditions that allow growth of diverse forest ecosystems. Most of them are located on rather small areas and often represent fragile ecological niches.

The National Forest Inventory (NFI) survey, provide quantitative information on abundance and diversity of forest resources on the Cape Verde islands. To facilitate the assessment on growing stock volume, biomass and carbon content, suitable local volume and/or biomass functions need to be developed.

This aim of this material is to describe a novelty approach of stem volume assessment that is based on stem shape models. This approach is perfectly suited to the conditions of the Cape Verde islands, because it is a non-destructive method that effectively utilizes advanced technology for remote sensing in combination with specific software solution for construction of tree stem shape models. This methodology is suited for single-stem tree species, but it is not applicable for multi-stemmed or shrubby trees such as Prosopis sp., forwhich a different methodology using biomass functions needs to be used.

Hereby, this material contains a description of the approach of stem shape model construction and the required working steps.

Concept of stem volume assessment based on stem shape profiles

The basis of all studies aimed at tree stem volume assessment is stem shape determined empirically for smaller or larger amount of sample trees. The stem shape is described by stem shape profile that provides stem diameters for corresponding positions along the stem axis. For simplification, a circular shape of stem cross-sectional area is commonly assumed. The radial increment allocated at different stem heights creates a specific stem shape. In this way, tree may react to changing growth conditions at the site. Stem shape is determined genetically, changes during the tree life span and it is significantly influenced by site growth conditions. This is also the reason why a locally specific stem shape models should be estimated.

The process of creating stem shape models requires an appropriate stem shape function. A suitable function was proposed by Riemer et al. (1995), which is shown as Eq. 1:

Eq. 1

where dh is stem diameter at height h, H is total tree height, d1.3 is stem diameter at breast height, and i, p, q are parameters to be fitted based on the collected empirical material. This function can well describe a wide spectrum of variations in stem shape and its parameters have reasonable interpretation.

Thanks to the advanced methods of remote measurements, stem diameters can be measured also on standing trees above the level accessible by callipering (above 2 m). In practice, stem shape can be effectively described by six points along the stem axis. The diameters located higher than 2 m are measured by laser rangefinder with optical scope.

Integrating the functions parameterized for individual trees and tree species provides the corresponding tree stem volumes. These data can also be used for assessment of aboveground or total tree biomass if converted to dry mass using the species-specific conventional wood density and adequate biomass expansion factors (BEF) to include branches and/or below-ground components.

Tree species and sample size

The tree species requested for volume assessment and construction of stem shape profiles are:

• Cupressus sp.
• Eucalyptus sp.
• Gravillea robusta
• Pinus canariensis

The sample size required for a practicable stem shape parameterization is suggested as 60 to 70 individual tree stems for each species. This sample should be collected from at least three different locations.

On each locality, the selected trees should evenly cover the entire range of stem diameters (DBH). Only straight trees with no obvious damage should be used for stem shape diameter measurement and stem shape assessment.

Technology used

The technology required for field measurement associated with data collection for stem shape models is as follows:

• Software Field-Map (IFER)
• Field computer Hammerhead (DRS)
• GPS SXBlue (Geneq)
• Laser rangefinder Impulse (LTI)
• Electronic compass MapStar (LTI)
• Scope for remote diameter measuring (IFER)
• Electronic caliper DigiTech (Haglof)
• Accessories (IFER)

Fig. 1: left -Field-Map technology is the key component needed for empirical data collection for stem shape models; right – remote measurement of stem diameter using a scale in optical scope.

Field work steps

The aim of the work in the field is to collect empirical data on stem diameters and corresponding heights. The working procedure at the selected site is as follows:

• GPS positioning to determine x, y, z coordinates of the site
• Tree position mapping
• Tree description (species)
• Tree height measurement
• Tree crown base height measurement
• Brest diameter at breast height (DBH) measurement with caliper
• Complementary diameter measurement for stem profiles at:
  • stump base and/or 50 cm using caliper
  • 2 m from ground using caliper
  • height at half distance between tree crown base and ground using scope for remote diameters
  • crown base height using scope for remote diameters

Other considerations for complementary diameter measurements:

• if crown base height is below 1/3 of tree height, the measuring point under 7c is omitted and replaced by diameter measurement at 3/5 of the total tree height
• if crown base height is above 3/5 of tree height, the measuring points under 7c and 7d are omitted and replaced by diameter measurements at 1/3 and 3/5 of tree height

Fig. 2: The measured stem diameters forming stem shape are displayed once entered in the computer.

It is important to note that the method is not sensitive to exact location of the diameter measurement along the stem axis, i.e., it is fully satisfactory to locate the measurement position with accuracy of ± 1 m. It is more important to ensure a good visibility of the measured stem section.

As the points are measured in the field, Field-Map software (Stem Profiler module) draws and displays the actual stem profile curve (local model) on the computer screen. This helps to visually control the measurement quality.

Data processing

The stem profile models for individual trees are sent to IFER for further analysis and creation of global models applicable for individual tree species.