2.1.1 Selection of the General Method for Destructive Tree Sampling

A first step to create a biomass model typically involves destructive sampling of trees. There is a multitude of biomass sampling methods which would warrant a book on their own. In the following section a pragmatic approach is followed since biomass sampling does not form the major focus of this chapter. The different approaches in assessing the biomass of trees can be roughly classified in bulk sampling approaches where more than one tree is sampled and individual tree sampling (Fig. 3.2). The first method is usually based on in-field chipping and is a

more industrial than scientific practice. Bulk sampling is frequently applied in short rotation plantations when e. g. the biomass of coppiced trees is to be measured and entire rows or stands are harvested and chipped for fresh weight calculation (e. g. Hytonen et al. 1987). Another typical application of bulk sampling is harvesting studies of invasive woody vegetation with a high proportion of multi-stemmed trees and bushes, where only a biomass value per area is required (Kitenge 2011). However, in most cases dry weight is usually not determined from all chips but from a randomised sub-sample, in order to minimise transport and handling losses and to increase the efficiency of the method. The advantage of the technique is that bulk sampling of many trees is highly efficient and mimics real harvesting conditions rather closely. But the results obtained by the bulk method are sometimes difficult to compare with other studies because the chipping usually involves as loss which is specific to the machine equipment used (see Chap. 6). A more scientific sampling approach is based on individual trees. In this instance a bulk sampling of the entire tree or the aboveground part can be done with all the positive and negative aspects mentioned above. To avoid chipping losses a full tree fresh weight can be determined for example by a harvester mounted scale (Pettersson and Njordfjell 2007). However, in this instance representative sub-sampling for dry weight determination might prove to be a challenge.

In many biomass studies an estimation of the different biomass components such as bark, wood, leaves and branches is a main objective because different proportions of biomass components will influence ash contents and calorific values of the tree (see Chap. 8). The quantification of biomass components is also a prerequisite to assess the impact on nutrient balances as a result of biomass export from the stand (see Chap. 10).

The practical choice of the individual tree sampling method is mainly determined by the number of components that should be differentiated between, but also by the

tree size and the available time and work force. A full measurement of the fresh weight of the stem is always preferable since it eliminates an upscaling step (part of the upscaling step 1 in Fig. 3.1) and thus a possible source of error. The method has been successfully applied in several biomass studies for example in short rotation Eucalypt plantations (du Toit 2008; Dovey 2009). While harvester mounted hanging scales can support the measurement of stem fresh weight, the separation of twigs and foliage on site is only feasible for smaller trees under the normal time and money constraints. A regression based sampling approach, which is introduced in detail in the following section, is thus necessary for bigger trees to maximise efficiency. In the end the choice of the method always depends on the scope and specific objectives of the biomass study (Table 3.1). In addition to the listed method archetypes a multitude of mixed approaches can also be applied.