Sample tree selection and characteristics

In order to allow the influence of a changing growth rate on the anatomical and mechanical properties of the sample trees in the 40 - 50 year age range to be evaluated, it was necessary to sample trees from the range of growth rates present within each of the even aged sites. To ensure that this criterion was fulfilled whilst also ensuring that as far as possible trees were selected at random, the methodology described in this section was adopted. The geographic information system ‘Forester’, a database driven package that gives a visual representation of sub-compartments1_{, boundaries and planting details within the publicly owned forestry estate,}

was used to divide each site into a series of circular survey plots. Each plot had an area of 300 m2_{, drawn such that its boundaries met with, but did not overlap, those which were}

adjacent to it. The plots were positioned within each site in order that a perimeter of 10 m was left around the site boundary, reducing the probability of selecting trees influenced by wind through effects which may have caused damage to the stem.

Each plot was assigned a unique number and a random number generator in Microsoft Excel was used to select four plots from each site to be used for the surveying of potential sample trees. The sole limitation placed on the selection of survey plots within a site was that they could not share a boundary. This restriction was imposed in order to avoid the formation of large openings within the canopy if several adjacent trees were felled. This occurred in one

1 _{A sub-compartment is a unit of measure in forestry, typically differentiated from surrounding sub-}

compartments by changes in species, age or management practices. In this work each sub-compartment is identified by the term ‘site’.

instance, and as a result the next randomly generated plot was selected as a replacement. Once four survey plots per site had been selected, the longitude and latitude positions of the plot centres were identified with the use of the Forester software.

The centre of each plot was located with a hand held GPS receiver. Prior to carrying out the survey, an inspection was made in which the conditions of the trees within a plot and the presence of any anomalies not representative of the site were evaluated. The two principle anomalies encountered were the presence of significant bow in the stem at the breast height sampling location and damage to the crown, with the latter typically occurring during extreme weather events. While trees exhibiting these features were included within the survey data for a plot, they were not included in the list of potential trees for sampling. The reason for this exclusion was to avoid the potential creation of large quantities of anomalous results within the data. These may have arisen due to excessive compression wood or unusual growth patterns in trees exhibiting the stated anomalies. The impact of these selection criteria on the intended random sampling methodology is discussed in Section 9.3.2. At one site, Nagshead, one of the survey plots initially selected was moved due to the presence of a footpath through its centre, resulting in only a limited number of trees, as shown in Figure 3-2. In this case the next randomly generated plot was selected as a replacement.

Once the condition of the plots was verified, each was measured out accurately by use of a 9.75 m length of cord attached to a wooden stake located at the centre. Any tree found to be located on the boundary of the plot was included if it was deemed that a proportion of the stem ≥ 50 % fell within the plot area. Each tree within a plot was tagged with a metallic tree marker for future identification. The diameter at breast height of each tree was measured and recorded with use of a diameter tape at 1.3 m above ground level, also referred to as breast height. An attempt to estimate the standing height of each tree was made by using a simple clinometer. However due to the dense nature of the canopy above many of the survey plots, the measurement was only successfully conducted on 18 trees. In total, a survey of 124 trees was conducted.

Figure 3-2: Rejected survey plot at Nagshead sampling site

Upon completion of the surveys the results from each site were collated and trees ranked according to their recorded breast height diameter. Given that all trees within a site were assumed to be of an even age, breast height diameter provided the best indication of the range of growth rates without taking increment core samples from each tree. In order to fulfil one of the key objectives of the study, to quantify the impact of rate of growth on anatomical and mechanical properties, trees were selected from within each site such that a broad spectrum of the growth rates present was sampled. This was done by selecting six trees for felling at regular intervals from the ranked diameter data, including the trees with both the lowest and highest diameters. Once trees for felling had been selected, an assessment was made to ensure that they were evenly distributed amongst the survey plots. This was done to avoid the creation of large openings within the canopy after felling and to guarantee a representative sample from across each site. As a result of this process it was necessary to reselect a number of trees; however this did not have a detrimental affect on the distributions of growth rates obtained from any sites. The characteristics of the trees selected for felling in the 40 - 50 year range are given in Table 3-2.

Table 3-2: Characteristics of sample trees in the 40 - 50 year age range

Site name Number of trees _selected DBH range (mm) Mean DBH (mm)

Nagshead 6 330 - 600 480

Tidenham 6 280 - 570 450

Highmeadow 6 260 - 510 400

Over Stowey 6 300 - 590 475

Following the selection of sample trees, each was marked with a unique number along with indicators to allow the cardinal compass directions of each tree to be identified once it had been felled. This was done with tree marking paint, with a single pink line used to designate the northern radius and pink and yellow the eastern, as shown in Figure 3-3.

Figure 3-3: Tree markings designating northern and eastern radii

Due to the limited number of sample trees with ages > 50 years to be felled, the methodology described above for their selection within sites was not employed. Instead, sample trees were randomly selected, with the cardinal compass directions again noted for each tree prior to felling. In total it was only possible to select three older trees for felling, rather than the target four. The total number of sample trees was therefore 27.