Sample site selection and characteristics

In order that the results obtained from this study were relevant to the Douglas-fir resource found within the South West of the United Kingdom, sample sites from which to extract test trees were split into two categories. The first was those containing trees in the age range of 40 - 50 years and the second those containing trees > 50 years of age. These age ranges were established based on the findings of an assessment of the conifer resource within the South West (Forest Research, 2009), which showed that selecting trees within the 40 - 50 year age range would capture the current peaks in the Douglas-fir age distributions in both public and private estates. An age of 50 - 55 years is also commonly regarded as an optimal felling age for Douglas-fir (e.g. Coed Cymru, 2007) when calculated from the perspective of mean annual volume increment1_{. However, as forest management practices develop towards those which}

utilise a mixed age stand structure, as discussed by Macdonald et al. (2010), a greater number of trees that have exceeded the optimal felling age calculated using the mean annual increment

1 _{The mean annual volume increment is the average volume production per year for a forest area, the}

method will be found. For this reason, a number of trees greater than 50 years of age were included within the sample set.

The review of literature presented in Chapter 2 showed that the large majority of the variations seen in anatomical and mechanical properties were found to occur within the juvenile region of the stem, typically at cambial ages less than 30 years. Trees in the 40 - 50 year age range would therefore capture these changes and were selected to form the basis of the experimental sample set, with a limited number of older trees used to establish if the trends observed continued with increasing age. The large number of variables that can influence the characteristics of the wood produced by trees within and between sites, made the selection of sample sites reflecting typical growing conditions within realistic logistical constraints difficult. It was therefore decided to select sites containing trees within the 40 - 50 year age range randomly from a list of those available and assessed for suitability according to the criteria described below. Younger trees were to be selected from five sites with six trees sampled per site, allowing for a good geographical range to be obtained within the region and also for trees to be selected from the range of growth rates present within each site. Four trees with ages > 50 years from two sites were to be tested to verify the continuation of trends in the results observed in older stems.

The selection of sample sites from those available was conducted according to the following criteria:

 Regional distribution: A sample site distribution reflecting the geographic range of Douglas-fir in the region was desirable.

 Site condition: An assessment was made of the overall condition of the trees within each site, noting any factors which could hinder the implementation of a consistent sampling strategy such as excess numbers of dead trees, storm damage, animal damage and a large number of stems with excessive bow1_.

 Availability of site management history: The availability of information detailing previous silvicultural interventions such as thinning, pruning and fertiliser applications was assessed.

 Even age: In order to allow the variations in growth rate between standing trees within a site to be assessed it was necessary that all trees within 40 - 50 year old sample sites were of an equal age.

 Site access: The ease with which individual trees could be marked, felled and removed was evaluated.

Due to unforeseen circumstances, it was not possible to obtain sample trees in the 40 - 50 year age range from one of the selected sites in the region of Dartmoor National Park. As a result of this only four sites were used. All trees within the 40 - 50 year age range were obtained from the publicly owned forest estate managed by the Forestry Commission, while older trees were obtained from privately owned estates. The location of the sites selected in the context of forestry within the South Western region of England, and the characteristics of each site is shown in Figure 3-2 and Table 3-1.

Figure 3-1: Sample site locations within the context of forestry in the South West of England from Forestry Commission (2002)

2 1

4

3

5

6 _{= 40 - 50 year old sites}

Table 3-1: Sample felling site characteristics

Site name _{number Latitude Longitude} Map Elevation _{(m) AOD} Age _(yr) Stems/ha

Nagshead 1 51º 38’ 2º 34’ 100 42 200 Highmeadow 2 51° 49’ 2° 40’ 75 50 250 Tidenham 3 51º 42’ 2º 38’ 210 48 200 Over Stowey 4 51º 07’ 3º 11’ 220 46 300 Quethiock 5 50º 27’ 4º 20’ 50 75 N/A Lostwithiel 6 50º 25’ 4º 39’ 80 78 N/A

The site latitude, longitude and elevation were determined by use of a GPS receiver and Ordinance Survey mapping. The number of stems per hectare was determined during the site surveys described in the following section. Data relating to the stems per hectare within the sites from which trees > 50 years of age were obtained was not available due to the different sampling methodology used.