Field data measurements

There is little consensus in the literature concerning the best indicators to use for assessment of condition at the individual tree and field plot-area scales. Those used were based on the above review of the available information (see section 2.2) and are listed in Table 3.1 for structural variables, Table 3.2 for regeneration and deadwood variables and Table 3.3 for disturbance and soil variables.

30

m

10mm

10

m

Table 3.1 – Forest structure variables recorded in the field

Metric: Measurement type: Plot area:

LAI Ring 4 Hemispherical camera 16 samples per plot

LAI Ring 5 Hemispherical camera 16 samples per plot

Canopy openness Hemispherical camera 16 samples per plot

Estimate of canopy layers Visual assessment Whole plot

Canopy density (%) Visual assessment Whole plot

X,Y coordinates of each tree Combined dGPS and total station Whole plot

Tree species Visual assessment Whole plot

Girth (m) (circumference at 1.3m height) Measurement tape Whole plot Tree height (m) Combined measurement tape and

clinometer Whole plot

Height of first live branch (m) (canopy base) Combined measurement tape and

clinometer Whole plot

North-South horizontal crown extent (m) Measurement tape Whole plot East-West horizontal crown extent (m) Measurement tape Whole plot Note if the tree has been 'pruned' Visual assessment Whole plot Assessment of canopy condition: [Open (1) -

Closed (4)] Visual assessment Whole plot

Number of saplings Visual assessment Whole plot

Number of seedlings Visual assessment Sub-plot

Table 3.2 – Ground vegetation and deadwood variables recorded in the field

Metric: Measurement type: Plot area:

Ground vegetation

Species of shrub vegetation (if >1m height) Visual assessment Whole plot Height of shrub individual (m) (if >1m height) Combined measurement tape and

clinometer Whole plot

Horizontal extent of shrub individual (m) (if >1m

height) Measurement tape Whole plot

Vascular plants percentage cover (%) Visual assessment Whole plot Estimate of percentage bare soil (%) Visual assessment Whole plot Estimate of percentage cover of moss on trees

(%) Visual assessment Whole plot

Estimate of percentage cover of moss on ground

(%) Visual assessment Whole plot

Deadwood

Height of snags (m) Combined measurement tape and

clinometer Whole plot

Girth of snag (m) Measurement tape Whole plot

Snag decay class: [Light (1) - Heavy (3)] Visual assessment Whole plot

Fallen tree length (m) Measurement tape Sub-plot

Fallen tree girth (m) Measurement tape Sub-plot

Fallen tree decay class: [Light (1) - Heavy (3)] Visual assessment Sub-plot

Fallen branch length (m) Measurement tape Sub-plot

Fallen branch girth (m) Measurement tape Sub-plot

Table 3.3 – Disturbance and soil variables recorded in the field

Metric: Measurement type: Plot area:

Disturbance

Evidence of bark stripping (%) Visual assessment Whole plot Obvious browse line on trees: [Light (1) - Heavy

(4)] Visual assessment Whole plot

Height of browse line (m) if different from crown

base Measurement tape Whole plot

Evidence of trampling of ground flora Visual assessment Whole plot Tree seedlings and saplings projecting above

ground vegetation height (y/n) Visual assessment Sub-plot Evidence of browsing of tree seedling or sapling

shoots (y/n) Visual assessment Sub-plot

Dung from animals: [Abundant (1) - None (4)] Visual assessment Whole plot Evidence of human activity (e.g. vehicle tracks in

site) Visual assessment Whole plot

Direct evidence of human activity (e.g. presence

of rubbish) Visual assessment Whole plot

Soil attributes

Soil pH Tecpel PH-707 meter 4 plot corners

Soil moisture Tecpel PH-707 meter 4 plot corners

Within each plot, the number and spatial location of each tree stem was recorded. Girth at Breast Height (GBH) of each tree >30cm GBH (i.e. approximately >10cm diameter) was measured to the nearest centimetre using diameter tape at a height from the base of 1.3m in accordance with Forestry Commission guidelines, as in Matthews and Mackie (2006). GBH can be used to determine Diameter at Breast Height (DBH) assuming the tree stem cross- section is that of a circle. Tree height and height of the first live branch (i.e. height of the living crown) was estimated for each tree (DBH>10cm) using a clinometer to measure angles from the surveyor and a measuring tape for determining distance from the tree base. Heights were derived through trigonometry (Matthews and Mackie, 2006). Tree crown horizontal extent was measured by projecting the crown onto ground-level, where the north-to-south and east-to-west extent was measured with tape (Suárez, 2004). A rough estimate of each tree’s canopy condition was then undertaken according to 4 classes: (i) very sick/dying; (ii) more sick than healthy; (iii) more healthy than sick; and (iv) healthy, with little sign of deadwood and lots of green leaves (Hendry et al., 2002). The number of canopy layers was estimated at the plot-level for all species present.

10x10m sub-plot, with seedlings defined as individuals <1.3m in height. The number of ground flora species and their percentage cover was recorded within the subplot; this included vascular plants and bryophytes.

Downed deadwood (DDW) was defined as deadwood logs of at least 10cm diameter lying on the ground (Spies et al., 1988). Measurements were made in the 10x10m sub-plot only (Cantarello and Newton, 2008). Length and girth around the maximum and minimum diameters of the log were recorded. Length and girth of fallen branches >2cm in diameter were also recorded for the sub-plot. To assess DDW decay class, logs and branches were divided into three decay classes according to the following criteria: (i) logs with a low decay state, no surface breakdown, bark still intact, wood structure firm; (ii) logs with a moderate decay state, with some surface breakdown, wood structure weaker but bole mostly sound; and (iii) logs with high decay state, extensive surface breakdown, bark mostly absent, bole with no sound wood present and colonised with vegetation (Cantarello and Newton, 2008).

Snags were defined as standing deadwood>10cm DBH (Spies et al., 1988). Girth was measured at a height of 1.3m from the base using measuring tape. Trigonometry was used to derive the heights of the snags using a clinometer and measuring tape. All snags within the 30x30m plot were measured. Snag decay class was assessed using a three-class system, identical to that described for DDW.

Soil pH and moisture measurements were taken in all of the 41 plots. Measurements were taken in each of the four corners using a Tecpel-707 (TECPEL Co. Ltd.), recording soil pH and soil moisture content. The sensitivity of the equipment could range from 3-8 pH, with an accuracy of ±0.2 pH, and soil moisture content could range between 10-80%. It should be noted that pH and moisture readings were taken on different days for each of the plots, and thus may not be representative.

Hemispherical photography was used in order to estimate Leaf Area Index (LAI) and Canopy Closure (CC). Neither LAI nor canopy closure can be estimated accurately from a single image; Weiss et al. (2004) suggest a minimum of 8 images is required. In this study a total of 16 images were taken for each field plot at a height of 1m from the ground using a horizontally levelled digital camera. A uniform 4x4 cell grid was laid out in each field plot (see Figure 3.8) where a photograph was taken in each cell as close to the centre as possible,

allowing for obstructions. All photographs were taken between 10am and 2pm under overcast conditions to ensure a homogenous illumination of the overstorey canopy and a correct contrast between the canopy and the sky, as in Riano et al (2004).

Hemispherical photography was acquired using a Nikon Coolpix 5400 digital camera and Nikon FC-E9 fisheye converter (combined focal length equivalent to 7.2mm, and the combined F number is f/2.4). The field of view of this lens is approximately 183°.The Nikon Coolpix 5400 employed a high density 1/1.8" colour CCD sensor with a maximum pixel density of 13 MP/cm².Photographs were taken at a resolution of 2592 x 1944 pixels, illustrated in Figure 3.9.Each of the photos was oriented north to south in the field, according to the markers present on the self-levelling mount (SLM5, Delta-T Devices Ltd.), so north is always to the top-left of the image.