Generating Ring-Width Chronologies

8.5.1.1 A brief overview of tree-coring

Cores from living trees are traditionally taken from a tree using an increment borer; a narrow, hollow tube with a threaded cutting end that is wound into the tree, enabling a cylinder of wood inside the borer to be retrieved (Fig. 3.1). Generally, at least two cores per tree are taken as growth anomalies can result in ‘partial rings’, i.e. rings that are ‘missing’ from parts of the tree’s circumference. A common example is a conifer growing on a steep slope may sometimes develop ‘reaction’ wood on the downhill/

stressed side but not on the uphill side (Stokes and Smiley, 1968). Similarly, in areas close to the loss or development of a branch, rings can be ‘wedged out’ making them locally ‘missing’ (Norton et al., 1987).

In other situations, during periods of limited photosynthesis, trees may concentrate their growth in the canopy, meaning that a ring does not form lower on the trunk where sampling generally occurs (Norton and Ogden, 1987). Conversely, trees can also produce ‘false’, or ‘double’, rings within a single growth season. Often they are not clearly delineated because the ‘false’ latewood blends with earlywood on either side and careful microscopic investigation can provide other clues such as the presence of earlywood cells throughout the ‘false ring’.

Obtaining multiple cores per tree facilitates the identification of missing and false rings, and therefore cross-dating. For stable isotope studies of tree-rings this practice has the further benefit of facilitating quantification of intra-tree isotopic varability and in particularly narrow-ringed species, multiple cores may be the only means of providing sufficient material for annual isotope analyses (e.g. Porter et al., 2009; Porter et al., 2013).

Evidence, both published and anecdotal, suggests that increment boring has little, if any, adverse effect on trees (Norton, 1998 and Appendix 3). The wound (just over 5mm in diameter in this study) is small enough that, depending on the species, it may be blocked by resin within hours or sometimes days of the coring taking place (Fig. 3.6). At a number of our field sites there was concern surrounding disease transport; since the identification of Phytophthora taxon Agathis (PTA) in native kauri stands any scientific activity involving kauri has been subject to rigorous biosecurity protocols (see Waipara et al., 2013). A field plan for minimising contamination risk was followed, including the sterilisation of field equipment, particularly borers and footwear. Full details are found in Appendix 4. Since this field-work was conducted, Dr Andrew Lorrey of NIWA has created an ingenious phytosanitary coring field-kit, and some details of its use are found in Lorrey et al. (submitted). Despite the decreased risk of disease transport when sampling/assessing other species, it was deemed best practice to continue to follow the biosecurity protocols relating to footwear and borers.

8.5.1.2 Increment boring and core preparation for this study

At all sites 2-3 cores per tree, >90^o apart where possible, were collected at breast height (~1.4m) using 40cm-long, 5.15mm diameter increment borers (Fig. 3.1). For the duration of the fieldwork, the cores were stored in plastic straws. They were then air dried over two or more days whilst tied loosely into grooved wooden mounts with string to prevent warping. Dried cores were then fixed into the mounts;

Fig. 8.1: The coring process in pictures, showing a increment borer being wound into a tree at DK, an extracted DK core being examined after extraction and a close-up of a borer alongside an old core wound.

Appendices

kauri samples in this study (section 3.2.1) were generally short cores with wide rings and it was sufficient to tape them in place with low-stick painter’s tape, which made later destructive isotope sampling much easier. The mature cedar sampled (section 3.2.2) generally produced longer cores with much narrower rings. For these cores tape did not sufficiently immobilise the cores in preparation for cross-dating; these were secured using water soluble PVA glue (drying overnight). The use of glue was not ideal as it has its own isotopic signature (Beghin et al., 2011). The contamination potential was minimised by avoiding/

removing lumps of glue when sampling cores for isotope analysis. Any residual glue was removed by the subsequent chemical extraction process (See 3.3.2) (Nakatsuka et al., 2004; Dodd et al., 2008).

Every effort was made to mount the cores with the transverse aspect facing upward in order to improve ring definition when the cores were sanded with 120, 240, 400 and 600 grit sandpaper. The common North-American approach to core surfacing (Stokes and Smiley, 1968), using a razor blade, has proved problematic with New Zealand’s often narrow-ringed natives (Norton and Ogden, 1987).

8.5.1.3 Cross-dating and measurement The measurement and cross-dating within this study was conducted by Dr Pavla Fenwick, a local dendrochronologist based in Christchurch with extensive experience analysing New Zealand’s native trees. A brief description of the dendrochronology process is provided here.

Both ‘missing’ and ‘false’ rings can generally be identified through cross-dating, the process of matching common growth patterns or marker (very narrow or wide) rings within and then between trees (Fig. 3.2). Even nearby trees of the same species can grow at very different overall rates due to a number of factors including age, canopy position, water availability etc. Comparison of the average ring-width does little to inform age determination or climatic reconstructions but, provided the tree’s growth is responding to climate, the width of a ring relative to those around it should be similar between trees.

Consequently, trees growing at vastly different rates can still be cross-dated by creating an internally relative standard ‘growth index’.

For this study, visual cross-dating was Fig. 8.2: The measuring process and an example of a

‘marked up’, cross-dated core

undertaken in the first instance, generally with the naked eye for kauri samples and under binocular microscope for cedar. Then, ring width measurements were recorded using a Velmex measuring stage with a precision of 0.001mm attached to a binocular microscope (Fig. 3.2). COFECHA software (Holmes, 1983) was then used to confirm or refine the visual cross-dating. COFECHA converted the raw ring-widths into growth-indices, averaged them into a site ‘master chronology’, and calculated correlations between the individual cores and that master.

The Southern Hemisphere growing season spans two calendar years, beginning in Austral spring of one and ending in Austral summer/autumn of the next. For dating purposes, we followed Schulman’s (1956) convention for the Southern Hemisphere, which assigns each tree-ring to the year in which radial growth started. For example, the ring that began growing in spring of 2000 and finished in autumn of 2001, is labelled 2000 in the chronology.