Assessing sapwood depth and wood properties in Eucalyptus and Corymbia spp. using visual methods and near infrared spectroscopy (NIR)

Abstract

Accurate measurement of sapwood depth (D S ) is essential for calculating volumetric water use of individual trees and stands. Various methods are available to measure D S but their accuracy is rarely cross-validated. We sampled 15 Eucalyptus and 1 Corymbia species along a gradient of aridity and obtained reference values of D S in fresh wood cores using light microscopy, which represents our reference method. We compared this method to the simpler and widely used macroscopic method: visual assessment of natural or induced colour change from sapwood to heartwood. In a third method, estimation of D S was based on species-specific models that rely on wood properties measured using near infrared spectroscopy (NIR). Microscopy allowed clear identification of D S based on the presence of blocked vessels. Measurement of D S using microscopic methods was possible for 78 of a total of 80 cores and ranged from 3.6 mm (E. loxophleba) to 43.8 mm (E. viminalis). Macroscopic assessment clearly differentiated sapwood and heartwood in 60 cores. Results from microscopic and macroscopic methods agreed closely (\10% deviation between estimates) in 35 of 78 cores. After elimination of clearly erroneous measurements ([50% deviation between estimates), macroscopic measurement across all species agreed well with microscopic assessment of D S (R 2 = 0.92). Models developed for differentiation between sapwood and heartwood using NIR spectroscopy were very robust (high coefficient of determination) for four species, but D S could only be predicted well for one (E. obliqua) of the four species. Even after elimination of apparent false estimates, prediction of D S by NIR across species was not as strong as for macroscopic assessment (R 2 = 0.88). D S can accurately be measured using microscopy if vessel occlusion is clearly visible. Although slightly overestimated, D S from macroscopic assessment was generally similar to that measured by microscopy. NIR spectroscopy was unable to predict D S with acceptable accuracy for the majority of species. Further improvements in the prediction of D S using NIR will require more intensive model calibration and validation, and may not be applicable to all species.

Elevated CO2 did not affect the hydrological balance of a mature native Eucalyptus woodland

“…Wood cores were 5 mm in diameter and were extracted using a standard Pressler increment borer (Haglöf Västernorrland, Sweden). Sapwood depth was measured with a digital calliper after staining wood cores with methyl orange that provided visual differentiation of the sapwood from the heartwood (Pfautsch, Macfarlane, Ebdon, & Meder, ). We calculated the correlation coefficients between basal area and A s measured in these trees to predict A s per unit of ground area from basal area inside each ring.…”

How Reliable Are Heat Pulse Velocity Methods for Estimating Tree Transpiration?

“…For example, sapwood area can be difficult to measure accurately due to heterogenous tree structure. Additionally, the transitional zone between conducting (sapwood) and non-conducting (heartwood) xylem can be ambiguous with different measurement techniques yielding different results [45]. In some studies, the sampled tree cannot be measured due to its heritage or inherent value and sapwood area must be estimated by measuring nearby trees or from allometric equations.…”

A new method for measuring stand sapwood area in forests

“…Traditionally, tree‐level SA has been estimated using a few small wood cores (5 mm in diameter) extracted manually from the outer 10–15 cm of the tree in a sample of stems. Sapwood thickness (ST) has been determined microscopically, on the basis of the presence of open vessels, or macroscopically, on the basis of natural colour changes or staining of sapwood using methyl orange (Pfautsch et al ., ). The SA of each sample tree is calculated assuming that the tree stem is perfectly circular in cross‐section and that mean ST measured in a small number of cores is representative of the true mean ST around the tree circumference.…”