Modelling variation in wood stiffness of Pinus ponderosa using static bending and acoustic measurements

Wood removed in Southwestern US forest restoration treatments currently has limited markets and thus low value. One important property of wood in structural products is its stiffness (measured as modulus of elasticity), which is known to vary systematically within trees. Directly measuring wood stiffness is expensive, time consuming and destructive. Therefore, we tested samples of ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) from northern Arizona destructively in bending and also non-destructively using acoustic velocity (AV) methods. In total, we tested multiple pith-to-bark small clear (2.54 × 2.54 × 40.64 cm) samples from up to four heights in 103 trees. We first measured the standing-tree AV of sample trees, then the AV of small clear samples, and finally measured wood stiffness using three point static bending tests. We found that a Michaelis–Menten curve was a good fit to the radial profile of wood stiffness, with a steep increase outward from the pith that approached an asymptote. The AV of small clear samples, coupled with measured volumetric density values, approximated the static modulus of elasticity values with high accuracy (r2 = 0.86). At the stand level, a model predicting standing tree AV from tree morphology fit the data well (r2 = 0.77). Results indicate that southwestern ponderosa pine contains outerwood with relatively high stiffness that could be suitable for structural products. However, when assessed using wood stiffness, the trees take a long time to reach maturity (~50 years) and thus the corewood proportion is large. AV measurements are a good way to assess variability within and among stands and thus could be employed to segregate the resource by expected stiffness values. Segregation could help identify stands not suitable for structural uses and direct them toward more appropriate products.