The use of multiple measurement techniques to refine estimates of conifer needle geometry

Knowledge of foliar surface area is important in many fields, but estimating the area of nonflat conifer needles is difficult. The primary goal of this study was to use optical scanning and immersion methods to test and refine the standard cross-sectional geometries assumed for black spruce (Picea mariana (Mill.) BSP) and jack pine (Pinus banksiana Lamb.) needles. Projected leaf area (PLA, measured using a flatbed scanner), and hemisurface leaf area (HSLA, estimated from water immersion) were compared for conifer samples from a 37-year-old even-aged stand in northern Manitoba, Canada. The HSLA–PLA relationship was used to infer information about needle cross-sectional geometry after assuming a basic form (rhombus for black spruce and hemiellipse for jack pine). The cross section of black spruce needles was best approximated as a rhombus with a major/minor diagonal ratio of 1.35. Jack pine needles were best described by a hemiellipse with major/minor axis ratio of 1.30. Minor but incorrect assumptions of needle cross-sectional geometry resulted in foliar area errors of 6–8% using scanning methods and 1–2% using immersion methods. Simple equations are presented to calculate hemisurface needle area from volume or projected needle area based on these refined parameters.