Measuring and modeling surface area of ponderosa pine needles

Two methods for needle area estimation were compared for Pinus ponderosa Dougl. ex P. Laws. & C. Laws., and models were developed to predict total, projected, and abaxial areas. Areas of needles were determined by using video capture - image analysis procedures (VCIA) and by direct measurement of needle sections. VCIA area estimates were 40-60% less than abaxial areas determined from direct measurements. Allometric models fit to VCIA area and mean needle width (W-v) explained 96% of the variation in measured sample areas; models omitting W-v explained 91% of the variation. Predictions for independently collected validation data were somewhat poorer and slightly biased but had similar residual patterns. Allometric models fit to midneedle width and total needle length explained 99% of the variation in directly measured needle areas, with root mean square error equal to 2% of the mean measured areas. Results were similar for the validation data. For both models, final parameters were estimated from the combined data. It is shown that fascicle areas estimated from predictions for the middle-sized needles are nearly as accurate as estimates based on measurements for entire fascicles. Direct measurement of needles is more portable than VCIA and provides more accurate needle area estimates with less measurement effort.