Spatial variation in black cottonwood (Populustrichocarpa) foliar nutrient concentrations at seven alluvial sites in coastal British Columbia

Abstract

Spatial variability of 13 foliar nutrients was assessed within and between individual black cottonwood (Populustrichocarpa Torr. & Gray ex Hook.) trees at seven alluvial sites in coastal British Columbia to help in the development of foliar sampling protocol for the determination of black cottonwood nutrient status. Foliar nutrients were divided into two groups based on concentration differences in foliage samples from within black cottonwood canopies: group 1 included N, P, K, S, SO4-S, Cu, and possibly Mg and B, mostly macronutrients that are mobile in the phloem; group 2 included mostly immobile micronutrients (Mn, Zn, Ca, and possibly Fe and active Fe, although these last two might best be included in a third group). Group 1 nutrient concentrations were significantly higher in the upper canopy, while group 2 nutrient concentrations were higher in the lower canopy at the two sites studied. Numbers of samples required for several combinations of accuracy (including both α and β significance) and precision were calculated based on mean coefficient of variation estimates from the seven stands. Fifteen foliar samples collected from the most recently matured late leaves on lateral branches in the top one-third of the canopy of dominant or codominant black cottonwood trees will estimate the mean with an allowable error of 10% at an α significance level of 0.95 for most macronutrients (N, P, K, S, Mg, and Ca). Including a β significance level of 0.95 at this level of precision and α significance would increase the number of required samples to at least 20 for these nutrients. Between 21 and 56 samples would be required to estimate the mean concentrations of SO4-S, Cu, Zn, Mn, B, active Fe, and Fe at the same levels of accuracy and precision.