Effects of thinning and nitrogen fertilization on xylem development in Douglas-fir

Abstract

The rates and duration of tracheid production of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) were studied by periodic marking of the cambium during two growing seasons. Trees were growing in plots that had been subjected to four treatments, 3 and 4 years previously when trees were 24 years old: (1) control; (2) thinned to one-third the original basal area; (3) fertilized at a rate of 448 kg N/ha; or (4) given the combined thinning and fertilization treatment. From prepared microscope slides of wood sections containing the markings and the previous year's growth, the diameter and wall thickness of tracheids and width of earlywood and latewood were measured for the years since treatment. Rates, but not duration, of tracheid production were highly affected by treatments with maximum radial file productions of 0.80, 1.27, 1.53, and 2.00 cells/day, for the above treatments, respectively. Seasonal changes were related to air temperature until mid-July; some effect of soil water stress was apparent in July–August, and growth cessation was attributed to a short photoperiod. Radial stem growth was more than doubled by the single treatments and this was primarily caused by an increase in cell number, the effects on cell diameter being small. Treatments, except thinning, tended to decrease the percentage of latewood, and thinning increased tracheid wall thickness of earlywood.