Phenology and nutrition of miombo woodland trees in Zambia

Seasonality in nitrogen (N) and phosphorus (P) concentration in soil and shoots of five Brachystegia-Julbernardia (miombo) woodland trees was studied from September 1991 to March 1993 at two regrowth miombo sites in central Zambia. Shoot growth started in the dry season (September–November) and lasted until April during the 1991/92 season but had virtually ceased by January 1993 during the 1992/93 season. The shoot growing season was associated with low foliar N/P ratios. These ratios were much lower (<5) during the 1991/92 season than in the 1992/93 season (12–15). The increase in foliar N/P ratios after the shoot growing season was caused by a sharp drop in foliar P concentration, apparently due to reabsorption before leaf fall. There were no annual variations in biomass N concentration in contrast to P. During the 1992/93 growing season P concentrations in foliage and wood were a quarter and a third, respectively, of the 1991/ 92 levels. Since the short shoot growing season observed during the 1992/93 season is typical of savanna woodland trees in southern Africa, the high biomass P concentration and longer growing season in 1991/92 season were exceptional and may have been related to reduced competition by shallow rooting herbaceous plants caused by the severe drought of that season.