Management implications of tree growth patterns in miombo woodlands of Zambia

Brachystegia-Julbernardia (miombo) woodlands of east and southern Africa are divided into dry (< 1000 mm annual rainfall) and wet (> 1000 mm). These woodlands first became established about 14,500 years ago. Since the migration of the Bantu people 5000 years ago, miombo woodlands have experienced widespread degradation and deforestation. There is little long-term data on tree growth and forest productivity for miombo woodlands and yet such data are required for the proper management of these woodlands. This study used tree growth data from 35 temporary plots in regrowth of 5-49 years old, and six permanent sample plots (PSPs) in old growth forest, which were monitored over a 28-year period (1990-2018). These data were used to investigate tree diameter growth and develop models of forest dynamics in wet and dry miombo woodlands. The study found that variations in tree diameter and stand basal area increments are strongly driven by stand age, tree size and density and that growth rates are higher in wet than dry miombo. Overall miombo woodland trees grew slowly at a rate of 0.34-0.50 cm yr(-1) in re-growth areas. Most of the species in uneven-aged old-growth stands in dry miombo grew by 0.15 cm yr(-1) with no significant annual differences although variations among conspecifics were large. Models predicted tree diameter increments of 0.10-0.24 cm yr(-1) for stands of 100 years old. Transitions from one 5-cm diameter class to the next was estimated to take over 20 years and significant shifts in stand tree diameter distribution curves require over 25 years because of the low annual diameter increments. Stand responses to degradation varied from site to site and were driven by the number of saplings transiting into small diameter size classes. On the temporary regrowth plots basal areas averaged 7.1 m(2) ha(-1) and 9.9 m(2) ha(-1) in dry and wet re-growth miombo, respectively, with positive increments estimated at 0.40 m(2) ha(-1) yr(-1) in dry miombo and 0.53 m(2) ha(-1) yr(-1) in wet miombo. By contrast, average basal area increment on old-growth PSPs was negative, at -0.26 m(2) ha(-1) yr(-1) over the 1990-2018 period, due to cutting and fire damage. Such drivers of forest degradation need to be controlled, especially in old-growth miombo, to maintain forest health and prevent decline in biomass.