Miombo Ecoregion Vision Report - Biodiversity Foundation for Africa

Miombo Ecoregion Report, page 32
the extent of releasing most of the carbon contained in roots. And although these species grow
slowly from seed because of the prolonged seedling phase lasting up to two decades or more,
regrowth from coppice is faster because of the already established root system. It is conceivable
therefore that regrowth woodland could be managed for carbon sequestration.
4.5 Nutrient Cycling
Low herbivory, high carbon content relative to nitrogen in plant biomass and a long dry season
when topsoil moisture is deficient, have resulted into a slow nutrient cycling system within the
ecoregion. Episodic outbreaks of insect herbivores, such as caterpillars and grasshoppers, may
occasionally speed up nutrient cycling, but as a rule the region is characterised by very slow
cycles. In fact, Brachystegia woodland trees show a high nutrient re-absorption rate (50–60%),
especially of N and P, from leaves prior to leaf fall as a strategy for conserving these critical
nutrients and reducing nutrient loss to other ecosystem components (Chidumayo 1997).
In some habitats, termites play an important role in nutrient cycling. They fall into three trophic
groups: humus feeders, lignin feeders and mixed feeders. Humus feeders ingest decaying organic
matter and mineral soil and produce a more stable organic matter in their faeces for the physicochemical
and bacterial agents of degradation. The lignin feeders, which include the large mound
builders, attack intact vegetation and dead material under the cover of access routes. Some of
these use the material to grow fungus in their mounds which produce termite mushrooms in the
rainy season. Although lignin feeders are active throughout the year, peak activity occurs in the
late rainy season. Other decomposers also show seasonal rhythms in their activity with most
microbial processes reaching a peak during the rainy season. Leaf litter decomposition is rapid
during the early rainy season due to the activity of bacteria, mites and Collembola.
Many woodland trees develop symbiotic relationships with fungi that live on or in their roots.
These root fungi (mycorrhizae) have been reported to improve the nutrition, especially
phosphorous absorption, and water uptake of the predominant tree species. Seedlings of
Brachystegia and Julbernardia trees that normally take up to two decades or more before
reaching the sapling stage, have been shown to grow faster in the presence of mycorrhizae.
Mycorrhizae are of two types. Brachystegia woodlands are dominated by ectomycorrhizal tree
species, while the drier or more eutrophic woodland types are dominated by endomycorrhizal
species (e.g. Colophospermum, Baikiaea, Burkea) (Högberg 1986, Högberg & Piearce 1986,
Munyanziza 1994). These symbiotic relationships are important in sustaining woodland
productivity.
4.6 Fire
Low herbivory, high carbon content in the plant biomass, seasonality in litter decomposition and
a long dry season (5–7 months) interact to create conditions in which fire plays an important role
in nutrient cycling. One of the key features of the ecoregion is the frequent occurrence of dry
season fires that are caused mainly by man. Annual fires tend to burn grass and woody litter and
therefore do not usually add much to the accumulation of carbon dioxide in the atmosphere as
emissions are recaptured the following year by annual regrowth.
Fire has been a component in the Miombo Ecoregion for at least 55,000 years (Davis 1971) when
it was first used as a tool for hunting and shaping the landscape during the Early Stone Age. It is
therefore one of the determinants of the ecoregion, which contains communities, both plant and