ALLOMETRIC RELATIONSHIPS BETWEEN SAPWOOD AREA AND SHRUB DIMENSIONS FOR 6
COMMON SOUTHERN AFRICAN SAVANNA BUSH ENCROACHER SPECIES: UNIVERSAL OR
SPECIES-SPECIFIC?
Abstract
Southern African savanna rangelands are facing a widespread degradation
pattern called bush encroachment. This is associated with implications
for various aspects of the water cycle and in particular canopy
transpiration. At the individual-tree scale, it is estimated by scaling
sap-flux density by sapwood area. However, the direct measurement of
sapwood area is impracticable at landscape scale and general allometric
equations of the West-Brown-Enquist (WBE) model relating sapwood area to
primary size measures seem to fail for some species and climates.
Therefore, we conducted intensive field measurements to establish
species-specific allometric relationships between sapwood area and sizes
(stem diameter, crown area) in six dominant shrub species involved in
bush encroachment in Namibia ( Colophospermum mopane, Senegalia
mellifera, Vachellia reficiens, Dichrostachys cinerea, Vachellia
nebrownii, Catophractes alexandri). We found strong allometric
relationships between sapwood area and stem diameter as well as between
sapwood area and crown area for all six species. These relations are
largely in line with the WBE theory but still provide estimates that are
more accurate. Only in D. cinerea, the sapwood area was
significantly smaller than predicted by the WBE theory, which might be
caused by a larger need for stabilizing heartwood. Our results are
useful to estimate water loss via transpiration at a large scale using
remote sensing techniques and can promote our understanding of the
ecohydrological conditions that drive species specific bush encroachment
in savannas. This is particularly important in the light of climate
change, which is considered to have major implications on
ecohydrological processes in savannas.