Page 17 - UNAM Virtual Graduation 2020 e-Book

FACULTY OF HUMANITIES & SOCIAL SCIENCES

DOCTOR OF PHILOSOPHY IN GEOGRAPHY

CANDIDATE: NAKANYALA Jesaya

CURRICULUM VITAE

Jesaya Nakanyala, a native of Elombe in the Onayena district, Oshikoto Region, is a
lecturer in the Department of Wildlife Management, University of Namibia. Prior to his
lecturing appointment, he was a postgraduate fellow at the University of Namibia’s
Multidisciplinary Research Centre (MRC), and previously occupied a research
assistant position at the Gobabeb Training and Research Centre. His academic journey can be traced back to 2009
when he graduated with a Bachelor of Arts degree in Geography at the University of Namibia. In 2012 he graduated
from his alma mater with a Masters of Arts degree in Geography with a specialization in Environmental Assessment,
focusing on vegetation in Etosha National Park. The same year, he received a United Nation University (UNU) fellowship,
and graduated with a Professional certificate in Land Degradation, Restoration and Global Environmental Change
from the Agricultural University of Iceland. He has published several refereed journal articles, technical reports and
presented numerous papers at local and international conferences. His PhD study was funded by the Southern African
Science Service Centre for Climate Change and Adaptive Land Use (SASSCAL) consortium.

CANDIDATE’S DISSERTATION

ROOTING STRATEGIES OF SAVANNA SHRUBS IN THE KALAHARI BASIN: IMPLICATIONS FOR COEXISTENCE OF WOODY
AND HERBACEOUS PLANTS AND SHRUB ENCROACHMENT IN THE AFRICAN SAVANNAS.
The doctoral study was undertaken and completed under the supervision of Professor M. Hipondoka of the University
of Namibia.
Bush encroachment, a replacement of palatable grasses with thicket bushes on grazing land, is a major land
degradation problem in Southern Africa. This phenomenon is only known to occur in the savannas, a unique biological
community where both trees and grasses intimately grow together and without displacing each other under a natural
setting. Environmental factors that make it possible for trees and grasses to grow together in the savannas are not
well understood by scientists. At the same time, principal factors that disrupt the harmonious distribution of both
trees and grasses to favour trees, resulting in bush encroachment, are still unknown. Consequently, scientists have
been confronted with this mystery for decades, and resulted in several models to explain the occurrence of both
phenomena, but without conclusive evidence. One of the challenges in understanding both the functioning of the
savannas and occurrence of bush encroachment rests in our limited ability to observe the behaviour and structure
of plants below the earth surface, owing to the soil opacity. Consequently, what we know today about the “hidden
half” of the savanna plants is based on uncertainties and assumptions, which in turn gave rise to competing models
that fuelled controversies. The most popular savanna model was derived in 1939 by a German ecologist, Heinrich
Walter, who, after visiting Namibia, suggested that the main important determinant of the savannas is the deployment
and separation of trees and grasses roots at different soil depths. It was this suggestion that prompted this study to
focus on the root structure of common savanna plants across the Kalahari rainfall gradient, where soils are relatively
homogenous. The overall objective of the study was to investigate how savanna shrubs develop and deploy their
root systems across a climatic gradient. Using direct excavation method, this study investigated the root system
architecture of common encroaching shrubs and their four nearest, non-encroaching neighbouring shrubs across a
climate gradient. In total 183 shrubs from 19 species were excavated.

Results indicate that savanna shrubs are not essentially deeper rooted as predicted by existing models. On the
contrary, savanna shrubs develop contrasting root systems, which vary across climatic gradients. Resultant root system
architecture was classified into three groups: (i) lateral root system (ii) dual root system, and (iii) taproot system. These
root systems are not necessarily unique to any plant species or environmental setting. Such behaviours may allow
savanna shrubs to develop rooting strategies adaptable to the prevailing, local environmental constraints such as soil
moisture. Of particular significance is that shrubs encroachment is largely attributed to shallow rooted shrubs such as
Dichrostachys cinerea (Sicklebush), Senegalia mellifera (Blackthorn) and Terminalia sericea (Silver cluster-leaf). This
study revised our understanding of the rooting behaviours of savanna shrubs. The study also set a new impetus to
rangeland ecology and management because plant species that develop contrasting root systems in response to
local environmental stimuli have inherent implications to the functioning of the savannas. Shrub encroachment is likely
a manifestation of this implication.
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