More than 64% of Namibia’s land is occupied by the savanna ecosystems which are vulnerable to climate change and variability. These ecosystems partly consist of forest ecosystem patches which are prime sources of many livelihoods in the country. The effects of climate change are likely to drive the majority of the country’s population to poverty if these resources are not sustainably managed. Therefore an understanding of forest dynamics and their responses to climate is important. Dendrochronology is a study that provides time-series data of climate change and variability, and the responses of trees to such changes. The data are used to reconstruct past events of climate and also to forecast possibilities. Dendrochronological studies have been carried out in Namibia, however, a review of such studies is lacking. This study thus aimed at closing this gap and carried out a literature review on the dendrochronological studies in Namibia. The review showed that the field of dendrochronology is relatively new and has been less applied in Namibia. There is a need for further dendrochronological studies in order to understand how the country can adapt better under the current and forecasted climate regimes. A focus on the marginal forests, encroachers and trees that are of indigenous use, is recommended. In cases of the absence of clear ring formation, which might be the case in arid to hyper arid, the use of staple isotopes is recommended.
Dendrochronology studies have been used by ecologists worldwide to understand the vegetation and environmental conditions of the surrounding, as trees record past conditions. Trees are sensitive to environmental abiotic and biotic factors. Abiotic factors such as temperature (severity of dry seasons) and amount of rainfall are the most important to the development of the vegetation structure [1-4]. Sensitivity to abiotic factors can be used as a sensor or proxy for climate as various tree species respond to climatic variations differently in the formation of their annual tree-rings, especially in areas with distinct seasonality of rainfall [1,5-9]. These features are more pronounced in the Northern hemisphere compared to the Southern hemisphere. However, Southern hemisphere trees also possess growth rings that correlate with rainfall [10,11].
Climatic conditions are characterised by erratic rainfall and recurring drought periods [12-14], reinforcing flood events, periods of drought and the rate at which evaporation might be occurring. Due to dry climate conditions that are so variable, the dense tropical forests, especially the indigenous forest, are becoming rare while bush encroachment is slowly taking over
[15,16]. Namibia is known for its remarkable variety of species and habitats and ecosystems and it is also recognised as a dryland with biodiversity hotspots . However, they are greatly influenced by the variability of the climate regimes, thus making the ecosystems more vulnerable. Forest resources do not only contribute to the national economy, but they are also crucial for reducing poverty in the country. As a result of poor rainfall, there is an increase in poor cereal harvesting and a rise of land degradation has been reported . The dating and reconstruction of long past periods from tree ring patterns to monitor vegetation growth and productivity during extreme climatic periods and specific events such as fire, and to also forecast the possible climate conditions in the country is therefore of utmost importance. Studies have been carried out in Namibia on tree rings and dating, relating to environmental conditions. However, there has not been a review of such studies in Namibia. This review paper therefore aimed at closing this gap by exploring the work done and outlining further dendrochronological opportunities in Namibia.
The study of tree annual growth rings by dating the ring formation to specific years is known as dendrochronology.
Several terminologies have stemmed from the concept of
dendrochronology, depending on the issues to be studied. This
includes terms such as dendroclimatology for particularly for
looking into past climate variability [17,18], dendroecology which
is the broader science field for drawing environmental information
from tree rings such as fires and insect outbreaks [19,20],
dendrogeomorphology which is a subfield of dendroecology and
focuses on geomorphological processes such as flooding, rock fall
activities or landslides, and river related vegetation processes
[21,22], and dendroarcheology which focuses on dating wooden
material from archaeological sites [23,24]. Decdrochronology has
also been used for studying snow avalanches, volcanoes eruptions
and earthquakes .
The use of dendrochronological studies to understand various
past environmental aspects is common and has been used for many
decades around the world. According to Worbes , Hartig 
has used it to look into the periodic wood formations in temperate
regions, whereas Brandis  focused on sustainable silvicultural
systems, and Coster  analysed the anatomical description of
the woods . The basic aspects, the applications and progress
in technical developments have been explained and documented
in various reviews (Rosendaal & Zuidema 2011; Scweingruber
1988) [26,30-32]. In tropical regions of Africa, an increasing
number of such studies have been carried by several scholars [2,9-
11,33-35]. Despite the early arguments about the lack of distinct
annual rings due to the absence of low temperatures and the
presence of dormant winter seasons faced by trees in Southern
Africa, particularly in tropical regions. Steenkamp, Vogel, Fuls, van
Rooyen and van Rooyen  analysed some species and showed
that trees are responsive to seasonal climate variations and this
lead to the formation of annual growth tree rings.
Namibia is located in the South West part of Africa and occupies
an areas of 825 418km. The perennial rivers are only located
at the borders, with Orange River at the South, Kunene at the
North West while Kavango river and Zambezi river are the North-
Eastern part of the country. The interior of the country consists
of only non-perennial rivers. The average annual temperatures
are observed to range from 16˚C at the coast to 20˚C and 22˚C in
the interior of the country (see Figure 1a) . The maximum
temperatures experienced are over 34˚C and the minimum in the
coldest months is less than 2˚C . The average annual rainfall
varies from the coast with less than 250mm to an average of 600m
in the areas with the most rainfall (Figure 1b). Of the total rainfall
that the country receives, it is estimated that 83% of the rainfall
evaporates, 14% ends up used by the vegetation, 2% goes to runoff
and 1% recharges the groundwater [15,37]. Despite the fact that
so much less rainfall contributes to groundwater recharge, around
80% of the country depends on the groundwater system.
In the last country census , the Namibian population was
estimated to be 2.1 million, and the current population estimation
is 2.5 million, making the country one of the least densely
populated in the world, with 3.13 people per square kilometre
. Traditional subsistence sectors are responsible for providing
for the livelihoods, with main activities including mining, tourism,
fishing and agriculture and at least 70% of the population depends
on agricultural activities [40,41].
Arid to semiarid areas occupy at least 44% of the world land
and these are responsible for nearly 38% of the global population
[42-44]. These regions are climatically stressed by the limited
rainfall, high temperatures and long dry seasons . According
to New , the mean temperature has increased by 0.25 degrees
per decade since 1960 in Southern African semi-arid areas.
Moreover, savanna trees, shrubs and grasslands cover most of
the arid and semi-arid regions worldwide. Due to the sensitivity of vegetation growth to timing and variations in the local climate
systems in Southern African tropical regions, savanna ecosystem
functioning is greatly affected and in some parts of the region, the
resultant changes are linked to decreased ecosystem productivity,
thus land degradation [10,46,47].
At least 20% of Namibia is occupied by a desert region, 33%
is arid, 37% is semiarid and 8% is covered by a sub-humid region
(see Figure 2a) [48,49]. In Namibia, savannah trees and shrubs
which are dominated by Acacia vegetation occupy up to 64% of
the land; savanna broadleaved dry woodlands and forests cover
20%, and the desert region makes up 16% (Figure 2b) [50,51].
Forests around the country indicate that rare dense forests are
generally scarce and they are described as dry, semi open to
woodlands and these are largely located mostly in the deep Aeolian
Kalahari sands (i.e. north-central-eastern part of the country)
. This is in spite of the fact that forest resources are used as
a means to reduce poverty in communities around the country.
The methodology employed in this study is a literature
review. First, the study provided a background and a time line
of dendrochronology. Furthermore, the study presented the
climatic, vegetation and population on the semi-arid savanna
ecosystems as a way to provide a Namibian context. In the results
section, the first part presents a summary of dendrochronological
researches carried out in Namibia in relation to their methods,
species analysed, regions covered and their findings in terms of
the tree ring observables. The second part presents the forecasted
climatic regimes to show the importance of dendrochronology
studies in the country. This is important as the majority of the
population depends on the vulnerable ecosystems, particularly on
agricultural resources. Climate change affects natural resources,
and this is projected to worsen in the future, especially in semiarid
to arid regions . The discussion drew from a context of climate
change, the application of dendrochronology in Africa, and the
need to focus on specific issues that are outlined to be of great
concern in Namibia.
A search for studies in dendrochronology yield eight studies
that are carried out in Namibia (Table 1). The studies were mostly
carried out from the central-northern Namibia, covering mostly
a rainfall gradient from the Kunene, Oshikoto, Kavango West and
East, Zambezi and Otjozondjupa regions. Figure 3 shows how the
ring rings are identified in one of the study. From these studies,
sixteen (16) species of trees were examined and distinctive tree
rings were successfully identified age determination was possible.
These indicate that the species are responsive to climatic seasons.
Using the staple-carbon isotope methodology, the trees showed
responsiveness to inter-annual variability as two samples showed
the same patterns (Figure 4). Climatic stress factors (wet and
dry) induce the cambial dormancy and promote growth in woods
. This results in observable anatomical changes as the trees
shed leaves and produce new leaves [63,64]. Large vessel zones
are produced during the wet periods where water availability is
in abundance, while the small vessel layers are defined by dry
seasons due to the cambial activities being reduced under low
water availability [64,65]. In addition to age determination, and
analyses of growth rings responsiveness to precipitation, the
latest study of tree rings in Namibia  also developed a master
chronology for Dichrostachys cinerea and Senegalia (Acacia)
mellifera, forming a basis for future dendrochronological studies
of the two species in Namibia.
New  carried out a study on the current state and
modelled future climate conditions. Rainfall variability is evident
within the last decades, with rainfall varying from 40% below
and 70% above the average (Figure 5). Their results further
showed faster increases in temperature over semi-arid areas in the future, indicating a warming rate of between 0.32 and 0.38
degrees per decade up to 2050 (Figure 6a) and projected trends
ranging from 35 to 40% and higher than the warming for the
wider Southern Africa region, which suggests that accelerated
warming will become worse in the future, depending on future
greenhouse gasses emissions. Their climate models also showed
reduced total rainfall in the future; however, 25% of the model
showed increased rainfall, which signifies rainfall variability in the
future (Figure 6b). With specific reference to Namibia, rainfall is
expected to decline with 10% in the southern part of the country
and with 15% in the central regions . For every 1% change in
the rainfall, it is estimated that there will be 1.2% -1.6% changes
in the carrying capacity, and around 1.3% changes in revenue from
livestock farming . Reduction on carrying capacity does not
only affect the livestock, it also affects wildlife, tourism activities,
and therefore livelihoods that directly and indirectly depend on
it, and as well as the country’s economy at large. A reduction on
the rainfall also means less harvests from traditional subsistence
farming which supports around 70% of the country’s population.
According to Gebrekirstos et al. , the application of
dendrochronology in Africa is mostly for the reconstruction
of climate change history, extreme events, geomorphological
related events, and this can also be used to assess the feedback
systems, teleconnections, impacts on growth and resilience of
species (Figure 7). This in the end gives quantified changes that
can be used to make decisions on adaptations, mitigations and
sustainable management of the resources.
The above outline application of dendrochronology is
significant to Namibia. This is because climate change, global
warming and rainfall variability, especially when it comes to
the reduction of rainfall, have great impacts on the Namibian
vulnerable recourses. First, the high temperatures promote more
aridity conditions; the variability either leads to flooding events or
drought periods. In addition, the various resources (water such as
the groundwater system, water from surface flow; agriculture and
land use including settlements areas) are significantly vulnerable
to these conditions. The ecosystems are highly marginal and any
relatively small changes are likely to lead to the systems to go
beyond the limit point of viability, may may lead to a possibility of
Namibian forests are rare and found largely along riverbanks
and mostly on remote locations. These forests support the majority
of livelihoods in the country, especially in rural communities. They
are also a way of “healing the dryland and protecting them from
desertification and droughts” . The use of dendrochronology
to understand their responses to climate variability Could yield
evidence based that can be used to understand their adaptation
strategies and resilience mechanisms. Based on Table 1 species
such as Pterocarpus anglensis and Terminelia sericea are more
responsive to climate signals in Oshikoto region  and in
Zambezi region [62,64], respectively. However, the ring growth
were much better in Oshikoto region than in Zambezi region. It is
less understandable for the former species to have better growth
ring in Oshikoto region with less rainfall than in the Zambezi
region with high rainfall amount and as temperatures are more
or less the same in the two regions. Therefore, other factors such
as soil and root structures have to be understood in studying
dendrochronology, to be able to set up proper sustainable
strategies in areas of concern.
Acacia mellifera (Senegalia mellifera) and Dichrostachys
cinerea are the most aggressive encroachers and are furthermost
widely distributed encroachers in rangelands in Namibia [66-68].
They are responsible for nearly 40% of the encroached lands in
rangelands and at least 15 million hectares in communal lands
[69-73]. Increased temperature favours the encroachers such as
A. mellifera due to their tap roots system that can tap onto deep
moisture during drought period [74,75] and the increased carbon
dioxide concentration in the atmosphere also favours them due
to their C3- photosynthetic systems that fixed carbon in elevated
CO2 conditions [76,77]. That results in less loss of energy through
photorespiration, therefore fast growth and recuperation from
damages [78,79]. The impacts of encroachers are greatly affecting
savanna ecosystems, biodiversity and groundwater systems, and
even contributing to desertification [66,80,81]. Based on Table 1,
only two dendrochronology study has focused on the aggressive
encroachers through analysing the growth rings and showed
that they are responsive to precipitation and could be used for
climatic signals. Studies using these species are therefore needed
to reconstruct long period of climatic conditions.
A literature review on dendrochronology studies shows that
the field of dendrochronology is relatively new and less applied
in Namibia. This is in spite of the country being one of the most
arid in the Southern Africa region, which makes the ecosystems
most highly vulnerable the effect of climate change. This is
worrisome when considering that such effects eventually impact
livelihoods due to their high dependence on agriculture resources.
The need for dendrochronology studies is therefore critical to
understand the long, past and possible future environmental
dynamics relating to the various resource the country depends
on. Time series data analysis is necessary to understand their
adaptation systems in relation to the climatic conditions, and
understand forest ecosystems. The study recommends a focus on
the marginal forest resources, further researches on encroachers
to possibly save the drylands and trees that of indigenous use in
Namibia.. There is high potential to expand the understanding on
the species used by past studies as many of them were limited to a
few areas and regions, and to a few (16) species. Vegetation need
to be understood, especially in arid and hyper-arid areas such as
around and in urbanised landscapes for sustainable management
and planning of urban forests; along river banks (especially the
ephemeral rivers as they support at least one-fifth of the Namibian
population); in conservancies and other human- wildlife
environments and lastly in areas of agricultural importance and
communal lands where indigenous trees are vital to livelihoods.
Clear ring formation growth ring determination might be difficult
in the arid to hyper-arid environments, therefore the use of staple
isotopes from cellulose to reconstruct past climate regimes, is
recommended. Another method is the use of xylem anatomical
features. This method focuses on functioning, growth and climaticgrowth
interactions, therefore providing information on most tree
resistant to the impacts of climate change.
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