Ounongo Repository
The Ounongo Repository (OR) is the institutional repository of Namibia University of Science and Technology. Ounongo means "knowledge. in the Oshiwambo and Otjiherero languages. The OR is administered by the Library, with technical assistance from DICT, and its aim is to collect, organize, manage, store, preserve, publish and make accessible worldwide, the knowledge assets or intellectual output of the University's researchers, staff and post-graduate students. Users may set up RSS feeds to be alerted to new content.
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Recent Submissions
Links across ecological scales: Plant biomass responses to elevated CO2
(2022) Maschler, Julia., Bialic-Murphy, Lalasia., Wan, Joe., Andresen, Louise C., Zohner, Constantin M., Reich, Peter B., Lüscher, Andreas., Schneider, Manuel K., Müller, Christoph., Moser, Gerald., Dukes, Jeffrey S., Schmidt, Inger Kappel., Bilton, Mark C., Zhu, Kai., Crowther, Thomas W.
The degree to which elevated CO2 concentrations (e[CO2]) increase the amount of
carbon (C) assimilated by vegetation plays a key role in climate change. However, due
to the short-term
nature of CO2 enrichment experiments and the lack of reconciliation
between different ecological scales, the effect of e[CO2] on plant biomass stocks
remains a major uncertainty in future climate projections. Here, we review the effect
of e[CO2] on plant biomass across multiple levels of ecological organization, scaling
from physiological responses to changes in population-,
community-,
ecosystem-,
and global-scale
dynamics. We find that evidence for a sustained biomass response
to e[CO2] varies across ecological scales, leading to diverging conclusions about the
responses of individuals, populations, communities, and ecosystems. While the distinct
focus of every scale reveals new mechanisms driving biomass accumulation under
e[CO2], none of them provides a full picture of all relevant processes. For example, while physiological evidence suggests a possible long-term
basis for increased biomass
accumulation under e[CO2] through sustained photosynthetic stimulation, population-scale
evidence indicates that a possible e[CO2]-induced
increase in mortality rates
might potentially outweigh the effect of increases in plant growth rates on biomass
levels. Evidence at the global scale may indicate that e[CO2] has contributed to
increased biomass cover over recent decades, but due to the difficulty to disentangle
the effect of e[CO2] from a variety of climatic and land-use-
related
drivers of plant
biomass stocks, it remains unclear whether nutrient limitations or other ecological
mechanisms operating at finer scales will dampen the e[CO2] effect over time. By
exploring these discrepancies, we identify key research gaps in our understanding of
the effect of e[CO2] on plant biomass and highlight the need to integrate knowledge
across scales of ecological organization so that large-scale
modeling can represent the
finer-scale
mechanisms needed to constrain our understanding of future terrestrial C
storage.
Rethinking the Plant Economics Spectrum for Annuals: A Multi-Species Study
(Frontiers, 2021) Kurze, Susanne., Engelbrecht, Bettina M. J., Bilton, Mark C., Tielbörger, Katja & Álvarez-Cansino, Leonor.
The plant economics spectrum hypothesizes a correlation among resource-use related traits along one single axis, which determines species’ growth rates and their ecological filtering along resource gradients. This concept has been mostly investigated and shown in perennial species, but has rarely been tested in annual species. Annuals evade unfavorable seasons as seeds and thus may underlie different constraints, with consequences for interspecific trait-trait, trait-growth, and trait-environment relations. To test the hypotheses of the plant economics spectrum in annual species, we measured twelve resource-use related leaf and root traits in 30 winter annuals from Israel under controlled conditions. Traits and their coordinations were related to species’ growth rates (for 19 species) and their distribution along a steep rainfall gradient. Contrary to the hypotheses of the plant economics spectrum, in the investigated annuals traits were correlated along two independent axes, one of structural traits and one of carbon gain traits. Consequently, species’ growth rates were related to carbon gain traits, but independent from structural traits. Species’ distribution along the rainfall gradient was unexpectedly neither associated with species’ scores along the axes of carbon gain or structural traits nor with growth rate. Nevertheless, root traits were related with species’ distribution, indicating that they are relevant for species’ filtering along rainfall gradients in winter annuals. Overall, our results showed that the functional constraints hypothesized by the plant economics spectrum do not apply to winter annuals, leading to unexpected trait-growth and trait-rainfall relations. Our study thus cautions to generalize trait-based concepts and findings between life-history strategies. To predict responses to global change, trait-based concepts should be explicitly tested for different species groups.
Evaluating grazing response strategies in winter annuals: A multi-trait approach
(Wiley, 2021) Kurze, Susanne., Bilton, Mark C., Álvarez-Cansino, Leonor., Bangerter, Sara., Prasse, Rüdiger., Tielbörger, Katja., Engelbrecht, Bettina M. J.
Plants minimize fitness losses through grazing by three fundamental strategies:
tolerance, avoidance and escape. Annual species have been traditionally assumed
to escape grazing through their short life cycle and seed dormancy; however, their
grazing response strategies remain almost unexplored. How traits and their coordination
affect species' grazing responses, and whether the generalized grazing
model, which posits convergent filtering by grazing and drought, is applicable to
this ecologically and economically important species group thus remain unclear.
2. We used a trait-based
approach to evaluate grazing response strategies of winter
annuals from the Middle East. Across 23 species, we examined the coordination
of 16 traits hypothesized to be relevant for grazing responses, and linked them to
species' fecundity responses, as proxy for fitness responses, to simulated grazing
in controlled conditions, to species' abundance responses to grazing in the field
and to species' distribution along a large-scale
rainfall gradient.
3. Winter annuals exhibited both grazing escape and to a lesser extent tolerance
indicated by (a) independent coordination of escape and tolerance traits, and (b)
maintenance of higher fecundity in species with more pronounced escape or tolerance
traits under simulated grazing. In the natural habitat, species with a more
pronounced escape but not tolerance strategy maintained higher abundance
under grazing in dry habitats, indicating convergent favouring of escape by both
grazing and drought. However, this finding at the local scale was not mirrored by
a strategy shift along a large-scale
rainfall gradient.
4. Synthesis. The convergent favouring of escape traits by grazing and drought in
annuals is consistent with the generalized grazing model. This model, which has
been developed for perennials based on the avoidance strategy, can thus be extended
to annuals based on escape, a finding that should facilitate projecting consequences
of global change in drylands dominated by annuals.
Understanding Rangeland Desertification at the Village Level: A Comparative Study with a Social-Ecological Systems Perspective in Namibia
(Springer, 2025) Schwieger, Diego Augusto Menestrey., Munyebvu – Chambara, Faith., Hamunyela, Ndamonenghenda., Tielbörger, Katja., Nesongano, Wellencia C., Bilton, Mark C., Bollig, Michael., Linstädter, Anja.
Desertification poses significant environmental and socio-economic threats to pastoral systems within the drylands of
sub-Saharan Africa. However, there remains a paucity of interdisciplinary studies delving into the anthropogenic drivers
of desertification at the local level of social-ecological systems, resulting in an inadequate understanding of its human-
induced causes. This research aims to bridge this gap by presenting three case studies from Namibia’s eastern communal
areas. Through an integrated approach drawing from rangeland ecology and anthropology, we offer a comparative analysis
revealing nuanced differences among individual pastoral settlements, shaped by their distinct social contexts. Our findings
elucidate the social determinants influencing varying degrees of desertification at the village level, highlighting local fac-
tors that mitigate the adverse impacts of grazing pressure and aridity on perennial grass populations. Notably, the study
identifies the role of social institutions in managing critical environmental conditions and physical infrastructures, such
as extensive pastures and cattle posts, which contribute to maintaining grassland resilience. Despite observable signs of
desertification, the presence of perennial grasses both aboveground and in the soil seed bank across all settlements suggests
that a tipping point has not yet been reached, emphasizing the window of opportunity for intervention. The discussion
extends to the potential transferability of these findings to other Namibian communities within the existing socio-ecolog-
ical framework, aiming to avert impending tipping points. Ultimately, the study challenges the notion of desertification
in pastoral social-ecological systems as solely a tragedy of the commons, emphasizing the imperative of developing and
implementing suitable social institutions within colonial and post-colonial contexts.
Winter annuals not only escape but also withstand winter droughts: Results from a multi-trait, multi-species approach
(Elsevier, 2025) Kurze, Susanne., Engelbrecht, Bettina M.J., Bilton, Mark C., Tielb¨orger, Katja., ´Alvarez-Cansino, Leonor.
Winter annual plants are a dominant life form in drylands. They evade seasonal drought through their life
history, but are also exposed to drought within their growing season. Across species, winter annuals differ in
traits allowing them to reproduce before a drought occurs (drought escape) as well as in traits minimizing tissue
dehydration (drought avoidance) and/or maintaining functioning under drought (drought tolerance). It is yet
uncertain how these traits are coordinated and influence winter annuals’ performance responses to drought
within the growing season and their distribution along rainfall gradients. Understanding these mechanisms is
crucial to predict global change impacts in drylands. We measured 22 traits hypothesized to influence wholeplant
performance responses to drought in 29 winter annuals common in the Eastern Mediterranean Basin.
We examined trait syndromes and linked species’ strengths of these trait syndromes with their fecundity responses
to an experimental within-season drought, their maximum growth rates (in 18 species), and their distribution
along a rainfall gradient. Four trait syndromes emerged: Two were largely consistent with drought
avoidance and tolerance, while the other two consisted of traits considered to confer drought escape. Both escape
syndromes were differently associated with plant size and therefore referred to as small and tall escape syndrome.
Species with a pronounced small escape syndrome showed, albeit weakly, higher fecundity losses under
experimental drought. Both species with a pronounced avoidance or tall escape syndrome exhibited higher
growth rates, but only annuals with pronounced avoidance traits tended to occur in moister conditions. Our
findings highlight that winter annuals, despite their common life history, exhibit several trait syndromes
conferring them similar ability to cope with drought in the growing season. Consequently, increasing withinseason
drought with global change may hardly affect community composition of winter annuals.