Nutrients in an African Savanna: the consequences of supply heterogeneity for plants and animals

C. van der Waal

Research output: Thesisinternal PhD, WU

Abstract

In savannas, trees and grasses co-exist and share resources such as water and
nutrients. The ratio between the tree and grass components (i.e., vegetation structure)
importantly controls productivity, animal assemblages and earth-atmosphere
feedbacks. As the structure of savanna vegetation is inherently unstable and easily
disturbed, finding out how the vegetation structure is controlled is of great importance
for rangeland management and conservation. Currently four factors are believed to
control the vegetation structure in savannas; namely, water, nutrients, herbivory and
fire. While the water and fire factors have been intensely studied, the roles of nutrients
and herbivores as factors are less well known. Improving our understanding of the
role of nutrients in savannas is relevant, because it is increasingly realised that global
change also alter the availability of nutrients, e.g., increased nitrogen deposition. How
savanna systems respond to nutrient perturbations is uncertain. Changes in soil
nutrient availability are also likely to feed back to changes in forage quality, which
may influence large herbivore use and vegetation impact patterns, thus indirectly
influencing vegetation structure. Moreover, it is increasingly realised that not only
changes in the availability of nutrients influence plants and subsequently herbivores,
but also how nutrients become spatially available. In fact, new ideas suggest that
differences in the scale of spatial resource heterogeneity may control how resources
are partitioned between co-existing species differing in size, e.g., large trees coexisting
with small grasses in savanna systems. To test how changes in nutrient
availability and spatial configurations influence savanna systems, several field
experiments were conducted in a semi-arid savanna in South Africa.
In the study area we found that nutrient (N, P and K) availability negatively
affected tree (Colophospermum mopane) seedling establishment in fertilization
experiments. Also, trees failed to re-colonize nutrient-rich kraal sites that were
abandoned almost half a century ago. In dry savannas, it is currently believed that the
success of tree seedling establishment exercises large control over the relative
dominance of trees, thus an increase in nutrient availability may feed back to a
structurally more open vegetation state. Different explanations may account for
constrained tree seedling establishment under fertile soil conditions. We tested the
hypothesis that the intensification of grass competition reduces tree seedling
recruitment in fertile environments. In controlled competition experiments it was
shown that negative nutrient effects on tree seedlings only occur when seedlings were
competing with grasses in mixtures. Furthermore, we found that adding both water
and nutrients to tree seedling-grass plant mixtures cancelled the negative effects of
added nutrients on tree seedlings. Thus the suppressing effect of increased nutrient
availability on seedlings appears to operate indirectly through the pre-empting of soil
water resources by vigorous herbaceous growth under fertile conditions. Since woody
seedlings are vulnerable to water stress, increased nutrient-induced water stress
translates into higher mortality rates and suppressed growth of tree seedlings in fertile
areas. In support, transplanted tree seedling mortality increased during a mid-season
drought as local fertilizer concentration increased. In conclusion, intensified
herbaceous competition under fertile soil conditions appears to be a viable
mechanism explaining poor tree seedling recruitment under fertile soil conditions.
Savannas and nutrients
While establishing tree seedlings suffer under fertile conditions, our data
suggest that established (mature) tree growth benefitted from an increase in nutrient
availability, especially following an increase in N availability. With regards to
increased atmospheric N deposition, we predict that tree cover may initially expand
following nutrient enrichment in dry savannas, although tree cover responses may be
insensitive to current levels of N deposition. However, in the long-term tree cover is
expected to decline, because of constrained tree recruitment that appears to be more
sensitive to small increases in N availability.
The relative availability of nutrients such as N and P may also influence how
resources are partitioned between co-existing trees and grasses. In a field experiment
we found evidence that trees were relatively more limited by N than P availability. An
East African study has shown that grasses underneath trees are more P than N limited
and we found evidence that the competiveness of sub-canopy grasses in our study
increased when only P was supplied. The relative availability of N vs. P may therefore
offer an additional axis governing resource partitioning between trees (non N-fixing)
and grasses in savanna systems. This supports the idea that organisms with a high
growth potential, apparently grasses in savannas, have relatively high P requirements
to sustain rapid protein synthesis, while slower growing organisms such as trees are
more limited by the availability of N.
The availability of soil nutrients strongly mediated where large herbivores
concentrated their impact in the landscape. Both grazers and browsers responded
positively to fertilization, apparently via the control that soil nutrient availability has
on forage quality. Tree and grass leaf N and P concentrations increased and
condensed tannin concentrations in trees decreased following fertilization. Under high
local soil nutrient concentrations the vegetation biomass was in some instances
reduced below control biomass by herbivores, indicating that top-down herbivore
effects potentially override bottom-up nutrient effects under fertile conditions.
In ecology, it is increasingly realized that it is not only the availability of
nutrients, but also how nutrients become spatially available that matters. Data from a
large field experiment where a gradient in the scale of nutrient patchiness (i.e., patch
grains sizes 2 x 2 m, 10 x 10 m or 50 x 50 m) was created, suggested that the scale of
nutrient patchiness controlled the partitioning of resources between co-existing trees
and grasses. For the same local fertilizer concentration, tree leaf quality was
unresponsive to fertilization in small patches, but responded in the larger patches.
Grass leaf quality increased with local fertilizer concentration regardless of patch size.
The differential responses of trees and grasses to scale differences subsequently
modulated the responses of the browser and grazer guilds. For the same high local
fertilizer concentration, grazers responded to both fine and coarse scales of nutrient
patchiness, while browsers responded only to the coarse scale of nutrient patchiness.
In turn, the selective grazing in the fine scale of nutrient patchiness treatment,
apparently stimulated tree growth. In the coarser scale of patchiness treatment both
browsers and grazer impact intensified locally. Thus the scale of nutrient patchiness
controlled nutrient partitioning between trees and grasses, which was apparently
closely tracked by the large herbivore assemblage, resulting in differential local
impacts on the tree and grass layers. Apart from local effects, the scale of nutrient
patchiness may also regulate the use and quality of forage resources at larger (e.g.,
landscape) scales. In the large field fertilizer experiment, calculations suggest that the
total herbaceous off-take by grazers peaked where the same fertilizer amount (15 kg N
plot-1) was spread over the whole plot surface area rather than concentrated in 10 x 10
m patches. Thus, how nutrients are distributed in an area controls secondary
productivity and where herbivores concentrate their impact.
The scale related patterns observed in the large fertilizer experiment may have
been reinforced by plastic responses (e.g., fine root proliferation in nutrient-rich soil)
to heterogeneous nutrient supplies of trees in the coarse scale treatments and grasses
in the fine scale treatments, respectively. To test this, the same nutrient amount was
supplied at two different scales of patchiness to focal trees with their associated
grasses. Two years after fertilization, large-scale fertilized trees showed increased
shoot growth and increased leaf N concentrations compared to small-scale fertilized
trees receiving the same N amount. Conversely, trees in a small-scale configuration
fertilized with P showed negative responses compared to large-scale counterparts.
These results suggest that differences in the local scale of nutrient patchiness also
influence how nutrients are partitioned between co-existing trees and grasses.
Herbivores not only respond to nutrient heterogeneity, but may also create
spatial heterogeneity in nutrient availability, which, in turn, may influence the
vegetation structure of savannas. We studied the current soil nutrient status, tree and
grass biomass patterns and large herbivore use of nine former livestock holding pen
areas (kraals) in a semi-arid, nutrient poor savanna. These were contrasted with
nearby control sites located in the surrounding landscape. The kraals, formerly
enriched by livestock dung and urine, were abandoned around 1970 and since then
wildlife replaced livestock in these parts. We found that around 40 years later, kraal
soil had elevated concentrations of inorganic N, extractable P, K, Ca and Mg
compared to control sites, which resulted in high quality forage in kraal sites. Trees
also failed to invade these sites, thus kraals remained as structurally open patches in
the otherwise dense savanna. Evidently, wild large herbivores maintain the high
nutrient status of kraal sites, probably by importing nutrients into these sites and by
accelerating local nutrient cycling. In turn, the increased local nutrient availability
prevents tree seedlings from establishing under these fertile conditions.
Finally, this study provided evidence that changes in the availability of
nutrients influenced the success of woody seedling establishment, which may feed
back to changes in the relative proportions of trees and grasses in dry savannas.
Furthermore, this study supports the new idea that the scale of resource heterogeneity
influences how resources are partitioned between co-existing trees and grasses, which,
in turn, modulated browser vs. grazer use and impact patterns on the vegetation.
In conclusion, this study provides new information on nutrient-plant-herbivore
interactions in a dry savanna with potentially important implications for the
management of dry savannas in general.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Prins, Herbert, Promotor
  • de Kroon, H., Promotor, External person
  • Heitkonig, Ignas, Co-promotor
Award date27 Sep 2010
Place of Publication[S.l.
Publisher
Print ISBNs9789085856740
Publication statusPublished - 2010

Keywords

  • nutrients
  • soil fertility
  • heterogeneity
  • savannas
  • trees
  • grasses
  • biological competition
  • herbivores
  • grazing
  • browsing
  • foraging
  • vegetation
  • semiarid zones
  • scaling
  • nutrient availability
  • south africa

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