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|Title: ||Below ground functioning of tropical biomes|
|Authors: ||Butler, André Joseph|
|Supervisor(s): ||Meir, Patrick|
|Issue Date: ||28-Jun-2011|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Within the field of ecosystem science, substantial progress has been made towards
our knowledge of the factors which shape the global distribution of vegetation.
However, factors which control the biogeography of belowground vegetation
structure and function remain less understood than their aboveground counterpart.
Vegetation types can differ substantially in terms of belowground processes such as
root growth, root turnover, and resulting vertical root distributions. Fine roots
provide an exchange surface, allowing transport of water and nutrients to the leaves.
On the other hand they also represent a significant sink for photosynthetically fixed
carbon to the soil in terms of maintenance and growth. Overall, root processes have a
major influence on fluxes of water, carbon and nutrients within ecosystems.
In this thesis, an electrical impedance method was used to determine the area
of ‘active’ root in contact with the soil for the purpose of absorption. These
measurements were compared to the leaf area of the trees, for the first time allowing
the aboveground and the belowground resource exchange areas of plant to be
contrasted. This approach was first developed to compare the exchange surface areas
of leaves and roots within a Sitka spruce (Picea sitchensis) managed forest, making
measurements in adjacent stands of differing tree density, but identical in age. Stem
density was found to significantly influence the proportion of absorbing root area
relative to leaves. Following the successful test of the method, it was used to
compare the resource exchange areas of eight stands of forest and savanna vegetation
in central Brazil. Across a broad gradient of vegetation structure, the results showed
progressively more investment in fine root area relative to leaf area across the
transition from dense forest to open savanna. However, a contrasting result showed
that the forests had a higher absorbing root area to leaf area ratio than savannas.
Furthermore, these measured ratios were strongly correlated with tree height across
the eight structurally contrasting stands. It appears that absorbing root area index
provides a physiologically meaningful way of characterising belowground water
uptake ability, it is possible that excessive investment in fine root area, relative to
leaf area, may reflect differences in the requirement for nutrient uptake in poor soils.
Complementary to the analysis of root absorbing area, measurements of root
activity and belowground carbon cycling were made by focussing on two of the eight
tropical study sites. Here, the carbon costs of root growth and respiration were
quantified to develop a belowground carbon budget for two structurally contrasting
Brazilian savannas, using soil respiration measurements and a root presence/absence
manipulation experiment. Annual estimates showed that at least 60% of the total
CO2 efflux from the soil was contributed by autotrophic processes, with this value
rising to 80% during the dry season. Seasonal fluctuations of soil respiration were
strongly correlated with soil moisture for both the autotrophic (R2=0.79, pvalue<
0.05) and heterotrophic (R2=0.90, p-value<0.05) components, with maximum
flux rates corresponding with 16.4 and 17.7% soil moisture content respectively.
Furthermore, autotrophic respiration was found to varied with phonological patterns
of fine root growth (R2=0.80, p-value<0.05). It follows that, the way in which
phenological processes respond to a changing climate is of potential importance
within seasonally dry regions. Diurnal fluctuations of heterotrophic CO2 efflux were
correlated with soil temperature (R2=0.74, p-value<0.05), demonstrating a Q10 value
of 1.6 across both sites. In contrast, total soil CO2 efflux was not correlated with
temperature (p-value=0.31), suggesting that autotrophic respiration is predominantly
limited by substrate supply.|
|Sponsor(s): ||Natural Environment Research Council (NERC)|
|Keywords: ||fine root area|
absorbing root areas
fine root turnover
|Appears in Collections:||Geography PhD thesis collection|
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