A broad corpus of previous research has sought to understand the role of
biodiversity as a driver of ecosystem structure and function. Although theory
suggests that increased biodiversity should increase ecosystem function by
niche complementarity among co-existing species, in natural systems wide
variation in the biodiversity effect exists among vegetation types and along
environmental gradients. In southern African woodlands and savannas, which
experience disturbance by fire and herbivory, drought and extreme temperatures,
it is unclear whether positive biodiversity effects should occur. In this thesis,
I explore the ecology of southern African woodlands through the lens of the
biodiversity-ecosystem function relationship, to improve our understanding of
the role of tree diversity as a mediator of ecosystem function, its interactions
with abiotic environment, and its effect on woodland structure.
In temperate and wet tropical forests, where the majority of biodiversity-ecosystem function studies in natural woody vegetation have been conducted,
the positive effect of niche complementarity hinges on the condition that conspecific competition is the limiting factor to ecosystem function. In highly disturbed
and environmentally stressed systems however, this may not hold true. I conducted a regional study investigating the role of tree species diversity and structural
diversity as mediators of woody biomass, using a plot network of 1235 plots
spanning wide climatic and biogeographic gradients across southern Africa.
Using Structural Equation Modelling, I determined that tree species diversity
has a positive effect on biomass, operating mostly via its effect on structural
diversity. I found that biodiversity itself increases with water availability, and that
positive biodiversity effects only arise under sufficiently high stem density.
To further understand the ecological mechanisms which drive positive
biodiversity-productivity relationships, I explored the effects of tree species
diversity and woodland demographic structure on patterns of land-surface phenology. I combined a dense plot-based tree census dataset across multiple
deciduous Zambian woodland types with remotely sensed measures of greenness, to understand drivers of variation in pre-rain green-up, growing season
length and productivity. I found that pre-rain green-up occurred earlier in more
diverse sites, across all woodland types, while in non-miombo woodlands, species richness also increased post-rain senescence lag and season length. I also
found that large-sized trees increase the degree of both pre-rain green-up and
post-rain senescence lag, across vegetation types, with an effect size similar to
that of species richness.
Southern African woodlands occur as a complex mosaic of open grassy
patches and closed canopy forest-like patches, driven by positive feedbacks of
fire-induced tree mortality and grass growth, but the biotic mechanisms causing
variation in canopy closure are unclear. I used terrestrial LiDAR at two sites, in
Tanzania and Angola, to understand at fine spatial scale the effects of species
composition and diversity on canopy architecture and canopy cover. Species
diversity was found to allow increased spatial clumping of trees, which drove
vertical canopy layer diversity and canopy height, demonstrating an indirect role
of species diversity on canopy cover via stand structure. Taken together with
the regional study of the biodiversity-ecosystem function relationship, these
findings suggest a nuanced role of tree species diversity on ecosystem function,
operating primarily via its effect on canopy structural diversity in southern
African woodlands. I propose that higher diversity communities are more likely
to produce forest-like closed canopy woodlands, with a higher upper limit on
biomass, and are more likely to transition from savanna to closed canopy forest
under conditions of atmospheric CO2 enrichment.
Finally, in an effort to increase our understanding of the variation in diversity
and structure of woodlands across southern Africa, I conducted a study of tree
species biodiversity and woodland structure in Bicuar National Park, southwest
Angola, with comparison to other woodlands around the miombo ecoregion.
Much of the published plot data and woodland monitoring infrastructure in
miombo woodlands is located in central and eastern regions of southern Africa,
while woodlands in the west of the region, which occur entirely within Angola,
remain poorly represented. I found that Bicuar National Park constitutes an
important woodland refuge at the transition between dry miombo woodland
and Baikiaea-Baphia woodlands. I recorded 27 tree species not recorded
elsewhere in the miombo ecoregion outside the Huíla plateau. An additional
study of one-off plots in areas previously disturbed by shifting cultivation, found
that this disturbance increases tree species diversity, but ultimately reduces
woody biomass, even after a period of regeneration, potentially representing a
directional shift to a different stable vegetation type.
Together, the findings of this thesis demonstrate multiple relationships among
tree biodiversity, ecosystem structure, and ecosystem function, measured
primarily through woody biomass and productivity, at multiple spatial scales.
I conclude that incorporation of diversity and canopy structural information
into earth system models, by scaling up plot data using cutting edge remotely
sensed datasets, could improve predictions of how climate change and biodiversity change will impact the functioning of different vegetation types across
southern Africa, with consequences for carbon cycle modelling, conservation
management, and ecosystem service provision. Finally, I suggest that biodiversity loss of large archetypal miombo tree species will have the greatest
impact on a number of ecosystem functions related to carbon cycling, raising
concerns over the impacts of selective logging of these species
