Browsing by Author "Lewis, Joel Robin"

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    Another look at the savannah conundrum : evidence of niche segregation and competition avoidance with grasses in establishing trees
    (2014) Lewis, Joel Robin; February, Edmund C
    The savannah vegetation, although iconic, is a curiosity due to the coexistence of the two dominant, but remarkably different life-forms: trees and grasses. However, despite an apparent competitive advantage grasses do not dominate over trees in these systems and this presents a conundrum: how are savannah trees are able to establish in the face of such strong competition from grasses? As this is when trees are most susceptible to competition from grasses. This makes the knowledge of establishment phase critical for understanding the tree-grass co-existence. Despite this, there is a paucity of knowledge of this phase and the long-term aspects of the competitive interactions between trees and grasses, where the focus should be on seedling establishment and survival rather than biomass accumulation This study investigated the natural levels of tree establishment amongst differing amounts of grass as we sought to identify the limitation, both aboveground for light and belowground for resources, imposed by grasses on tree establishment. To uncover this limitation we searched for naturally established Acacia nigrescens (the dominant savannah tree species in our research area) seedlings at 12 sites in the Limpopo Province, South Africa. We also collected data on seedling health to determine the effect of proximate grass canopy on their performance. Finally, we compared the root lengths of established seedlings and co-occurring grasses to determine if seedlings were rooted in the same soil horizons as grasses. We hypothesised that tree establishment would be facilitated by low levels of grass biomass, as the lower the grass biomass, the more gaps would be present in the grass layer for seedlings to establish. We found that establishing seedlings were more resilient to grass competition than had previously been appreciated, as we found established seedlings (at a density of 9 seedlings per 100m2) with a grass canopy cover of 65%. We also found no correlation between our measure of grass competition and seedling performance. Finally, we found that even very young tree seedlings consistently root deeper than co-occurring grasses, despite establishing on different geologies, soils and across a rainfall gradient (p < 0.0001) - demonstrating that this resilience to grass competition is due to a specialised rooting strategy present in seedlings that allows them avoid belowground competition with grasses. In light of this, we propose a dynamic two-niche hypothesis of savannah tree-grass co-existence.
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    Functional divergence between Vachellia and Senegalia could underpin differences in invasiveness and Eltonian niche partitioning in African savannas
    (2019) Lewis, Joel Robin; February, Edmund C; Verboom,Tony G
    For several decades Vachellia and Senegalia have been assumed to be ecological equivalents in African savannas. Their supposedly close evolutionary relationship has, however, been totally revised in light of recent molecular phylogenetic data. These data highlight the deep divergence (± 30 Ma) between these genera and also their independent transitions into African savannas. Distant divergence and independent transitions into savannas between Vachellia and Senegalia suggest their possible ecological niche divergence, which could explain the observed differences in invasiveness between them. This thesis provides the first ecophylogenetic study to assess the level of ecological niche divergence between these economically, socioeconomically and ecologically important tree genera in Africa. I address this question by first determining the scale(s) of coexistence between Vachellia and Senegalia and determine their phylogenetic community structure at both the landscape-regional scale and the plot-scale. I then conduct a large glasshouse experiment using seven species from each genus to determine conserved functional differences between them during seedling establishment in the presence, and also the absence, of grass competition. While I find evidence of some Grinnellian niche differentiation between Vachellia and Senegalia at the landscape-regional scale, their greater-than-expected co-occurrence in plots suggests they generally partition an Eltonian niche in savannas. Glasshouse trait data also reveal that Vachellia has a conserved strategy for rapid vertical growth when free from grass competition and Senegalia has a conserved root tissue density response to grass competition. I argue that the rapid growth (bolting) strategy of Vachellia in the absence of grass competition provides a functional explanation for why this genus is observed to be relatively more invasive in grassy ecosystems. I also argue that the root tissue density response of Senegalia enables stable coexistence at the plot scale with Vachellia as it increases competitive ability among grasses leaving establishment less tied to times of low grass biomass. I conclude that this study provides overwhelming support for ecological niche divergence between Vachellia and Senegalia and that this divergence may underpin their observed differences in invasiveness. That the coexistence of these genera is largely attributable to Eltonian niche partitioning also provides fresh support for the contention that tree-tree competition structures tree communities in African savannas. Finally, niche divergence suggests that, when promoting diversity and managing invasion, Vachellia and Senegalia species will require different management practices.