The research contained in this thesis was completed by the candidate while based in the School of Life Sciences, of the College of Agriculture, Engineering and Science. The thesis is written as papers and follows the format of the journal where each paper has been/will be submitted for publication, except for the Introduction (Chapter 1) and Conclusions (Chapter 6).
PLAGIARISM
PUBLICATIONS
We investigated the effects of manipulated water availability and passive heating on the productivity of the common grass species Eragrostis curvula and the emergence and success of seedlings of the woody species Acacia sieberiana. The putatively beneficial effects of disturbance on litter and plant species richness were more pronounced in areas with high rainfall.
Introduction
Rationale for the research
Primary areas of research in savanna ecology are the mechanisms of coexistence between two contrasting life forms (grasses and trees), and the factors that influence the relative abundance of each life form (Accatino et al. 2010). Anthropogenic activities, mainly the burning of fossil fuels for energy, transport and industrial activities, have increased global emissions of carbon dioxide, methane and nitrogen dioxide (Wenig et al. 2003).
Woody plant encroachment and future climate change
In grasslands and savannas, precipitation events are usually patchy, both on spatial and temporal scales (Wiegand et al. 2006). Warmer temperatures increase the rate of virtually all chemical and biochemical processes in plants and soil (Saxe et al. 2001).
Woody plant growth and performance: Land-use drivers of encroachment
Furthermore, fire is known to cause large amounts of soil nutrient loss (especially N loss) to the atmosphere through the process of volatilization (Aranibar et al. 2003). Through this association, the legume receives otherwise unavailable biologically fixed N2 from the soil bacteria (Liu et al. 2010).
Aims and objectives of the study
For example, if N fixation carries a high carbon cost, low fixation rates can be expected in scenarios with low resource availability, or under intense intraspecific competition (Kambatuku et al. 2013). Therefore, while reproductive fitness is the most important measure of performance (Goldberg et al. 1999), it is not always possible to measure a metric such as fecundity in savanna trees.
Study sites
Therefore, for the purpose of this thesis, we used physiological traits as performance indices (Lavorel et al. 2007; Pérez-Harguindeguy et al. 2013).
Thesis structure
Here, we found that increased nutrient availability is only significant in high-rainfall areas, likely due to water scarcity in low-rainfall areas. The overall putatively beneficial effects of litter intervention and plant species richness were more pronounced in high rainfall sites. All chapters in the thesis, except for the introduction (Chapter 1) and conclusions (Chapter 6), are designed according to the style of each target journal, as indicated on the title page of that chapter.
The effects of fire on invasion of woody plants are exacerbated by the succession of trees with reduced palatability. The effects of seed intake by livestock, manure fertilization, trampling, grass competition and fire on seedling establishment of two woody plant species.
Drought suppresses grass productivity and warming promotes tree performance
Abstract
Introduction
Methods
Results
Discussion
Conclusions
Abstract
Introduction
Materials and Methods
Results
Discussion
Conclusions
Acknowledgments
Soil, litter and plant dynamics across a precipitation gradient: the effects of fertilization
Abstract
We compared differential responses to fertilizer addition and invasion in terms of soil N stocks and P, litter % N and C:N ratio (index of decomposition), plant productivity and species richness. Soil N stocks were on average 15 times higher in the higher precipitation sites regardless of nutrient addition or intrusion. Greater variation in soil P was noted at the high-precipitation sites, with some evidence of P limitation in the affected area with the highest precipitation.
Overall, increased nutrient availability was found to be significant only at high rainfall sites, possibly due to water scarcity at low rainfall sites.
Introduction
Differences in litter quality and plant productivity associated with woody encroachment (Vitousek et al. 1997; Phoenix et al. 2006) can be explained by MAP. Studies examining the effects of woody plant encroachment along a precipitation gradient have revealed a number of significant relationships between precipitation and changes in litter quality and plant productivity. We designed a two-year field experiment to investigate the effects of fertilizer addition and woody plant encroachment on soil (N and P stocks), litter (N and C:N ratio), and plant.
Fertilizer addition will increase productivity and reduce plant species richness in higher rainfall sites due to interspecific competition (Ward and others 2017).
Methods
Plant species richness (sampled as the total number of species per plot) was recorded before the start of the experiment and after two years. Xu and others 2015) were calculated to quantify the impacts of fertilizer application and woody plant impact. a) comparison of the richness of the species in the second year with that of the richness of the species before fertilization, and. We then calculated the relative effects of each treatment (fertilizer addition and disturbance) on species richness.
The effects of treatment variables. fertilizer and land cover type) on forb ratio, grass biomass and plant species richness var.
Results
At the site with the lowest rainfall, Middelburg, we found a significantly higher litter C:N ratio in sown plots than in open grasslands. Significant site and fertilizer treatment interactions were found only in areas with high rainfall (ANOVA: F = 4.64; p < 0.012). We found significantly higher grass biomass with N+P and P for the two high rainfall sites (KwaMbonambi and Bergville; Figure 7).
We also found significant negative relative effects of fertilization on species richness (ANOVA: F = 2.85; p = 0.049) due to N and N+P addition at the two high-rainfall sites (KwaMbonambi and Bergville; Figure 9) .
Discussion
For example, Blaser and others (2014) found that total P and extractable P pools increased along a shrub cover gradient. Lü and others (2014) found that water addition rather than N addition favored plant species in a semi-arid grassland. Lower C:N ratios in invaded areas have been reported by several studies, possibly due to the presence of N2-fixing legumes (Hudak and others 2003;.
Hudak et al (2003) found that areas with heavy encroachment had lower C:N ratios throughout the soil profile than areas with less encroachment.
Conclusions
Acknowledgments
Soil chemical properties and plant species diversity along a rainfall gradient in semi-arid grasslands of South Africa. Effects of rain, nitrogen, fire and grazing on tree recruitment and early survival in shrub-infested savanna, South Africa. Storage and dynamics of carbon and nitrogen in physical soil fractions after woody plant invasion in grassland.
Yield and species composition of a mesic grassland savanna in South Africa are affected by long-term nutrient addition.
Fertilizer and fire influence biological nitrogen fixation and soil respiration in Acacia
Abstract
BNF can also change soil properties by increasing soil fertility, thereby affecting the rate of soil respiration. We then evaluated the effects of fertilizer addition, simulated fire, and grazing on nitrogen (N) fixation rate, soil respiration, and root nodule number and weight. There was a strong positive correlation between tree size and number and weight of nodes.
In contrast, fire reduced soil respiration by removing biomass, thereby reducing soil C input.
Introduction
Assuming that South African savannas do not become P limited with increased woody plant density (Blaser et al. 2014), one might expect that. Fynn et al. 2003), nitrogen lost through fire or grazing can be replenished by biological nitrogen fixation, especially by legumes, which are capable of The high density of nitrogen-fixing legumes can change soil properties by increasing soil fertility (Schleicher et al. 2011).
Fire is predicted to reduce soil respiration by removing aboveground plant material and thereby reducing C inputs into the soil (Richards et al. 2012).
Methods
The purpose of the fire treatment was to remove the grass cover and "top kill" the seedlings, to observe the post-fire response. While there is controversy regarding the use of the ARA method, ARA is a sensitive and cost-effective method for estimating the rate of nitrogen fixation in trees (Danso et al. 1992). The size of the "burst" is measured after 24 hours with a digital color reader in ppm CO2 (Haney et al. 2008).
Such a relationship violates the assumption of conventional least squares regression that there is homogeneity of variance (by definition an 'envelope' leads to lower .. variance at higher values of the independent variable, Ward et al. 2000). Grass biomass was grouped into 20 g width classes, and regressed against the maximum rate of nitrogen fixation per width class.
Results
However, when belowground grass biomass exceeded ± 80 g, BNF was limited to less than 0.5 mol ethylene plant-1h-1. We found a significant positive relationship between nitrogen fixation rate and tree root biomass (r2 = 0.74, F = 4.302, p < 0.001), with higher fixation rates in saplings with greater root biomass (Fig. 3).
Discussion
Fire is known to cause large losses of soil nutrients (especially nitrogen loss) to the atmosphere through the process of volatilization (Aranibar et al. 2003). These include nutrient redistribution and positive feedbacks of nitrogen through manure and urine deposition (Tjelele et al. 2015). BNF rates are also expected to decrease if grass competition is reduced (Cramer et al. 2007; Kambatuku et al. 2013).
Our results are contrary to the findings of Kambatuku et al. 2013), who showed that BNF increased in the presence of grass competition for A.
Conclusions
Menge DN, Levin SA, Hedin LO (2009) Facultative versus obligate nitrogen fixation strategies and their ecosystem consequences. Schulze ED, Gebauer G, Ziegler H, Lange OL (1991 ) Estimates of nitrogen fixation by trees on an aridity gradient in Namibia. Woldemeskel E, Sinclair FL (1998 ) Variations in seedling growth, nodulation and nitrogen fixation of Acacia nilotica inoculated with eight rhizobial strains.
Closed diamonds (♦) indicate regression using the maximum nitrogen fixation rate against grass biomass pooled in 20 g latitude classes.
Conclusions and Recommendations for Future Research
- Introduction
- Summary and future directions
- Conclusions
- References
The positive effects of high precipitation on annual net primary productivity (ANPP) are well documented in the literature (O'Connor et al. Some studies have revealed significant effects of nutrient addition in a single year (Vourlitis et al. 2007) .Woody encroachment is generally associated with desertification and reduction in species diversity (Ratajczak et al. 2012).
A large body of work points to important mechanisms that will be affected (Archer et al. 2017;
