<p>Drylands, comprising arid and semi-arid areas and the dry subtropics, over some 40% of the world's land area and support approximately 2 billion people, including at least 1 billion who depend on dryland agriculture and grazing. 10-20% of drylands are estimated to have already undergone degradation or desertification, and lack of monitoring and assessment remains a key impediment to preventing further desertification. Change in vegetation cover, specifically in the spatial organization of vegetation may occur prior to irreversible land degradation, and can be used to assess desertification risk. Coherent spatial structures arise in the distribution of dryland vegetation where plant growth is localized in regular spatial patterns. Such "patterned vegetation" occurs across a variety of vegetation and soil types, extends over at least 18 million ha, occurs in 5 continents and is economically and environmentally valuable in its own right.</p> <p>Vegetation patterning in drylands arises due to positive feedbacks between hydrological forcing and plant growth so that the patterns change in response to trends in mean annual rainfall. Mathematical models indicate that vegetation patterns collapse to a desertified state after undergoing a characteristic set of transformations so that the condition of a pattern at any point in time can be explicitly linked to ecosystem health. This dissertation focuses on the mathematical description of vegetation patterns with a view to improving such predictions. It evaluates the validity of current mathematical descriptions of patterning for the specific case of small-scale vegetation patterns and proposes alternative hypotheses for their formation. It assesses the significance of seed dispersal in determining pattern form and dynamics for two cases: vegetation growing on flat ground with isotropic patterning, and vegetation growing on slopes and having anisotropic (i.e. directional) patterning. Thirdly, the feedbacks between local biomass density and infiltration capacity, one of the positive feedbacks believed to contribute to patterning, are quantified across a wide range of soil and climatic conditions, and new mathematical descriptions of the biomass-infiltration relationship are proposed. Finally the influence of land surface microtopography on the partitioning of rainfall into infiltration and runoff is assessed.</p> / Dissertation

The objectives of my research were twofold: 1) to evaluate new technologies (camera traps and a new type of GPS collar) for studying large mammals in tropical forests, and 2) to study the ecology of the lowland tapir (Tapirus terrestris) in the Peruvian Amazon. Camera traps proved to be an efficient tool for mammal inventories in tropical forests. They recorded 24 out of 28 terrestrial medium and large sized mammal species with a survey effort of 2340 camera days. Camera traps were also able to reveal important information on habitat use, activity patterns and the use of mineral licks for five Amazonian ungulate species. There was a high spatial overlap between all the species with the grey brocket deer being the only species that was restricted to terra firme forest. White-lipped peccaries, tapirs and red brocket deer frequently used mineral licks, whereas collared peccaries and grey brocket deer were hardly ever observed at licks. A new type of GPS collar (TrackTag) tested in this study performed well under the dense canopy of a tropical forest. Position success rates of 87% for stationary collars and 48% for collars placed on tapirs were comparable to data obtained with GPS collars in temperate forests. The mean location error for stationary collars inside the forest was 28.9 m and the 95% error was 76.8 m. GPS collars placed on six tapirs for seven to 182 days showed home range sizes of 102 to 386 hectares. Tapirs were mainly nocturnal and areas used for foraging during the night differed from resting sites used during the day. Tapirs could walk up to 10 km to visit a mineral lick. Visits were irregular at intervals of a few days up to 36 days. The analysis of 135 tapir dung samples showed that tapirs ingest seeds of over 120 plant species. Seeds were found throughout the year but monthly species diversity was related to fruit availability. The size distribution of ingested seeds was related to availability. Most seeds were less then 10 mm wide, but seeds up to 25 mm were found. Both camera traps and TrackTag GPS collars greatly increased the possibilities for studying large rainforest mammals. The two technologies collect complementary information and each one is suited for a different set of questions.

Palms are ecologically important and charismatic trees of the tropics. They are important to the livelihood of local communities and are key resources for the frugivore community in tropical forests. These frugivores are in turn hunted by humans for food. This ecological connection between human, palms, and frugivores provides a unique setting to study how cultural and ecological components within this multitrophic interaction influences palm populations. In chapter 1, I explored the traditional and ecological knowledge behind the cultivation of palm-weevil larvae for food. I found the Joti people, cultivated two species of weevil-larvae differently, which also determined whether palms were logged before or after reproductive maturity. The cultivation of each weevil-larvae species therefore had a differential impact on palm populations. In chapter 2, I investigated how frugivores mediate interactions between two dominant and co-occuring palms in the Peruvian Amazonia-- Attalea phalerata and Astrocaryum murumuru. I found frugivores codispersed seeds of the two palm species, which contributed to aggregated spatial patterns of their juveniles. Spatial patterns suggested associations between heterospecific palms experienced lower density-dependent mortality than associations between conspecifics and this likely contributes to the coexistence of the two palm species in their early life-history. These findings highlight the importance of dispersers to species coexistence and suggest over-hunting can lead to shifts away from species codominance. In chapter 3, I examined the contribution of dispersal, distance-and density-dependent to spatial ecology of Attalea phalerata. Using microsatellite-based parentage analysis, I found high levels of seed movement mediated by frugivore dispersers. Despite this, I found dispersal limitation remains strong enough to cause spatial aggregation between offspring and parents. As individuals grew towards maturity, distance and density dependent mortality contributed to increasingly disaggregated patterns between older offspring cohorts of parents, non-parent adults, and siblings. These results provide a foundation for assessing the impacts of hunting on the spatial ecology of palm populations. In chapter 4, we characterized 14 microsatellite loci for A. phalerata that were used in the parentage analysis of chapter 3. These loci amplified reliably and were sufficiently polymorphic and will be useful for future studies addressing population-level questions for this species. / text

Seed size is regarded as a functional trait with very important consequences for the fitness of plant species. Seedlings emerging from larger seeds are more competitive but are more costly to produce than seedlings from smaller seeds. Seed size is also a trait with transgenerational effects, affecting both the fitness of the parent as well as that of the offspring. Theory on the evolution of offspring size predicts an optimum balance between size and number, seen from the parent's perspective; while empirical studies often show selection for larger seeds, seen from the offspring's perspective. Seed size selection arising from post germination traits is, however, often not unidirectional, nor operating with the same strength in all life history stages of the plant. Seed size selection is also environmentally dependent. Even environmental influence might not operate with the same consistency and strength uniformly through the plant's life cycle. This dissertation is intended to study these questions concerning the dynamics of seed size selection in the wild. This work is to my knowledge, the first to document how seed size selection operates through the whole life cycle, with naturally germinated annual plants from the Sonoran Desert. In my first chapter I explored the offspring fitness consequences of seed size in a multiyear observational study using plant demography and relating vital rates (germination, survival, and fecundity) to the size of the seeds that originate individual plants and the environmental variables of precipitation and competition. I detected positive directional selection operating both through survival and fecundity. Water availability increased both survival and fecundity but also strengthened survival selection and had no effect on fecundity selection. Competition detrimental effects were only observed in fecundity but not in plant survival. In my second chapter I ask whether seed size-specific germination could influence seed size selection later in the life cycle. We found that because germination is differential in relation to seed size, the time of optimal conditions for germination in the field would determine the variance of seed size in the germinated fraction and thus influencing the strength of seed size selection operating through survival. In my third chapter I explored the dispersal consequences of phenotypic plasticity in seed provisioning. We found that mother plants that experienced more competition made smaller seeds and affected the seed dispersal process. Smaller seeds were better able to disperse farther away from their mothers and therefore increased their probability of escaping competition in the next growing season. These studies demonstrated that seed size selection varies through the life cycle and in intensity depending on interactions with the environment.

A central issue in climate change research is to identify what species will be most affected by variations in temperature, precipitation or CO2 and via which underlying mechanisms. Dynamic global vegetation models (DGVMs) have been used to address questions of habitat shifts, extinctions and changes in carbon and nutrient cycling. However, DGVMs have been criticized for assuming full migration and using the most generic of plant functional types (PFTs) to describe vegetation cover. My doctoral research addresses both of these concerns. In the first study, I added two new tropical PFTs to an existing regional model (LPJ-GUESS) to improve vegetation representation in Central America. Although there was an improvement in the representation of some biomes such as the pine-oak forests, LPJ-GUESS was still unable to capture the distribution of arid ecosystems. The model representations of fire, soil, and processes unique to desert vegetation are discussed as possible explanations. The remaining three chapters deal with the assumption of full migration, where plants can arrive at any location regardless of distance or physical barriers. Using LPJ-GUESS, I imposed migration limitations by using fat-tailed seed dispersal kernels. I used three temperate tree species with different life history strategies to test the new dispersal functionality. Simulated migration rates for Acer rubrum (141 m year-1) and Pinus rigida (76 m year-1) correspond well to pollen and genetic reconstructed rates. However, migration rates for Tsuga canadensis (85 m year-1) were considerably slower than historical rates. A sensitivity analysis showed that maturation age is the most important parameter for determining rates of spread, but it is the dispersal kernel which determines if there is any long distance dispersal or not. The final study demonstrates how northerly refugia populations could have impacted landscape recolonization following the retreat of the last glacier. Using three species with known refugia (Acer rubrum, Fagus grandifolia, Picea glauca), colonization rates were faster with a northerly refugia population present. The number of refugia locations also had a positive effect on landscape recolonization rates, which was most pronounced when populations were separated. The results from this thesis illustrate the improvements made in vegetation-climate models, giving us increasing confidence in the quality of future climate change predictions.

No description available.

A central issue in climate change research is to identify what species will be most affected by variations in temperature, precipitation or CO2 and via which underlying mechanisms. Dynamic global vegetation models (DGVMs) have been used to address questions of habitat shifts, extinctions and changes in carbon and nutrient cycling. However, DGVMs have been criticized for assuming full migration and using the most generic of plant functional types (PFTs) to describe vegetation cover. My doctoral research addresses both of these concerns. In the first study, I added two new tropical PFTs to an existing regional model (LPJ-GUESS) to improve vegetation representation in Central America. Although there was an improvement in the representation of some biomes such as the pine-oak forests, LPJ-GUESS was still unable to capture the distribution of arid ecosystems. The model representations of fire, soil, and processes unique to desert vegetation are discussed as possible explanations. The remaining three chapters deal with the assumption of full migration, where plants can arrive at any location regardless of distance or physical barriers. Using LPJ-GUESS, I imposed migration limitations by using fat-tailed seed dispersal kernels. I used three temperate tree species with different life history strategies to test the new dispersal functionality. Simulated migration rates for Acer rubrum (141 m year-1) and Pinus rigida (76 m year-1) correspond well to pollen and genetic reconstructed rates. However, migration rates for Tsuga canadensis (85 m year-1) were considerably slower than historical rates. A sensitivity analysis showed that maturation age is the most important parameter for determining rates of spread, but it is the dispersal kernel which determines if there is any long distance dispersal or not. The final study demonstrates how northerly refugia populations could have impacted landscape recolonization following the retreat of the last glacier. Using three species with known refugia (Acer rubrum, Fagus grandifolia, Picea glauca), colonization rates were faster with a northerly refugia population present. The number of refugia locations also had a positive effect on landscape recolonization rates, which was most pronounced when populations were separated. The results from this thesis illustrate the improvements made in vegetation-climate models, giving us increasing confidence in the quality of future climate change predictions.

Abstract The restoration of degraded landscapes has become one of our most valuable tools for conservation, however there are many factors which can restrict natural regeneration and impede active restoration attempts. The purpose of this study was to investigate three key processes which commonly limit the establishment of forest tree species into abandoned pasture in tropical forests: i) dispersal limitation, ii) seed predation, and iii) competition from the grass sward. Seed dispersal I identified 59 species of birds that were using the grassland habitat. Through 216 hours of focal tree observations I established that isolated trees in the grassland that had larger canopies, and those that were providing a food source (i.e. flowers or fruit) had significantly higher bird visitation rates and average stay lengths. I found evidence of the “perch effect” as patches of remnant trees encouraged more birds into areas of grassland, and the density of seedlings under tree canopies was significantly positively correlated with bird visitations. 95% of the seedlings found beneath tree canopies in grassland were of a different species to that of the tree canopy above them, demonstrating the dispersal of seeds from elsewhere into these microhabitats. 98% of these seedlings are grassland or forest edge species showing forest core species are still dispersal or microsite limited despite the effect of these trees. Seed predation Removal rates of seeds from experimentally laid out seed piles varied among seed species, the habitat the pile was in, and the predator guild able to access the piles. Preliminary results indicate that these trends are driven by the ecology of the seed predator. Removal of seeds by vertebrates was highest in the core forest, while ant predation was constant across all habitats. Vertebrates removed the larger seeds (Entandrophragma angolense and Sterculia tragacantha) while ants preferred the smaller Celtis gomphophylla and Croton macrostachyus. Overall predation rates in grassland were lower than those in the forest, and the presence of remnant trees did not influence predation rates, a positive sign for regeneration and the survival of seeds dispersed into these areas. Competition from the grass sward While the grass sward provides shade for seedlings of forest tree species it is also a harsh environment for them, as the grass competes with seedlings for water. Removing the grass and covering planted seedlings with artificial shading structures significantly increased both the survival and growth of these seedlings. Recommendations From this study I was able to make recommendations for a low input restoration program at Ngel Nyaki. Planting seedlings in small „islands‟ takes advantage of the natural increase in dispersal of seeds under isolated trees, while low seed predation rates increase the chance of survival of these seeds to germination. Planting these seedlings under shade will lead to increases both their growth rates and their survival. Once the secondary forest develops, under-planting seedlings of core forest trees will introduce them to the system, as the natural establishment of these seeds appears to be limited in the current environment. This study has also served to remind us how little we know about this particular forest-grassland system, and has led to the development of ideas for further investigations into several more aspects of regeneration.

Woody plant encroachment is a phenomenon whereby trees and shrubs invade grasslands or increase in an already wooded area, resulting in lower yields of herbaceous plants and a reduction in the carrying capacity of rangelands. It is not only the extent of woody plant encroachment, but also the rate at which it occurs, that is a major concern for livestock ranchers interested in herbaceous production. The question of what causes woody plant encroachment still remains unanswered. Animals consume a considerable amount of woody plant seeds during the dry season and could be responsible for spreading undesirable species to new areas or they could enhance the dispersal of species already established. However, under natural conditions, seed dispersal does not guarantee seed germination, which depends on appropriate environmental and seed survival conditions. To better understand the mechanisms involved in woody plant encroachment, we studied the effects of livestock (goats, sheep and cattle), diet quality (high-quality (Medicago sativa hay) vs. low-quality (Digtaria eriantha hay)), seed characteristics (size, shape and hardness), grass competition and fire on germination and seedling recruitment of Dichrostachys cinerea and Acacia nilotica seeds. These objectives were achieved by conducting the following trials: a) recovery and germination of D. cinerea seeds fed to goats, b) diet quality on germination of Dichrostachys cinerea and Acacia nilotica seeds fed to ruminants, c) the effects of gut passage and dung fertilization on seedling establishment of Dichrostachys cinerea and Acacia nilotica seeds and d) the effects of gut passage, dung fertilization, trampling, grass competition and fire on seedling recruitment of the two woody plant species. During the first trial, seed recovery rate of D. cinerea seeds consumed by goats, either voluntarily after mixing them with feed pellets (mixed), or by force-feeding (gavaged) and germination percentage were measured. Seed recovery for the gavaged treatment (32.7%) was significantly higher than for the mixed treatments (9.9%; P < 0.001). Seeds that passed through the digestive tract (mixed (35.5%)) and gavaged (31.2%) treatments) had a significantly higher germination percentage than untreated seeds (19.0%). A non-negligible proportion of D. cinerea seeds remained intact after chewing and passage through the digestive system, and their germination percentage increased. In the second trial, I tested the effects of animal species (goats and sheep, goats and cattle), diet quality (Medicago sativa hay and Digitaria eriantha hay), and seed characteristics (size, shape and hardness) on the effectiveness of animal seed recovery and germination of D. cinerea and A. nilotica seeds. The trial was divided into two experiments. In the first experiment, a significant interaction effect of animal species (goats, sheep), diet (high-quality hay, low-quality hay) and seed species (A. nilotica seeds, D. cinerea seeds) was found on percentage germination (P < 0.0001). There was also a higher percentage seed recovery (P < 0.009) when animals were offered high-quality hay (47.4% + 4.65) compared to low-quality hay (30.2% + 3.24). In goats fed D. eriantha hay, A. nilotica seed germination (9.38% + 3.66) was higher (P < 0.05) than goats fed D. eriantha hay and D. cinerea seeds (6.78% + 1.13). A greater germination percentage was observed in goats fed M. sativa hay with D. cinerea seeds (6.71% + 1.53) than goats fed M. sativa hay with A. nilotica seeds (2.50% + 0.97) (P < 0.05). In the second experiment, animal species had a positive impact, both on seed recovery (P < 0.0325; goats 32.0% + 6.44; cattle 50.3% + 4.27) and germination percentage (P < 0.055; goats 14.1% + 1.48; cattle 9.3% + 0.94; control: D. cinerea 0.64 + 0.06; A. nilotica 0.59 + 0.07). Animal species in experiment one (goats and sheep) and two (goats and cattle) was most important for seed recovery and germination. However, diet (M. sativa hay and D. eriantha hay) and seed species (D. cinerea, A. nilotica seeds) also had important effects on germination of seeds retrieved from experiment one. The interaction of animal species and size, diet quality, and seed characteristics (size, hardness) all played a major role in recovery of viable and scarified seeds either alone or in combination. I also studied the effects of seedling emergence, seedling establishment and recruitment of dispersed A. nilotica and D. cinerea seeds by goats and cattle under natural conditions. The interaction effect of animal species, seed recovery day and seed germination treatment/planting method was significant on seedling recruitment. Seeds retrieved from goats in the last four days and planted 2 cm in the soil with dung (25.85% ± 0.46) and seeds planted 2 cm in the soil with no dung (24.77% ± 0.35), recruited significantly better than seeds planted on top of the soil (16.98% ± 0.46). The results also indicated significant differences in percentage seedling recruitment among goats, cattle and control, with goats and cattle having the highest percentage recruitment than controls or untreated seeds. Overall, seeds can potentially germinate and recruit following passage through the gut, thereby facilitating woody plant encroachment. The results of the last trial showed that seed passage through the digestive tract of goats and cattle compared to untreated seeds (i.e. not ingested) played an important role in improving germination through seed scarification. However, seed recovery by livestock does not guarantee seedling establishment survival, survival and recruitment. Fire and grass mowing treatments affected seedling emergence, seedling survival and recruitment, most probably because of reduced grass competition, thereby reducing competition for resources (light, water and nutrients) between grasses and seedlings. In conclusion, this study indicated that animal species (goats, sheep and cattle), associated diet (low-quality vs. high-quality), seed species (D. cinerea and A. nilotica) and seed characteristics (size, shape, hardness) all played an important role in seed germination. The interactions of animal species, fire, dung, and season either directly or indirectly were pivotal in the emergence, survival and recruitment of D. cinerea and A. nilotica seedlings. Thus, acid scarification in the gut of herbivores in combination with their indirect effects (dung fertilization) and removal of grass competition (either by fire or mowing) can facilitate seedling emergence, seedling survival and recruitment of woody plant species, which may lead to woody plant encroachment. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2014.

Diss. (sammanfattning) Alnarp : Sveriges lantbruksuniv. / Härtill 5 uppsatser.