Browsing by Subject "Competition"

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A study of pasture cropping as an alternative cropping system for sub-saharan Africa
(2020) Orford, Rohan; Asch, Folkard
With food security and soil degradation being a major concern and hurdle in the development goals of sub-Saharan Africa (SSA), there has been and continues to be an attempt to find an alternative cropping system to conventional monocropping that rehabilitates soils whilst increasing productivity and efficiency of the subsistence cropping system. Such a cropping system needs to be realistically adoptable within the SSA social and ecological constraints. An alternative Australian winter rainfall relay cropping system coined pasture cropping (PaCr) was identified as an option that may surmount some of these limitations.This research involved completing a field trial through to model scale introductory assessment of the water dynamics in PaCr and the implications thereof in yield, water use efficiency (WUE) and competition for water; ultimately assessing the potential of PaCr in SSA. PaCr was adapted to an intercropping system for SSA summer rainfall conditions. The three treatments included the representative subsistence crop cowpea (Vigna unguiculate) and a common indigenous pasture (Eragrostis curvula) and an additive PaCr setup of cowpea directly seeded into pasture in water limited (rainfed) field trials in Pretoria, South Africa between 2013-2015. The DM yields of PaCr were 17% and 293% higher in both seasons compared to the conventional cowpea monocrop yield. When comparing PaCr yield to conventional pasture, there was a 12% and 89% higher yield in both seasons compared to the conventional pasture monocrop yield. The greater yield advantage in 2015 with the limited rainfall indicates that PaCr was most advantageous in terms of DM yield in a drier year which is a time of greatest risk and food insecurity. PaCr was also more WUE in both seasons, being significantly higher than the cowpea monocrop in 2015. Competition also showed a higher degree of competitiveness by cowpea in the wetter 2013-14 season and lower competitive ability in the drier 2015, whereas pasture showed little competitive response in 2013-14 and attaining significantly higher yields than the monocrop in 2015. The results of the field trials were used to adapt the University of Pretoria’s Soil Water Balance (SWBsci) crop model to simulate an intercropping system. Observed field results were compared to simulated results and statistical goodness of fit indicators were assessed, concluding that with all the variations of season and systems, the results were acceptable as an inaugural adaptation of the Soil Water Balance model. Other relevant crop water use parameters were extrapolated from the simulated data allowing for a more complete insight into the field trials. With the adapted SWBsci model, 14-year simulations were run in three different climates and on three different soil types for all three cropping systems to map out the viability of PaCr across an aridity index continuum as a reference for further application in research or in industry and to stress test SWBsci. Results demonstrated that PaCr was only advantageous in dry sub-humid to humid conditions on clay-loam to sandy soils, whereas pasture was dominant in more semi-arid conditions on the three different soils. Cowpea only performed better on clay soils in dry-sub humid to sub humid conditions. These advantages are attributed to differing plant water availability at various root depths suiting growth and/or competition of either one or both crops. These plant water availability differences were determined by water holding capacity of various soil types and rainfall volumes. From a WUE perspective, the pasture and PaCr did have a higher WUE but with the extreme variation in rainfall there was no significant difference. But pasture and PaCr both had a very high WUE in arid to semi-arid conditions due to the deeper roots of pasture accessing stored soil water. Competition also showed insignificant results due to the variation in the rainfall. However, in more arid to semi-arid conditions on clay-loam and sand competition outweighed facilitation thus resulting in land equivalent ratios (LER) of below 1, whereas on clay for the same aridity levels the average LER was greater than one. This was attributed to cowpea have a better competitive ability when clay water holding capacity confined plant available water to the top soil layers. The converse is true in the dry sub-humid conditions and wetter conditions because LER was less than one on clay soils while being greater than one on clay-loam and sand. This was attributed to the lower water holding capacity of sand spreading the plant available water through the profile allowing for niche root partitioning to be effective. For subsistence farmers, PaCr out-yielded the cowpea monocrop in arid conditions on all three soil types and on clay in semi-arid conditions. In the wetter dry sub-humid conditions, PaCr out-yielded cowpea on sand. In the wet sub-humid conditions PaCr does well on clay-loam and sand, but cowpea yields under these conditions are more than adequate to make the choice of PaCr debatable form a yield point of view. However, if soil rehabilitation is a necessity in the sub-humid areas, this makes PaCr a very realistic option.
Crop yield and fate of nitrogen fertilizer in maize-based soil conservation systems in Western Thailand
(2021) Wongleecharoen, Chalermchart; Cadisch, Georg
The increase in food demand and land scarcity in high-potential lowland areas have forced cropping intensification with a transformation of land use from subsistence to permanent agriculture in remote hillside in Southeast Asia. This change and inappropriate land use are the prime cause of soil degradation by erosion, which have negatively affected the agricultural systems productivity and sustainability in Thailand. Therefore, vulnerable land in sloping terrain is classified as unsuitable for continuous production of arable crops unless conservation measures are introduced to stabilize the landscape. Even though conservation practices can stabilize sloping land, farmers have not been widely adopted the measures due to various constraints, such as crop area loss and crop-tree competition. To improve land use management, a two-year study (2010-2011) was conducted at the Queen Sirikit research station (13°28’N, 99°16’E), Ratchaburi Province, Thailand, on a hillside with a slope of around 20%. The treatments consisted of (T1) maize (Zea mays L.) mono-crop under tillage and fertilization, (T2) maize intercropped with chili (Capsicum annuum L.) under tillage and fertilization, (T3) maize intercropped with chili, application of minimum tillage plus Jack bean (Canavalia ensiformis (L.) DC) relay cropping and fertilizer application, (T4) maize intercropped with chili, application of minimum tillage with Jack bean relay cropping and fertilizer application plus perennial hedges of Leucaena leucocephala (Lam.) de Wit, (T5) as T3 but without fertilization, and (T6) as T4 but without fertilization. There was an additional plot of chili sole cropping to calculate the land equivalent ratio (LER). The first part of the study evaluated yield performance and nitrogen use efficiency (NUE) of crops using the 15N isotope technique under diverse fertilized cropping systems during the first year. Maize grain yields were lower in T2 (3.1 Mg ha-1), T3 (2.6 Mg ha-1) and T4 (3.3 Mg ha-1) than in the control (T1) (6.7 Mg ha-1). The total returns from maize and chili yields were 1,914, 5,129, 3,829, 3,900, 3,494, and 2,976 USD ha-1, for T1, T2, T3, T4, T5 and T6, respectively. Higher economic returns in mixed crop systems, by selling both maize and chilies, compensated for the maize area loss by intercropping. Maize 15NUE was highest in T2 (53.5%), being significantly higher than in T1 (47.0%), T3 (45.5%), and T4 (45.7%). Overall system’s NUE in T2 (56.8%) was comparable to T1 (53.8%) and T4 (54.5%) but significantly lower in T3 (48.6%). Minimum tillage and hedgerows (despite their positive filter effect) did not increase NUE but adversely affected maize growth during the establishment phase. The second part of the study examined nitrogen fertilizers fate and quantified partial nitrogen budgets at plot level over two cropping seasons for various maize-based cropping systems with or without fertilizer application. Overall plant uptake of fertilizer 15N applied to maize was 48.6-56.8% over the first season, while residual fertilizer 15N recovery of plants was only 2.3-4.9% over the subsequent season. The quantity of applied labelled N remaining in the soil at the end of season 1 and season 2 was 6.2-28.1% and 7.7-28.6%, respectively. Thus, 60.0-76.0% in season 1 and 12.7-31.3% in season 2 of the applied fertilizer 15N were accounted for within the plant-soil system. Consequently, 24.0-40.0% and 12.9-16.1% of labelled fertilizer N were not accounted for at the end of season 1 and season 2, respectively. The derived N balance over two years revealed severe soil N depletion under T1 (-202 kg N ha-1), T5 (-86 kg N ha-1) and T6 (-48 kg N ha-1), and a slightly negative N budget under T2 (-5 kg N ha-1). In contrast, T3 (87 kg N ha-1) and T4 (62 kg N ha-1) had positive N balances. The increase of N input via additional N fertilizer applied to chili and symbiotic N2 fixation of legumes, and the reduction of N losses by soil erosion and unaccounted fertilizer N (probably lost via leaching, volatilization and denitrification) were the main factors of the positive N balances under maize-chili intercropping systems with conservation measures and fertilization (T3 and T4). Maize yield decline under T1, T2, T5 and T6 in season 2 was related to negative N balances, while maize yield increase under T3 and T4 was related to positive N balances. However, maize-chili intercropping with fertilization had some advantage (LER > 1.0) relative to sole species cropping. Moreover, total returns from crop yields in season 2 of all maize-chili intercroppings (1,378-1,818 USD ha-1) were higher than chili sole cropping (1,321 USD ha-1), which pointed to its crucial role in decreasing production risk by reducing yield loss by pests and diseases observed in chili plants. The third part of the study used combined data of stable isotope discrimination and electrical resistivity tomography (ERT) to improve understanding of competition at the crop-soil-hedge interface. Hedges significantly reduced maize grain yield and aboveground biomass in rows close to hedgerows. ERT revealed water depletion was stronger in T1 than in T4 and T6, confirming time domain reflectometry (TDR) and leaf area data. In T4, water depletion was higher in maize rows close to the hedge than rows distant to hedges and maize grain δ13C was significantly less negative in rows close to the hedge ( 10.33‰) compared to distant ones ( 10.64‰). Lack of N increased grain δ13C in T6 ( 9.32‰, p ≤ 0.001). Both methods were negatively correlated with each other (r= 0.66, p ≤ 0.001). Combining ERT with grain δ13C and %N allowed identifying that maize growth close to hedges was limited by N and not by water supply. In conclusion, the results suggested a significant positive interaction between mineral N fertilizer, intercropping systems and soil conservation measures in maintaining or improving crop yields and N balances in Thailand’s hillside agriculture. Simultaneously, combining ERT imaging and 13C isotopic discrimination approaches improved the understanding of spatial-temporal competition patterns at the hedge-soil-crop interface and pointed out that competition in maize-based hedgerow systems was driven by nitrogen rather than water limitation. Therefore, sustainable agriculture might be achieved if farmers in Thailand combine soil conservation measures with appropriate and targeted N fertilizer use.
Measuring and modelling resource use competition at the crop-soil-hedge interface on a hillside in Western Thailand
(2015) Hussain, Khalid; Cadisch, Georg
Thailand’s western uplands are facing severe soil loss and runoff problems due to intensive cultivation of cash crops for high food, feed, fiber, and fuel demand by an increasing population. Thus the Land Development Department and the International Board for Soil Research and Management in Thailand are promoting the use of soil conservation measures such as contour hedgerows, grass barriers and agroforestry systems based on fruit trees and annual crops. Although such measures have been shown to be effective in controlling soil erosion, farmers often are reluctant to adopt such practices as inclusion of trees reduces the cropped area and yields competition for available resources with crops. Hence, a better understanding of the underlying processes at the crop-soil-hedge interface is needed to design soil conservation systems that are more attractive for farmers. It was hypothesized that soil conservation systems with hedgerows and intercropping will induce spatial patterns of resource use which can be linked to competition while planting patterns affect canopy characteristics and light distributions. This study focused on the following objectives; (i) to improve understanding of competition at the crop soil hedge interface by combining stable isotope discrimination, electrical resistivity tomography and time domain reflectometry, (ii) to identify the effects of intercropping and hedgerows on maize biomass accumulation, nitrogen concentration and light use efficiency, (iii) to evaluate the competition between maize hedges at crop-soil-hedge interface, (iv) to explore possible mitigating options to cope with competition between hedge and maize by using a modelling approach. A field trial was laid out in randomized complete block design with three replicates at Queen Sirikit research farm, Ban Bo Wi village (13°28´ N and 99°15´ E), Suan Phueng District, Ratchaburi province in western Thailand with 20-25% slope magnitude. The experiment was established in 2009 while the research presented here was carried out during the 2010 and 2011 maize growing seasons. Six cropping treatments with following management practices were investigated: (T1, control) maize [Zea mays L.) monocrop, tillage, with fertilizer application (farmer’s practice; (T2) maize intercropped with chili (Capsicum annuum L. cv. Super Hot), tillage and fertilizer application; (T3) maize intercropped with chili, minimum tillage, fertilizer application, and Jack bean (Canavalia ensiformis (DC) L.) relay cropping; (T4) maize intercropped with chili, minimum tillage, fertilizer application, Jack bean relay cropped, and leucaena (Leucaena leucocephala (Lam.) de Wit) hedgerows; (T5) and (T6) as (T3) and (T4), respectively, but both without fertilizer application. Tillage was carried out manually by hoe to around 0-20 cm depth. Plots were 13 x 4 m. Fertilizer was applied to maize at a rate of 62, 22, 36 kg ha-1 of N, P, and K, respectively. Urea (N) application to maize was done in two splits as 31 kg ha-1 of N one month after sowing maize and another split of 31 kg ha-1 of N two months after sowing maize Chili received 92 kg N ha-1 at the time of transplanting and 92 kg ha-1 N as top dressing one month after transplanting. The impact of competition at the crop-soil-hedge interface was studied in 2011, two years after establishment of soil conservation measures, to exclude the establishment period of leucaena with a potentially weak impact on maize. At this time, highest above ground biomass (AGB) production of maize of 1364 g m-2 was witnessed in T2 being statistically different from all other treatments, except T4 and T3; while lowest above ground biomass production of 1034 g m-2 was observed in T5. In hedgerow treatments, maize rows planted distant to hedges produced 46% and 73% higher AGB than maize growing in rows close to the hedgerow (p≤0.0001) in T4 and T6, respectively. Similar effects were observed for plant height, grain nitrogen concentration and grain yield. Mean grain δ13C was significantly higher in T6 (-9.32‰) than in T4 (-10.49‰) and T1 (-10.55‰). Generally, higher δ13C signals mean higher water availability; however the higher δ13C signals in unfertilized T6 treatment imply that lack of nutrients may have led to increased δ13C values. Similarly in T4, δ13C signals were significantly higher in maize grains originating form rows planted close to hedges (10.33‰, p≤0.0001) than samples obtained from maize rows distant to hedges (-10.64‰). These results point out that competition at the crop-soil-hedge interface was driven by nutrient rather than water limitation. The electrical resistivity tomography (ERT) imaging further supported this finding showing that maize monocrop showed higher soil moisture depletion patterns than hedge intercrop with fertilizer (T4) treatment, while hedge intercrop without fertilization (T6) depleted soil moisture least. Light use efficiency (LUE) for maize above ground biomass production was statistically higher LUEAGB (1.56 g DM MJ-1) in maize and chilli intercrop (T2) than in maize sole cropping LUEAGB (1.23 g DM MJ-1). In T4 and T6 maize rows planted close to hedgerows had lower LUEAGB than rows distant to hedgerows. The land equivalent ratio showed that inclusion of hedgerows with fertilizer application in the intercropping treatment enhanced land utilization by 21%. The Water, Nutrient and Light Capture in Agroforestry (WaNuLCAS) model simulated AGB with R2= 0.83, RMSE=6.3, EF=0.82 and CD=1.4 during calibration while model validation also showed promising results with R2= 0. 76, P<0.001, RMSE=4.6 and EF=0.69. Simulations also pointed to major nutrient limitation between maize rows planted close to hedgerows. Simulations showed that negative impacts of hedgerows on crops can be reduced by managing fertilizer application in hedge adjacent maize rows leading to a successful application of agroforestry systems on a long-term basis not only for soil conservation but also for sustainable crop production in tropical uplands. The study figured out the scope of stable isotopic discrimination, ERT, light use efficiency and modelling approaches for evaluating resource use competition at crop-soil-hedge interface on hillside agriculture. The combination of isotopic discrimination and ERT measurements provided useful information for identification of cause-impact relationships. Spatial LUE patterns provided insights for canopy light harvest under various soil conservation options. Furthermore, light use data was also helpful in validation of WaNuLCAS model which did not only provide multiple options to figure out resource use competition at crop-soil-hedge interface but also allowed to test mitigation options for sustainable crop production in tropical uplands. Model scenarios showed that negative impacts of hedgerows on crops growing close to hedges can be reduced by applying minute additional doses of fertilizer only to the crop rows planted close to hedgerows, leading to a sustainable crop production along with soil conservation. Productivity evaluation of investigated cropping systems showed that inclusion of hedgerows and intercropping in tropical hillside agriculture is promising in enhancing crop production and thus can be adopted by farmers with yield advantage.
Spatial and functional determinants of long-term fecundity in serotinous shrub communities
(2016) Nottebrock, Henning; Schurr, Frank
Understanding the dynamics of biological communities is a central aim of ecological research. Contemporary environmental change reinforced this interest: in order to predict how communities will react to environmental change, we have to understand the processes driving their dynamics. Ultimately, the dynamics of a community depends on the reproduction, mortality and dispersal of its component individuals, and on how these demographic processes are altered by environmental factors and biotic interactions. A general understanding of biological communities is unlikely to arise from a species-specific approach that attempts to quantify all pairwise interactions between species. Instead, it seems promising to pursue a trait-based research program that quantifies how variation in the performance of species and individuals is shaped by the interplay of functional traits, biotic interactions and environmental factors. In this thesis, I investigated how functional plant traits determine plant-plant, plant-pollinator and plant-herbivore interactions in space and time, and how these spatiotemporal interactions affect the long-term fecundity of plants. In the South African Fynbos biome (a global biodiversity hotspot), I studied a species-rich, ecologically and economically important group of woody plants (genus Protea) and its interactions with pollinators and seed predators. The objectives of this thesis were: (1) to combine plant traits and high-resolution maps of Protea communities in order to quantify the landscapes of nectar sugar and seed crops that plant communities provide for pollinators and seed predators, (2) to examine how sugar landscapes shape pollinator behaviour, and how pollinator behaviour and pollinator-mediated interactions between plants affect the reproduction of Protea individuals, (3) to study how the spatial structure of plant communities and seed crop landscapes determine direct and predator-mediated interactions between plants, and (4) to understand how the interplay of these biotic interactions shapes the dynamics of plant communities. I addressed these objectives by analysing spatially-explicit data and high-resolution maps from 27 sites of 4 ha each that contained 129,750 plants of 22 Protea species. The results show that Protea plants and their pollinators interact on several spatial and temporal scales, and that these interactions are shaped by sugar landscapes. Within plants, inflorescences compete for pollination. At a neighbourhood scale, Protea reproduction benefits from nectar sugar of conspecific neighbours but not from heterospecific neighbour sugar. Seed set also increases with the amount of nectar sugar at the scale of entire study sites. This corresponds with the finding that the abundance and the visitation rates of key bird pollinators strongly depend on phenological variation of site-scale sugar amounts. Nectar sugar furthermore influences the strength of interactions between Protea species and bird pollinators: Protea species that provide nectar of high sugar concentration depend more strongly on bird pollinators to reproduce. When foraging in sugar landscapes, these bird pollinators show both temporal specialization on single plant species and a preference for common plant species. In addition to these pollinator-mediated interactions, the long-term fecundity of Protea individuals is reduced through both competition and apparent competition mediated by seed predators. Competition is stronger between conspecifics than between heterospecifics, whereas apparent competition shows no such differentiation. The intensity of competition between plants depends on their size and the intensity of apparent competition between plants depends on their seed crops. Moreover, competition has a stronger effect on plant fecundity than apparent competition. These findings have interesting implications for understanding the dynamics of Protea communities and the maintenance of plant diversity in the Fynbos biome. The positive interspecific density-dependence resulting from pollinator-mediated interactions causes community-level Allee effects that may lead to extinction cascades. My analyses also imply that competition stabilizes the coexistence of Protea species (because intraspecific competition is stronger than interspecific competition), whereas apparent competition via seed predators does not have such a stabilizing effect. In summary, this study highlights the benefits of ‘community demography’, the demographic study of multiple interacting species. Community demographic studies have the potential to identify general determinants of biotic interactions that act across species and communities. In this thesis, I identified nectar sugar and seed crops as interaction currencies that determine how multiple plant species interact through shared pollinators and seed predators. In megadiverse systems such as Fynbos, such generalizations are urgently needed to understand and forecast community dynamics. The analysis of community dynamics with respect to such interaction currencies provides an alternative to the classical species-specific approach in community ecology.
The performance of and interactions between multiple co-occurring alien and native plant species
(2023) Ferenc, Viktoria; Sheppard, Christine
This thesis focuses on investigating species interactions in the context of alien species establishment, which poses severe threats to species, communities, and ecosystems due to climate change and globalization. The study emphasizes the need to understand the effects of multiple co-occurring alien species and their potential explanations, such as niche or fitness differences. The outcome of competition can also be influenced by priority effects, where earlier emerging species affect later emerging species in a given environment. Positive interactions, like facilitation, are often overlooked but play a significant role in species interactions. Legumes, known for their ability to fix atmospheric nitrogen, can facilitate alien species, leading to both severe negative effects on natives and reduced resource competition. The research conducted three common garden pot experiments to address various aspects of alien species interactions. The first experiment examined pairwise combinations of 20 alien annual plant species in Germany to identify the mechanisms driving these interactions. It tested the prevalence of competition versus facilitation and assessed the effectiveness of individual traits, hierarchical or absolute trait distances, multivariate trait or phylogenetic distance, and trait plasticity in explaining plant performance. Results indicated that while interspecific competition was more common, interspecific facilitation occurred in 24% of cases. Hierarchical trait distances provided better explanations for interactions than phylogenetic or multivariate trait distances. Accounting for trait plasticity did not necessarily improve plant performance predictions. Notably, taller individuals with lower specific leaf area than their alien neighbours exhibited increased biomass and seed production when growing together. The second experiment focused on interactions between five pairs of alien and native species. It evaluated the impact of growing with one or two alien neighbour species on native plants and manipulated the arrival time of alien or native neighbours. Generally, native species performed worse when surrounded by two alien species compared to one, although the effect varied among species. Both native and alien species experienced significant performance decreases when arriving second in the pot, while alien species tended to benefit more from early arrival. However, further studies are required to generalize these findings across multiple alien and native plants regarding neighbour species and arrival time responses. The third experiment delved into legume facilitation in more detail. It involved growing 30 annual Asteraceae species (neophytes, archaeophytes, and natives) in communities with or without legume presence. The study measured functional traits, fitness, and nitrogen characteristics to understand how legume presence affected Asteraceae fitness and the relationship between traits, nitrogen concentration, and fitness. Using the δ15N natural abundance method, the research explored whether facilitation mechanisms differed among native phytometer, neophyte, and archaeophyte Asteraceae. Specific leaf area negatively affected aboveground biomass and seed production, with a stronger effect in the absence of legumes. Nitrogen concentration was positively correlated with biomass but did not significantly increase seed production. The results suggested direct legume facilitation for the native grass phytometer Festuca rupicola when growing alongside archaeophytes but not neophytes. This indicated varied mechanisms of competition for nitrogen between natives and alien species of different residence times and deepened understanding of altered facilitative leguminous effects in the presence of alien species. Overall, this research demonstrates the application of community ecology concepts and theories to investigate alien species interactions, particularly when multiple co-occurring alien species are involved. As the rate of alien species arrival in new habitats continues to increase, understanding their combined impact on native species, communities, and ecosystems becomes increasingly crucial.