Exploring the ecological intensification of weed management in cropping systems of South Africa’s winter rainfall region

Taking an ecological intensification approach to weed management could increase agriculture’s ability to meet human needs for food, fuel and fibre whilst avoiding impacts on the integrity of Earth’s life-supporting ecosystems. Ecological intensification would achieve this by replacing anthropogenic inputs with ecosystem function. In this thesis, ecosystem processes are sought that could replace conventional weed control inputs and actions such as herbicides and tillage, and so reduce the environmental impacts associated with these. First, there is a need to shift thinking away from isolated mechanisms that seek to remove weeds, and to instead aim for ‘agroecosystem resilience to weeds’ in which farming systems would be designed to incorporate ecological processes and properties that inherently limit the negative impacts and promote the positive roles of weeds. To understand how this resilience could be achieved, recent advances in weed ecology are reviewed to identify relevant processes and properties, and to consider how these might be implemented in farm design and management. In sum, practices that could increase the diversity of filters applied to weed communities while decreasing filter strength, and that could reduce resource availability, would be expected to confer agroecosystem resilience to weeds. This thesis explores the practical implementation of some of these practices in South Africa’s winter rainfall region through field studies and trials. The practices were chosen for their suitability for conservation agriculture systems (the dominant farm management style in the region) and for their potential to harness or enhance ecological processes for weed management. The first practical study, a field survey of weeds in 15 vineyards with differing weed management practices, confirmed that the use of management techniques imposing lower disturbance leads to more diverse weed communities composed of less competitive species, and this effect can be enhanced by using specific management techniques to select for specific weed traits. The second practical study was linked to a long-term crop rotation experiment, and explored the effects of increasing crop diversity and integrating livestock as methods to increase the variability experienced by weeds in these rotation systems. Combining these two practices substantially reduced weed abundance and conserved weed diversity over the twelve years investigated. They also reduced herbicide and fertiliser requirements, and sustained cash crop yields, thus contributing to both profitability and sustainability. The final practical study applied theories of biotic resistance from invasion ecology to investigate how best to design cover crop mixes for weed suppression, that could be used in field crop systems or vineyards. Mixes composed of highly productive species were most effective at resource capture, and thus most effective at reducing resource availability to suppress weeds. In sum, reducing herbicide use in favour of grazing or mowing, increased crop and management diversity, and competitive crops (in mixes or monoculture) are viable practices that constitute first steps toward the ecological intensification of weed management in South Africa’s winter rainfall region. They offer improvements to agricultural sustainability through sustaining yields and farm incomes whilst reducing the environmental impacts and health risks associated with conventional weed control such as herbicides and tillage. Overall, the findings of this thesis suggest that ecological intensification offers a promising direction for future weed management to achieve agricultural sustainability, both in South Africa’s winter rainfall region and around the world. Weed researchers can assist farmers in this challenge by drawing on global advances in weed ecology to design and test locally appropriate weed management techniques and strategies

Timing of Arrival and Native Biomass Are Better Proxies of Invasion Suppression in Grassland Communities

A challenge in many restoration projects, in particular when establishing de novo communities, is the arrival and later dominance of invasive alien plants. This could potentially be avoided by designing invasion-resistant native communities. Several studies suggest achieving this by maximizing trait similarity between natives and potential invaders (“limiting similarity”), but evidence supporting this approach is mixed so far. Others pose that the relative time of arrival by native and invasive species (“priority effects”) could play a stronger role, yet this factor and its interaction with trait similarity is not fully understood in the context of ecological restoration. Thus, we hypothesized that multi-trait similarity would increase suppression of invasive species by native communities, and that the effect would be stronger when natives arrive first. We established two distinct communities of native central European grassland species based on native–invasive trait similarity, and then tested the introduction of invasive Ambrosia artemisiifolia and Solidago gigantea separately when arriving in the native communities at two times, i.e., sown either at the same time as the natives or 2 weeks after. For the traits selected, our data did not provide evidence for a limiting similarity effect, but rather supported priority effects. Both native communities more effectively suppressed invaders that arrived after the natives. In addition, the native community that produced the most biomass suppressed both invasive species more than the most ecologically similar community. This effect of biomass revealed that prioritizing native–invader ecological similarity can fail to account for other community characteristics that affect invasion resistance, such as biomass. Instead, native communities could be designed to enhance priority effects through the inclusion of early and fast developing species. We conclude that native community composition plays a significant role in the establishment success by invasive species, and resource pre-emption seems more significant than trait similarity. In terms of grassland restoration, native species should be selected based on plant traits related to fast emergence and early competitiveness

Tillage practices affect weeds differently in monoculture vs. crop rotation

Reduced tillage practices are widely considered to be more sustainable than conventional tillage practices, but many producers remain reluctant to reduce tillage due to difficulties controlling weeds. Crop rotation is often put forward as the best means to manage weeds in reduced tillage systems, but uncertainties remain around how different tillage practices and crop rotations interact. Here, we assess the effects of four different tillage practices on weed seedbank density and composition in wheat (Triticum aestivum) monoculture (WWWW), and two different rotations, wheat-medic-wheat-medic (annual medic, Medicago spp.; WMWM), and wheat-canola-wheat-lupin (Brassica napus, Lupinus spp.; WCWL). We use data across a whole four-year rotation period from a long-term experiment replicated at two sites in South Africa's winter rainfall region. The four tillage practices assessed follow a gradient of soil disturbance: conventional tillage (CT, soil inversion through ploughing), minimum tillage (MT, shallow soil loosening), no tillage (NT, direct drilling with tine openers) and zero tillage (ZT, direct drilling with disc openers)

Moving Away From Limiting Similarity During Restoration: Timing of Arrival and Native Biomass Are Better Proxies of Invasion Suppression in Grassland Communities

A challenge in many restoration projects, in particular when establishing de novo communities, is the arrival and later dominance of invasive alien plants. This could potentially be avoided by designing invasion-resistant native communities. Several studies suggest achieving this by maximizing trait similarity between natives and potential invaders (“limiting similarity”), but evidence supporting this approach is mixed so far. Others pose that the relative time of arrival by native and invasive species (“priority effects”) could play a stronger role, yet this factor and its interaction with trait similarity is not fully understood in the context of ecological restoration. Thus, we hypothesized that multi-trait similarity would increase suppression of invasive species by native communities, and that the effect would be stronger when natives arrive first. We established two distinct communities of native central European grassland species based on native–invasive trait similarity, and then tested the introduction of invasive Ambrosia artemisiifolia and Solidago gigantea separately when arriving in the native communities at two times, i.e., sown either at the same time as the natives or 2 weeks after. For the traits selected, our data did not provide evidence for a limiting similarity effect, but rather supported priority effects. Both native communities more effectively suppressed invaders that arrived after the natives. In addition, the native community that produced the most biomass suppressed both invasive species more than the most ecologically similar community. This effect of biomass revealed that prioritizing native–invader ecological similarity can fail to account for other community characteristics that affect invasion resistance, such as biomass. Instead, native communities could be designed to enhance priority effects through the inclusion of early and fast developing species. We conclude that native community composition plays a significant role in the establishment success by invasive species, and resource pre-emption seems more significant than trait similarity. In terms of grassland restoration, native species should be selected based on plant traits related to fast emergence and early competitiveness

A heteroskedastic model of park grass spring hay yields in response to weather suggests continuing yield decline with climate change in future decades

UK grasslands perform important environmental and economic functions, but their future productivity under climate change is uncertain. Spring hay yields from 1902 to 2016 at one site (the Park Grass Long Term Experiment) in southern England under four different fertiliser regimes were modelled in response to weather (seasonal temperature and rainfall). The modelling approach applied comprised: (1) a Bayesian model comparison to model parametrically the heteroskedasticity in a Gamma likelihood function; (2) a Bayesian varying intercept multiple regression model with an autoregressive lag one process (to incorporate the effect of productivity in the previous year) of the response of hay yield to weather from 1902 to 2016. The model confirmed that warmer and drier years, specifically autumn, winter and spring, in the 20th and 21st centuries reduced yield. The model was applied to forecast future spring hay yields at Park Grass under different climate change scenarios (HadGEM2 and GISS RCP 4.5 and 8.5). This application indicated that yields are forecast to decline further between 2020 and 2080, by as much as 48-50%. These projections are specific to Park Grass, but implied a severe reduction in grassland productivity in southern England with climate change during the 21st century