On the stability of mixed grasslands

<p>Recent years have seen a renewed interest in the use of white clover (Trifolium repens) in grasslands, as a more sustainable alternative to fertiliser nitrogen inputs. However, mixtures of grasses and white clover have frequently been associated with unstable and hence unreliable herbage yields. The maintenance of a stable production from mixed grasslands requires the prototyping of new grassland management strategies.</p><p>Temporal yield fluctuations may result from intrinsic ecosystem fluctuations, from environmental fluctuations, or from both. The stability of ecosystems in isolation of their environment is defined as the intrinsic stability. The stability of systems subjected to seasonal environmental fluctuation only is defined as the extrinsic stability, while the stability of systems under a regime of both seasonal and stochastic environmental fluctuations, as measured in field experiments, is defined as the actual stability. The actual stability depends on coincidental weather events and is hence an unreliable reflection of the grassland ecosystem or of the management imposed.</p><p>A mathematical framework is presented with which the extrinsic and intrinsic stability levels of grassland ecosystems can be deducted from their yields during a large number of years, and with which the effects of intrinsic system properties and of environmental fluctuations on the stability of their yields can be segregated. Intrinsically stable systems remain stable in the face of seasonal environmental fluctuations, but are destabilised by stochastic environmental events. Intrinsically unstable systems are instead stabilised by environmental seasonality, and may be further destabilised or stabilised by stochastic fluctuations, depending on the timing of individual environmental events.</p><p>This framework was applied to the yield data of two long-term experiments. Regular lime applications and a grazing regime increased the extrinsic stability of grassland ecosystems, whereas fertiliser treatments only had small or inconsistent effects. It was consequently hypothesised that the extrinsic stability of ecosystems depends largely on the availability and turnover rate of nutrients.</p><p>Vertical spatial interactions between grass species and white clover depend on the vertical distributions of their lamina and total leaf material. A mechanistic model is presented, with which the distributions of perennial ryegrass (Lolium perenne) and white clover can be accurately simulated for a wide range of sward types subjected to various management regimes. This model uses simple morphological features of both species as input variables.</p><p>The lateral spatial heterogeneity of mixed grasslands was quantified, using data generated by the Dry Weight Rank method for botanical assessment of grasslands. The heterogeneity of the total herbage mass was highest after cutting and topping events, and under lenient grazing, and lowest under strip-grazing. Opposite responses to the grassland management were found for the heterogeneity of the white clover herbage mass.</p><p>The performance of white clover may be compromised by infestations of creeping bentgrass (Agrostis stolonifera). A mathematical methodology, based on the Dry Weight Rank method, is presented with which the effects of niche-differentiation and direct competition between white clover and creeping bentgrass can be discriminated. The exclusion of white clover by creeping bentgrass foremost resulted from direct competition. Both species only showed niche-differentiation in response to two extreme management strategies. Whereas the clover performance was enhanced under a permanent cutting regime, creeping bentgrass prevailed under a regime of lenient grazing.</p><p>Finally, two types of management strategies were proposed to maintain the production stability of mixed grasslands. Structural management strategies, such as lime applications, the maximisation of the grazed area, and the targeted use of mixtures of grassland species and varieties, aim to increase the extrinsic stability of ecosystems, and can be prototyped. Dynamic management strategies are required to counteract the fluctuations of the abiotic and the biotic environment, and cannot be generalised due to the individuality of each farm ecosystem.</p><p><strong>Additional key-words:</strong> stability, variability, ecosystem, grassland, white clover, Trifolium repens, legumes, mixtures, model, competition, methodology, dry weight rank, organic farming.</p&gt