Pursuing pasture tolerance and resilience through species with different functional traits and soil-plant-water interactions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatu, New Zealand

Abstract

Higher stability, persistence and yield can be achieved through increasing the biodiversity of pasture plants. The combination of species with different functional traits confers niche differentiation (e.g. different root depth). Otherwise species compete for the same resources in the same location and time. In diverse pasture, agricultural needs should overlap between species, enhancing species survival during critical periods. Productive ecosystems with low complexity (low plant functional diversity) show more the negative impacts of climate change, being less stable through the stress periods. 𝘉𝘳𝘰𝘮𝘶𝘴 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 Phil. is regarded as having high potential for grazing systems, due to its high yield and good nutritive quality. It also has high tolerance to periods of soil water restriction, maintaining a higher growth rate during summer in comparison to 𝘓𝘰𝘭𝘪𝘶𝘮 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 L. Alongside 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦, 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 co-dominates permanent perennial pastures in the South of Chile, indicating that it is a good competitor in mixed cool-temperate pastures. However, key parameters associated with recovery from defoliation, such as watersoluble carbohydrate (WSC) reserves, growth rate, tiller number and persistence, are unknown for 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴. Therefore, the first step in this thesis was to determine these defoliation criteria in relation to similar defoliation criteria of other highly productive species, 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦e and 𝘗𝘭𝘢𝘯𝘵𝘢𝘨𝘰 𝘭𝘢𝘯𝘤𝘦𝘰𝘭𝘢𝘵𝘢, to determine if and when any overlap occurred. This thesis consisted of three main experiments, which cover the physiological, morphological and competitive traits of 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴. The first experiment was designed to determine a theoretical optimum defoliation interval for 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴, and it was concluded that defoliation at leaf stage 4 (LS-4) was the optimum defoliation in terms of highest shoot and root growth rates, and accumulation of WSC. The second experiment was designed to determine the resilience and tolerance between monocultures and mixtures of 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴, 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 and 𝘗. 𝘭𝘢𝘯𝘤𝘦𝘰𝘭𝘢𝘵𝘢. All three species were defoliated when 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 reached LS-4, which coincided with approximately 3.5 regrowth leaves/tiller for 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦, and over 6 leaves/plant for 𝘗. 𝘭𝘢𝘯𝘤𝘦𝘰𝘭𝘢𝘵𝘢. Measurements included biomass production across critical periods, botanical composition, physiological response against water stress (waterlogging and soil water restriction) and water uptake at different depths, and it was concluded that a more diverse pasture (𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 + 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 + 𝘗. 𝘭𝘢𝘯𝘤𝘦𝘰𝘭𝘢𝘵𝘢) maintained higher biomass under soil water restriction and also had a more effective water uptake from the soil profile. The third experiment was designed to determine the tiller population dynamics, photosynthetic carbon fixation capacity (PCFC) and competitive ability of 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 in relation to 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦. It was concluded that 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 was a better competitor than 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴, however, 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 was able to recover its tiller population during a period of soil water restriction and reached a full recovery at the end of the experimental period. Also, a 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 + 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 mixed pasture had the highest values for PCFC during the waterlogging and soil water restriction periods. Pasture plants such as 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 and 𝘗. 𝘭𝘢𝘯𝘤𝘦𝘰𝘭𝘢𝘵𝘢 can access water from deeper in the soil than 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦, having a direct effect on their physiological traits. Water accessibility (root depth) plays a key role in maintaining their photosynthesis, production, and improving their survival, during periods of soil water restriction, relative to 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦. On the other hand, 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 tolerate waterlogging and maintain a relatively high growth rates during winter. Pasture establishment and performance, in a climate with dry and wet seasons and in soils with a high percentage of silt and/or clay (low gas permeability), is related to species tolerance to drought and waterlogging conditions. Thus, increasing species diversity is a good strategy that confers stability to the pastoral ecosystem, especially when global warming has enhanced droughts and unpredictable rain events. Mixtures of 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 + 𝘛. 𝘳𝘦𝘱𝘦𝘯𝘴 + 𝘗. 𝘭𝘢𝘯𝘤𝘦𝘰𝘭𝘢𝘵𝘢 or 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 + T. repens + 𝘉. 𝘷𝘢𝘭𝘥𝘪𝘷𝘪𝘢𝘯𝘶𝘴 can reach higher growth rates during water restriction periods, in comparison to 𝘓. 𝘱𝘦𝘳𝘦𝘯𝘯𝘦 + 𝘛. 𝘳𝘦𝘱𝘦𝘯𝘴 pastures, along with relatively high growth rates during winter. Therefore, combining species with the aim of complementary resources uptake, and depending on the contribution of each species within the pasture, will change the seasonal herbage growth rate under the stress periods. However, to keep a great contribution of the desirables pasture species it is essential to use a defoliation criterion that allow them to replenish its water soluble carbohydrates, only then, the persistence, survival and yield of the pasture and the desirables species within it will be maximized.