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
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. perenne 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.