A kānuka silvopastoral system in New Zealand hill country : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Manawatū, New Zealand
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2023
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Massey University
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Abstract
Soil erosion, water quality issues, low production and climate change are some of the current challenges facing land managers and farmers in New Zealand hill country. ‘Tree-pasture’ silvopastoral systems that build soil resources could be integral land management practices for mitigating these issues and improving the health and production of these systems. Silvopastoral trees are already planted in New Zealand, although primarily used as soil conservation trees. Nevertheless, there are many other potentially facilitative effects of silvopastoral systems on other under researched silvopastoral outcomes. Researching these is vital for realising the full potential of silvopastoralism in New Zealand.
The native genus kānuka (Kunzea spp.) in New Zealand has the potential to form intergenerational and multifunctional silvopastoral systems that build soil resources and positively impact pasture production. This is because of the genus’s potentially advantageous bio-physical tree attributes, such as its longevity, potentially reduced competition for soil water and nutrients compared to faster-growing and more resource intensive trees typically planted in hill country, and evergreen nature, potentially influencing livestock behaviour and soil organic matter return to the soil. Despite being locally very common in New Zealand hill country, this study is the first to measure the influence of kānuka silvopastoral trees on the pastoral environment at field scale.
The study begins by presenting a novel framework that links bio-physical tree attributes to a wide range of silvopastoral outcomes. Poplar (Populus spp.), the most commonly planted soil conservation tree in New Zealand hill country, and kānuka, are then reviewed as silvopastoral trees within this framework. This process clearly conveyed the complexity of silvopastoral systems and highlights that there may be potential for kānuka to positively impact many silvopastoral outcomes such as longevity, pasture production, livestock welfare, biodiversity conservation and carbon sequestration.
The study then investigated the impact of kānuka on pasture production and pasture stability, soil condition and surface runoff and sediment and nutrient losses within a kānuka silvopastoral system. At two sites over two years, there was on average 107.9% more pasture production under kānuka trees compared to open pasture. This pasture production increase was associated with significantly greater Olsen-phosphorus, potassium and porosity. Soil moisture was similar between kānuka pasture and open pasture positions. The improvements to the agricultural environment were hypothesised to be because of livestock excreta deposition under the trees in the sheltered tree environment and tree litterfall.
The increased pasture production under the trees was the result of trees facilitating the growth of a few dominant and competitor pasture functional groups via the mass ratio effect such as perennial ryegrass (Lolium perenne), cocksfoot (Dactylis glomerata), soft brome (Bromus hordeaceus) and barley grass (Critesion murinum). Moreover, despite reduced species richness and functional richness in kānuka pasture, there was evidence that pasture stability was maintained under the trees because functional evenness and functional dispersion was statistically similar in kānuka pasture and open pasture, and the functional groups that grew had mixed (cocksfoot) or annual (soft brome and barley grass) survival strategies. This indicates that kānuka has the potential to increase pasture production sustainably by not negatively impacting the pasture’s response to stress.
There was 53.8 mm annual surface runoff in kānuka pasture and 7.5 mm in open pasture, despite the improved soil conditions in kānuka pasture. Moreover, sediment and nutrient losses were 10–100 times greater in kānuka pasture. Sediment and nutrient losses were a function of surface runoff, and these differences were hypothesised to be because significantly less pasture biomass was present under the trees, decreasing surface runoff attenuation. The pasture biomass difference was likely because of livestock preferentially grazing the pasture under kānuka because of the sheltered environment and good condition pasture. This suggests that a choice between good condition pasture under trees and poor condition pasture away from trees can lead to negative impacts in terms of sediment and nutrient management under isolated silvopastoral trees.
Overall, this study shows that tree configuration is a fundamental aspect in silvopastoral systems, and gives evidence that pasture biomass under silvopastoral trees is important for mitigating surface runoff and sediment and nutrient losses. The improved pasture production and pasture species composition under kānuka, in conjunction with the other potential environmental and cultural benefits of a kānuka silvopastoral system identified in the framework, shows that this genus may have potential to transform hill country landscapes by adding economic, environmental and cultural value to New Zealand farms. Nevertheless, because of the limitations of this study, such as the potential impact of site specific conditions and compounded livestock effects, more research is required to provide a full evaluation of the potential of kānuka silvopastoral systems in New Zealand hill country.
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Silvopastoral systems, Hill farming, Kunzea, New Zealand