Composition, structure and restoration potential of riparian forest remnants, Hawke's Bay, New Zealand
West, M. (2021). Composition, structure and restoration potential of riparian forest remnants, Hawke’s Bay, New Zealand (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14104
Permanent Research Commons link: https://hdl.handle.net/10289/14104
Extensive modification of riparian zones across the globe has seen a reduction in the important functions and services provided by the vegetation. Maintaining water quality, sediment control, nutrient cycling, habitat provision, climate change mitigation, and increased biodiversity value are a few of the services provided by riparian vegetation. Within New Zealand, approximately 16,000 ha of native forest has been cleared in recent times. This forest loss, compounded by historical forest loss over the previous seven centuries (14 million ha as of 2002), alongside the important services, gives native forests occurring within riparian zones increased value. This research assessed the species composition, community structure, restoration potential, and restoration needs of native riparian forest remnants in the Hawke’s Bay Region of New Zealand. Eleven riparian forest remnants that were predominately comprised of native species and were greater than 0.25 ha in size were selected to be surveyed. Empirical data was collected through vegetation surveys, including 10 x 10 m RECCE plots, forest health assessments, bird counts, and visual soils assessments. Bagged regression trees were utilized to determine which explanatory variables were the most important for the species composition and community structure of the sites surveyed. Variable importance plots and partial dependence plots were used to interpret the results from the bagged regression trees. Of the 127 vascular plant species identified, 66 were native or endemic and 61 were exotic. Only one species, Kunzea robusta, was deemed threatened. Species found in the greatest number of plots included, Melicytus ramiflorus, Coprosma robusta, K. robusta, and Salix fragilis. Bagged regression tree models indicated that the six key explanatory variables for the community structure and species composition of the sites surveyed were annual rainfall, elevation, mesotopographic index, road density, slope, and distance from the river. Partial dependence plots indicated that sites with high native species richness and importance, low exotic species richness and importance, high understory density and canopy cover were those with an annual rainfall between 1050 mm and 1200 mm, an elevation above 150 m above sea level (a.s.l.), a mesotopographic index of 45 or above, a road density of 0.60 km² or below, a slope between 15° and 30°. Desirable or good species composition and community structure was associated with high native species richness and importance, low exotic species richness and importance, a high understory density, and a high canopy cover. Moderate forest health, good soil quality, and a mixture of good and poor community structure and species composition was observed across the Ngaruroro, Tutaekuri, and Tukituki Rivers. Sites or portions of a site with an annual rainfall outside of the 1050 mm to 1200 mm desirable range, that are present further than 190 m from the river’s edge, have an elevation less than 150 m a.s.l., have a mesotopographic index of greater than 45, a road density of 0.60 km² or below, and have a slope outside of the 15° to 30° range, may require more intensive restoration efforts and greater investments. Overall, sites with good current community structure and species composition were deemed to have the highest restoration potential as such sites had fewer restoration needs and would require fewer interventions and lower investments. Two of the Tukituki River sites, named TT-4 and TT-5, had the highest restoration potential of all the sites surveyed. This was largely due to the presence of diagnostic species from the ecological reference used, and the close proximity to each other and to a managed forest remnant. Sites TT-4 and TT-5 could be made a priority for restoration projects utilising more passive methods. The remaining Tukituki River site named MK-3 and one of the Ngaruroro River sites NG-6 were deemed to have the lowest restoration potential, particularly due to the lack of seed sources and poor community structure and species composition. Sites MK-3 and NG-6 could be made a priority in restoration projects utilising more active methods. Future restoration projects should aim to improve species composition and community structure. The improvements could be made by removing undesirable species such as exotic and weed species and introduce desirable native and site-appropriate species. Interventions could utilize canopy manipulation, nurse plants, and ecological references. This research contributes to the national reporting of quantitative data describing the community structure and species composition of native riparian forests in New Zealand, provides a quantitative model of the most important explanatory variables for community structure and species composition in the riparian forests surveyed, and contributes to the understanding of the restoration potential of riparian forest remnants in the region. Future studies could further explore the suitability of the restoration methods and ecological references outlined in this research, the future risk of forest clearance. 1 MK stands for the Makaretu River which feeds into the Tukituki River.
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