title: Community composition, assembly and priority effects in a volcanically disturbed alpine herbfield, Mount Taranaki, New Zealand

authors: Moore, Theresa Grace

subjects: alpine flora

abstract: The New Zealand alpine flora is highly diverse, having evolved extensively over only c. 2 Ma. Examining how communities assemble and the effects of historical contingency may provide evidence for the mechanisms of rapid diversification of the New Zealand alpine flora. Evolutionary priority effects, in which the sequence of colonisation during community assembly can drive radiation of early arriving clades, has been tested in one South Island alpine community. The results showed genera belonging to older evolutionary lineages were more dominant and diverse in those communities, supporting the presence of priority effects.
Mount Taranaki in the North Island is geologically the youngest alpine zone in New Zealand, and supports a depauperate assemblage of alpine species compared to other New Zealand mountains. Mount Taranaki has had approximately 10 moderately sized eruptions in the last 500 years and is separated from other New Zealand alpine regions by 130 km. The present study sought to determine how the vegetation in the alpine herbfield of Mount Taranaki has been affected by various eruptions, and to test if priority effects have influenced those communities.
To quantify community composition in the Taranaki alpine herbfield, estimates of percent cover from 55 (2 x 2 m) plots were used to calculate relative species abundance at three sites within an altitudinal range of 1450–1650 m above sea level. Study sites were located at North Egmont, East Egmont and Fantham’s Peak. Multivariate analysis using non-metric multidimensional scaling ordination of relative abundances was used to compare variations in composition between sites and elevations. To test for priority effects, time-calibrated molecular phylogenies of clade divergence (as age estimates) were compared to the relative abundances of genera within the plots using linear regression. 
The results showed significant variation of species composition between sites, which correlates with the spatial pattern of the AD 1655 Burrell eruption. The eruption was directed toward East Egmont, and the plots there showed considerably lower species diversity compared to the other two sites. In addition, the inconsistency of species distribution between the sites indicated a lack of community convergence, suggesting herbfield vegetation is still in the early ages of assembly following eruptions. The test of priority effects showed no age-related abundance or richness relationships, therefore priority effects are unlikely to be contributing to community assembly in the Taranaki alpine herbfield. Instead, assembly appears to be influenced by the geological age and frequency of eruptions of Mount Taranaki, and its isolation from other alpine systems. 
This is the first study to determine the effects of volcanic eruptions on the herbfield vegetation of Mount Taranaki, and it is recommended that plots be re-measured in the future to determine how those communities develop. Because no priority effects were evident on Mount Taranaki, further research on a range of older volcanic and non-volcanic alpine systems in the North Island would be valuable. This would further contribute to our understanding of how communities assemble and if and when priority effects become significant.

text: Community composition, assembly and priority effects in a volcanically disturbed alpine herbfield, Mount Taranaki, New Zealand The New Zealand alpine flora is highly diverse, having evolved extensively over only c. 2 Ma. Examining how communities assemble and the effects of historical contingency may provide evidence for the mechanisms of rapid diversification of the New Zealand alpine flora. Evolutionary priority effects, in which the sequence of colonisation during community assembly can drive radiation of early arriving clades, has been tested in one South Island alpine community. The results showed genera belonging to older evolutionary lineages were more dominant and diverse in those communities, supporting the presence of priority effects.
Mount Taranaki in the North Island is geologically the youngest alpine zone in New Zealand, and supports a depauperate assemblage of alpine species compared to other New Zealand mountains. Mount Taranaki has had approximately 10 moderately sized eruptions in the last 500 years and is separated from other New Zealand alpine regions by 130 km. The present study sought to determine how the vegetation in the alpine herbfield of Mount Taranaki has been affected by various eruptions, and to test if priority effects have influenced those communities.
To quantify community composition in the Taranaki alpine herbfield, estimates of percent cover from 55 (2 x 2 m) plots were used to calculate relative species abundance at three sites within an altitudinal range of 1450–1650 m above sea level. Study sites were located at North Egmont, East Egmont and Fantham’s Peak. Multivariate analysis using non-metric multidimensional scaling ordination of relative abundances was used to compare variations in composition between sites and elevations. To test for priority effects, time-calibrated molecular phylogenies of clade divergence (as age estimates) were compared to the relative abundances of genera within the plots using linear regression. 
The results showed significant variation of species composition between sites, which correlates with the spatial pattern of the AD 1655 Burrell eruption. The eruption was directed toward East Egmont, and the plots there showed considerably lower species diversity compared to the other two sites. In addition, the inconsistency of species distribution between the sites indicated a lack of community convergence, suggesting herbfield vegetation is still in the early ages of assembly following eruptions. The test of priority effects showed no age-related abundance or richness relationships, therefore priority effects are unlikely to be contributing to community assembly in the Taranaki alpine herbfield. Instead, assembly appears to be influenced by the geological age and frequency of eruptions of Mount Taranaki, and its isolation from other alpine systems. 
This is the first study to determine the effects of volcanic eruptions on the herbfield vegetation of Mount Taranaki, and it is recommended that plots be re-measured in the future to determine how those communities develop. Because no priority effects were evident on Mount Taranaki, further research on a range of older volcanic and non-volcanic alpine systems in the North Island would be valuable. This would further contribute to our understanding of how communities assemble and if and when priority effects become significant.

year: 2017

title: Assessing dredge spoil dispersion on the inner shelf off the coast of Tauranga Harbour, Aotearoa New Zealand

authors: Blackler, Charlotte

abstract: Dredging began in Te Awanui/Tauranga Harbour, the largest port in Aotearoa New Zealand, in 1919 to allow continued development of the port. The dredged material gets disposed of by depositing it into open water in a process known as spoiling. It is more important than ever to understand the impact of dredging as ships are getting ever larger, which require deeper and wider channels for safe passage into ports. This thesis investigated the composition and distribution of the dredge spoil on the Bay of Plenty inner shelf for the first time since 1992. Between August 2023 and February 2024, I used seismic and sedimentological analyses, including shallow seismic surveys, vibracoring, and surficial sediment sampling to determine the size and shape, and the grain size and organic content comprising the dredge spoil mound. Results showed that the sediments are predominantly fine to medium-grained moderately sorted sand with low organic content. The dredge spoil mound sits on average 1.5 m above the pre-deposition seafloor, and has no discernible surface texture or bedforms. This study establishes that the composition of the dredged sediment remains comparable to that of the pre-deposition seafloor. The results are compatible with past studies, suggesting that dredge spoil deposited on the inner shelf is stable, and is not subject to resuspension and transport under fair weather conditions.

text: Assessing dredge spoil dispersion on the inner shelf off the coast of Tauranga Harbour, Aotearoa New Zealand Dredging began in Te Awanui/Tauranga Harbour, the largest port in Aotearoa New Zealand, in 1919 to allow continued development of the port. The dredged material gets disposed of by depositing it into open water in a process known as spoiling. It is more important than ever to understand the impact of dredging as ships are getting ever larger, which require deeper and wider channels for safe passage into ports. This thesis investigated the composition and distribution of the dredge spoil on the Bay of Plenty inner shelf for the first time since 1992. Between August 2023 and February 2024, I used seismic and sedimentological analyses, including shallow seismic surveys, vibracoring, and surficial sediment sampling to determine the size and shape, and the grain size and organic content comprising the dredge spoil mound. Results showed that the sediments are predominantly fine to medium-grained moderately sorted sand with low organic content. The dredge spoil mound sits on average 1.5 m above the pre-deposition seafloor, and has no discernible surface texture or bedforms. This study establishes that the composition of the dredged sediment remains comparable to that of the pre-deposition seafloor. The results are compatible with past studies, suggesting that dredge spoil deposited on the inner shelf is stable, and is not subject to resuspension and transport under fair weather conditions.

year: 2024-10-17

title: The role of mānuka (Leptospermum scoparium) riparian plantings in the mitigation of diffuse agricultural nitrogen

authors: Adamson, Olivia María

subjects: Manuka

abstract: The disruption of nutrient cycles in agricultural settings, particularly pastoral farming, is responsible for up to 70% of the nitrogen (N) loads entering streams in New Zealand (NZ) — resulting in the widespread degradation of freshwater ecosystems. Riparian plantings are one strategy to mitigate the losses of N from land to water, removing N by denitrification and plant uptake. Some types of vegetation are more effective than others at intercepting N, due to their ecology, impact on soil quality and root exudates. Mānuka (Leptospermum scoparium) is a plant native to NZ, and the species could be a good candidate to mitigate N losses due its ability to tolerate high N loads and co-benefits such as farm diversification through apiculture or essential oil production. Previous work also suggests that L. scoparium could be a biological nitrification inhibitor (BNI), limiting nitrate (NO₃⁻) production in soil.

The aim of this study was to investigate the potential of mānuka to intercept and remove N in an experimental riparian buffer in Lake Waikare. Currently, the lake is eutrophic due to high nutrient and sediment inputs. Significant restoration efforts have been made in recent years, led by many hapuu around the lake. All are very passionate and determined to recover the mauri of this ecosystem and reconnect the people to the lake, as well as provide economic opportunities. The riparian band had been established for four years and was set up as a series of experimental plots with different vegetation types, including plots solely in mānuka, and grassed controls. The plots are on a Perch- Gley Ultic Soil underlain by a slowly permeable clay layer— the Hamilton Ash. Further, the study sought to explore the relationship between soil physical properties and N cycling, and to identify influences expected by the local hydrology— as there was evidence of perching and lateral flow in the site. A series of suction-cup lysimeters were installed and pore water was sampled seven times between May and July 2021. Samples were analysed for total N (TN), total Kjeldahl N (TKN), NO₃⁻ and ammonium (NH₄⁺).

The results show that the riparian buffer is effectively removing N from subsurface flows, with TN declining from an average of 9.32 mg/l at 1 m into the buffer to 2.03 mg/l at 7 m. Although N concentrations were higher under mānuka, the total amount of N extracted from those plots was 21% less due to high rainfall interception by the canopy (63.9%), limiting the transport of the solute. Indicators of soil physical quality, bulk density (ρb) and macroporosity (MP), were investigated as potential causes for differences in the movement of N. There was a 17% improvement in ρb, and 38% improvement in MP relative to 2017 measurements, before the riparian band was established. No appreciable differences in soil physical properties were found between vegetation types at this early stage in the inception of the riparian buffer, although changes are likely to occur as the mānuka trees mature and the impact of roots is more developed. Shallow wells were deployed to monitor the dynamics of the water table, as there was evidence of winter perching. Dip wells demonstrated the existence of a short- lived perched water table, resulting in lateral flow, and likely impacting the degree of soil gleying in different parts of the riparian band. Water and contaminants are likely to move laterally over the Hamilton Ash towards the drain, supported by the accumulation of NO₃⁻ at the boundary with the clay layer.
Future work should be devoted to better quantifying NO₃⁻ leaching from the riparian plot by incorporating a drainage model, as well as further investigating the relative importance of N removal pathways at this site (i.e. denitrification and plant uptake)— including the potential for the lower part of the riparian band, closest to the drain, to become a hotspot for nitrous oxide (N₂O) emissions. It would also be of value to revisit this study in 4-10 years to explore how the buffer impacts soil quality and N dynamics in the subsurface as it matures.

text: The role of mānuka (Leptospermum scoparium) riparian plantings in the mitigation of diffuse agricultural nitrogen The disruption of nutrient cycles in agricultural settings, particularly pastoral farming, is responsible for up to 70% of the nitrogen (N) loads entering streams in New Zealand (NZ) — resulting in the widespread degradation of freshwater ecosystems. Riparian plantings are one strategy to mitigate the losses of N from land to water, removing N by denitrification and plant uptake. Some types of vegetation are more effective than others at intercepting N, due to their ecology, impact on soil quality and root exudates. Mānuka (Leptospermum scoparium) is a plant native to NZ, and the species could be a good candidate to mitigate N losses due its ability to tolerate high N loads and co-benefits such as farm diversification through apiculture or essential oil production. Previous work also suggests that L. scoparium could be a biological nitrification inhibitor (BNI), limiting nitrate (NO₃⁻) production in soil.

The aim of this study was to investigate the potential of mānuka to intercept and remove N in an experimental riparian buffer in Lake Waikare. Currently, the lake is eutrophic due to high nutrient and sediment inputs. Significant restoration efforts have been made in recent years, led by many hapuu around the lake. All are very passionate and determined to recover the mauri of this ecosystem and reconnect the people to the lake, as well as provide economic opportunities. The riparian band had been established for four years and was set up as a series of experimental plots with different vegetation types, including plots solely in mānuka, and grassed controls. The plots are on a Perch- Gley Ultic Soil underlain by a slowly permeable clay layer— the Hamilton Ash. Further, the study sought to explore the relationship between soil physical properties and N cycling, and to identify influences expected by the local hydrology— as there was evidence of perching and lateral flow in the site. A series of suction-cup lysimeters were installed and pore water was sampled seven times between May and July 2021. Samples were analysed for total N (TN), total Kjeldahl N (TKN), NO₃⁻ and ammonium (NH₄⁺).

The results show that the riparian buffer is effectively removing N from subsurface flows, with TN declining from an average of 9.32 mg/l at 1 m into the buffer to 2.03 mg/l at 7 m. Although N concentrations were higher under mānuka, the total amount of N extracted from those plots was 21% less due to high rainfall interception by the canopy (63.9%), limiting the transport of the solute. Indicators of soil physical quality, bulk density (ρb) and macroporosity (MP), were investigated as potential causes for differences in the movement of N. There was a 17% improvement in ρb, and 38% improvement in MP relative to 2017 measurements, before the riparian band was established. No appreciable differences in soil physical properties were found between vegetation types at this early stage in the inception of the riparian buffer, although changes are likely to occur as the mānuka trees mature and the impact of roots is more developed. Shallow wells were deployed to monitor the dynamics of the water table, as there was evidence of winter perching. Dip wells demonstrated the existence of a short- lived perched water table, resulting in lateral flow, and likely impacting the degree of soil gleying in different parts of the riparian band. Water and contaminants are likely to move laterally over the Hamilton Ash towards the drain, supported by the accumulation of NO₃⁻ at the boundary with the clay layer.
Future work should be devoted to better quantifying NO₃⁻ leaching from the riparian plot by incorporating a drainage model, as well as further investigating the relative importance of N removal pathways at this site (i.e. denitrification and plant uptake)— including the potential for the lower part of the riparian band, closest to the drain, to become a hotspot for nitrous oxide (N₂O) emissions. It would also be of value to revisit this study in 4-10 years to explore how the buffer impacts soil quality and N dynamics in the subsurface as it matures.

year: 2021