This thesis investigates landscape disturbance history in Westland since 1350 AD. Specifically, I test the hypothesis that large-magnitude regional episodes of natural disturbance have periodically devastated portions of the landscape and forest, and that these were caused by infrequent earthquakes along the Alpine Fault.
Forest stand history reconstruction was used to determine the timing and extent of erosion and sedimentation events that initiated new forest cohorts in a 1412 ha study area in the Karangarua River catchment, south Westland. Over 85 % of the study area was disturbed sufficiently by erosion/sedimentation since 1350 AD to initiate new forest cohorts. During this time four episodes of catchment-wide disturbance impacted the study area, and these took place about 1825 AD ± 5 years (Ruera episode), 1715 AD ± 5 years (Sparkling episode), 1615 AD ± 5 years (McTaggart episode), and 1445 AD ± 15 years (Junction episode). The three most recent episodes disturbed 10 %, 35-40 % and 32-50 % respectively of the study area. The Junction episode disturbed at least 6 % of the study area, but elimination of evidence by more recent disturbances prevented an upper limit being defined.
The three earliest episodes correspond to the date-ranges for three Alpine Fault earthquakes from geological data, and are the only episodes of disturbance within each date-range. An earthquake cause is also consistent with features of the disturbance record: large portions of the study area were disturbed, disturbance occurred on all types 'of landforms, and terrace surfaces were abandoned upstream of the Alpine Fault. On this basis erosion/sedimentation induced by Alpine Fault earthquakes has disturbed 14-20 % of the land surface in the study area per century. Storms and other non-seismic erosional processes have disturbed 3-4 % per century.
To examine the importance of the Alpine Fault earthquakes to forest disturbance throughout Westland, I collated all available data on conifer stand age structures in the region and identified dates of disturbance events from 55 even-aged cohorts of trees. Three region-wide episodes of forest disturbance since 1350 AD were found in this sample, and these matched the three Alpine Fault earthquake-caused episodes found in the Karangarua. Forest disturbance at these times was widespread across Westland over at least 200 km from Paringa to Hokitika, and originated from both tree fall and erosion processes. This disturbance history can explain the long-observed regional conifer forest pattern in Westland, of a predominance of similar-sized stands of trees and a relative lack of small-sized (young) stands. The many similar-sized stands are a consequence of synchronous forest disturbance and re-establishment accompanying the infrequent Alpine Fault earthquakes, while the dominance of mature stands of trees and relative lack of young small-sized trees in stands is explained by the long lapsed time since the last Alpine Fault earthquake (c. 280 years).
I applied the landscape disturbance history information to the existing geological data to reconstruct the paleoseismicity of the Alpine Fault since 1350 AD. Best estimates for the timing of the most recent three rupture events from these data are 1715 AD ± 5 years, 1615 AD ± 5 years and 1445 AD ± 15 years. Earthquake recurrence intervals were variable, ranging from about 100 years to at least 280 years (the lapsed time since the last event). All three events caused forest and geomorphic disturbance over at least a 200 km section of Fault between the Karangarua and Hokitika Rivers, and were probably single rupture events. Suppressions in cross dated tree-ring chronologies in the western South Island suggest that the last rupture occurred in 1717 AD, and extended as a single rupture from Haupiri to Fiordland, a distance along the Fault of 375 km.
Mixed conifer, beech and hardwood forests are relatively common in Aotearoa/New
Zealand, but are not well studied. This thesis investigates the coexistence, regeneration
dynamics and disturbance history of a mixed species forest across an environmental
gradient of drainage and soil development in north Westland.
The aim was to investigate whether conifers, beech and non-beech hardwood species were
able to coexist on surfaces that differed in their underlying edaphic conditions, and if so to understand the mechanisms that influenced their regeneration on both poorly drained and
well drained soils. The site selected was an area of high tree species diversity on a lowland
0.8 km² post-glacial terrace at the base of Mount Harata in the Grey River Valley.
My approach was to use forest stand history reconstruction at two spatial scales: an
intensive within-plot study of stand dynamics (chapter 1) and a whole-landform approach
(chapter 2) that examined whether the dynamics identified at the smaller within-plot scale
reflected larger patterns across the terrace.
In chapter 1, three large permanent plots (0.3-0.7 ha) were placed at different points along
the drainage gradient, one plot situated in each of the mainly well-drained, poorly drained
and very poorly drained areas along the terrace. Information was gathered on species age
and size structures, spatial distributions of tree ages, species interactions, microsite
establishment preferences, patterns of stand mortality, and disturbance history in each plot.
There were differences in stand structure, composition and relative abundance of species
found between the well drained plot and the two poorer drained plots. On the well drained
site conifers were scarce, the beeches Nothofagus fusca and N. menziesii dominated the
canopy, and in the subcanopy the hardwood species Weinmannia racemosa and Quintinia
acutifolia were abundant. As drainage became progressively poorer, the conifers
Dacrydium cupressinum and Dacrycarpus dacrydioides became more abundant and
occupied the emergent tier over a beech canopy. The hardwoods W. racemosa and Q.
acutifolia became gradually less abundant in the subcanopy, whereas the hardwood
Elaeocarpus hookerianus became more so.
In the well drained plot, gap partitioning for light between beeches and hardwoods enabled
coexistence in response to a range of different sized openings resulting from disturbances
of different extent. In the two more poorly drained plots, species also coexisted by
partitioning microsite establishment sites according to drainage.
There were several distinct periods where synchronous establishment of different species
occurred in different plots, suggesting there were large disturbances: c. 100yrs, 190-200
yrs, 275-300 yrs and 375-425 yrs ago. Generally after the same disturbance, different
species regenerated in different plots reflecting the underlying drainage gradient. However,
at the same site after different disturbances, different sets of species regenerated,
suggesting the type and extent of disturbances and the conditions left behind influenced
species regeneration at some times but not others. The regeneration of some species (e.g.,
N. fusca in the well-drained plot, and Dacrydium in the poorer drained plots) was periodic
and appeared to be closely linked to these events. In the intervals between these
disturbances, less extensive disturbances resulted in the more frequent N. menziesii and
especially hardwood regeneration. The type of tree death caused by different disturbances
favoured different species, with dead standing tree death favouring the more shade-tolerant
N. menziesii and hardwoods, whereas uprooting created a mosaic of microsite conditions
and larger gap sizes that enabled Dacrycarpus, N. fusca and E. hookerianus to maintain
themselves in the poorly drained areas.
In chapter 2, 10 circular plots (c. 0.12 ha) were placed in well drained areas and 10
circular plots (c. 0.2 ha) in poorly drained plots to collect information on species
population structures and microsite preferences. The aims were to reconstruct species'
regeneration responses to a range of disturbances of different type and extent across the
whole terrace, and to examine whether there were important differences in the effects of
these disturbances.
At this landform scale, the composition and relative abundances of species across the
drainage gradient reflected those found in chapter 1. There were few scattered conifers in well drained areas, despite many potential regeneration opportunities created from a range
of different stand destroying and smaller scale disturbances.
Three of the four periods identified in chapter 1 reflected distinct terrace-wide periods of
regeneration 75-100 yrs, 200-275 yrs and 350-450 yrs ago, providing strong evidence of
periodic large, infrequent disturbances that occurred at intervals of 100-200 yrs. These
large, infrequent disturbances have had a substantial influence in determining forest
history, and have had long term effects on forest structure and successional processes.
Different large, infrequent disturbances had different effects across the terrace, with the
variability in conditions that resulted enabling different species to regenerate at different
times. For example, the regeneration of distinct even-aged Dacrydium cohorts in poorly
drained areas was linked to historical Alpine Fault earthquakes, but not to more recent
storms. The variation in the intensity of different large, infrequent disturbances at different
points along the environmental drainage gradient, was a key factor influencing the scale of
impacts. In effect, the underlying edaphic conditions influenced species composition along
the drainage gradient and disturbance history regulated the relative abundances of species.
The results presented here further emphasise the importance of large scale disturbances as a
mechanism that allows coexistence of different tree species in mixed forest, in particular
for the conifers Dacrydium, Dacrycarpus and the beech N. fusca, by creating much of the
environmental variation to which these species responded. This study adds to our
understanding of the effects of historical earthquakes in the relatively complex forests of
north Westland, and further illustrates their importance in the Westland forest landscape as
the major influential disturbance on forest pattern and history.
These results also further develop the 'two-component' model used to describe
conifer/angiosperm dynamics, by identifying qualitative differences in the impacts of
different large, infrequent disturbances across an environmental gradient that allowed for
coexistence of different species. In poorer drained areas, these forests may even be thought
of as 'three-component' systems with conifers, beeches and hardwoods exhibiting key
differences in their regeneration patterns after disturbances of different type and extent, and
in their microsite preferences.
Liquefaction features and the geologic environment in which they formed were carefully studied at two sites near Lincoln in southwest Christchurch. We undertook geomorphic mapping, excavated trenches, and obtained hand cores in areas with surficial evidence for liquefaction and areas where no surficial evidence for liquefaction was present at two sites (Hardwick and Marchand). The liquefaction features identified include (1) sand blows (singular and aligned along linear fissures), (2) blisters or injections of subhorizontal dikes into the topsoil, (3) dikes related to the blows and blisters, and (4) a collapse structure. The spatial distribution of these surface liquefaction features correlates strongly with the ridges of scroll bars in meander settings. In addition, we discovered paleoliquefaction features, including several dikes and a sand blow, in excavations at the sites of modern liquefaction. The paleoliquefaction event at the Hardwick site is dated at A.D. 908-1336, and the one at the Marchand site is dated at A.D. 1017-1840 (95% confidence intervals of probability density functions obtained by Bayesian analysis). If both events are the same, given proximity of the sites, the time of the event is A.D. 1019-1337. If they are not, the one at the Marchand site could have been much younger. Taking into account a preliminary liquefaction-triggering threshold of equivalent peak ground acceleration for an Mw 7.5 event (PGA7:5) of 0:07g, existing magnitude-bounded relations for paleoliquefaction, and the timing of the paleoearthquakes and the potential PGA7:5 estimated for regional faults, we propose that the Porters Pass fault, Alpine fault, or the subduction zone faults are the most likely sources that could have triggered liquefaction at the study sites. There are other nearby regional faults that may have been the source, but there is no paleoseismic data with which to make the temporal link.
Knowledge of past climate variability is essential for understanding present and future climate trends. This study used Halocarpus biformis (pink pine) ring-width chronologies to investigate palaeotemperature history in Westland, New Zealand. The ensuing reconstruction is among the longest palaeoseries produced for New Zealand to date. It is in good agreement with other tree-ring-based records, and with instrumental (both local and hemispheric) data.
Thirteen pink pine chronologies were developed. Ring-width measurements were detrended using the Regional Curve Standardisation method to retain as much low-frequency variance as possible. Crossdating revealed the existence of a strong common signal among trees. Inter-site comparison indicated that a common control mechanism affected tree growth not only within sites, but also across sites.
To determine whether climate was the main factor that controlled the growth of pink pine in Westland, correlation and response function analyses were employed. Temperature, precipitation and the Southern Oscillation Index were tested for their relationship with tree growth. Mean monthly temperature was identified as the primary growth-limiting factor. Chronologies were positively correlated with temperature over an extended period (5-17 months), and climate response modelling showed that temperature explained 11-60% variance in the tree-ring data. The highest and most stable correlations occurred between tree growth and summer (January-March) temperatures.
Tree-ring data from the six sites that contained the strongest temperature signal were combined, and the Westland Regional Chronology (WRC) was developed. The WRC was then used to reconstruct January-March temperatures back to A.D. 1480. The calibration model explained 43% of the variance in temperature, and all calibration and verification tests were passed at high levels of significance. The reconstruction showed that temperatures in Westland have been following a positive trend over the last 520 years. The coolest 25-year period was 1542-1566, while temperatures reached their maximum in 1966-1990. Spectral analysis of the Westland palaeotemperature record revealed cycles at periods of about 3, 5-6, 11, 14, 22, 45 and 125 years.
This study also confirmed that climate response is species-dependent. A separate exercise, which compared two species from the same site, demonstrated that while pink pine's growth was mainly influenced by summer temperatures, Libocedrus bidwillii was affected by conditions at the beginning of the growing season. However, the temperature signal in Westland's Libocedrus bidwillii was insufficient to produce a reliable reconstruction. It might be because the climate signal in this species was obscured by disturbances, as was shown in the final section of this project. Frequent growth releases and suppressions implied that Libocedrus bidwillii integrated both major (Alpine Fault earthquakes) and minor (windthrow) disturbances in its ring widths. Pink pine, on the other hand, was not sensitive to disturbance, and was therefore a better indicator of palaeotemperatures in Westland.
This research has strengthened the New Zealand network of chronology sites, and confirmed that pink pine has great dendroclimatic value. The last 520 years of temperature fluctuations were reconstructed with a high degree of fidelity - the model developed in this thesis is currently the most accurate estimate of a temperature-growth relationship in the country.
