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Floodplain and regional scale variation in earthquake effects on forests, Westland, New Zealand Louise E. Cullen H. Stewart

a c

a

b

, Richard P. Duncan , Andrew Wells & Glenn

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a

Ecology and Entomology Group, Soil, Plant and Ecological Sciences Division , Lincoln University , P.O. Box 84, Canterbury, New Zealand E-mail: b

Department of Conservation , Private Bag 701, Hokitika, New Zealand c

School of Plant Biology, Faculty of Natural and Agricultural Sciences , University of Western Australia , 35 Stirling Highway, Crawley, WA, 6009, Australia Published online: 30 Mar 2010.

To cite this article: Louise E. Cullen , Richard P. Duncan , Andrew Wells & Glenn H. Stewart (2003) Floodplain and regional scale variation in earthquake effects on forests, Westland, New Zealand, Journal of the Royal Society of New Zealand, 33:4, 693-701, DOI: 10.1080/03014223.2003.9517753 To link to this article: http://dx.doi.org/10.1080/03014223.2003.9517753

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© Journal of the Royal Society of New Zealand, Volume 33, Number 4, December 2003, pp 693-701

Floodplain and regional scale variation in earthquake effects on forests, Westland, New Zealand


Louise E. Cullen1,3, Richard P. Duncan1, Andrew Wells2, and Glenn H. Stewart1

Abstract Using existing and new tree age data, we assess spatial variation in earthquake effects in Westland at regional and floodplain scales. In Westland there have been four major episodes of forest cohort establishment, three of which appear to relate to Alpine Fault earthquakes in AD C. 1460, 1610-1620, and 1717, and the fourth of which coincides with a Fiordland earthquake in AD 1826. The AD 1826 event had its greatest effect in south Westland. In contrast, the impacts of the AD 1610-1620 and 1717 earthquakes were more extensive, with forest cohort initiation following both events occurring throughout Westland. However, these two events differed in their effects at the floodplain scale, with the AD 1610-1620 event triggering a larger aggradation event on floodplains than the AD 1717 event. Variation in earthquake effects at a regional scale most likely reflects variation in earthquake magnitude and the location of the epicentre. At a floodplain scale, variation in effect could reflect differences in both earthquake magnitude and shaking and the amount of sediment stored in upland catchments as a consequence of the time elapsed since the last event. Keywords aggradation events; Alpine Fault; earthquakes; earthquake impact; forest cohort establishment; forest disturbance; Westland INTRODUCTION The Alpine Fault is the largest active fault in New Zealand, extending for 650 km from Milford Sound in the south to Blenheim in the north (Berryman et al. 1992; Yetton et al. 1998). Much of the landscape east of the Alpine Fault is predisposed to earthquake-induced mass movements and associated floods due to a combination of steep topography, weak schist bedrock, and extreme precipitation (up to 12 000 mm yr–1) (Whitehouse 1988). The landscape on both sides of the Alpine Fault is largely forested and the effects of earthquakes can be clearly seen in these forests, which contain long-lived (>700 years), shade-intolerant trees that establish on surfaces disturbed by mass movement, flooding, or following major forest canopy collapse (Wardle 1974; Veblen & Stewart 1982; Duncan 1993; Stewart et al. 1998). Forested areas that have been affected by major disturbance typically comprise approximately even-aged groups of trees (cohorts), with the trees in a single cohort having established at about the same time in a disturbance opening (Wells et al. 1998). The age of the

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Ecology and Entomology Group, Soil, Plant and Ecological Sciences Division, P.O. Box 84, Lincoln University, Canterbury, New Zealand. Email: [email protected] 2 Department of Conservation, Private Bag 701, Hokitika, New Zealand. 3 Present address: School of Plant Biology, Faculty of Natural and Agricultural Sciences, University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia. R02017 Received 19 June 2002; accepted 28 May 2003; online publication date 19 November 2003


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oldest tree in a cohort can be used to estimate the date of the disturbance event (with adjustment for the time that trees take to colonise a newly formed opening). Using this approach, Wells et al. (1999) identified three regionally extensive episodes of massive forest and landscape disturbance in Westland during the last 700 years, centred around AD 1460, 1630, and 1715. The two most recent disturbance episodes fall within the radiocarbon date bands of the two most recent Alpine Fault earthquakes, identified independently from fault trenching. Using tree ring growth anomalies, the dates of these two most recent earthquakes have been refined to AD 1610-1620 and 1717 (Wells et al. 1999, 2001). The event centred around AD 1460 is assumed to represent a third Alpine Fault earthquake. These earthquakes will have had variable effects across the landscape due to variation in the intensity of shaking and the susceptibility of different parts of the landscape to disturbance (Vittoz et al. 2001). Our aim in this paper is to examine the effects of these earthquakes on Westland forests at two scales. First, we examine the extent of aggradation surfaces formed on floodplains following Alpine Fault earthquakes, by dating the formation of these surfaces using tree ages. We show that aggradation surfaces dating from the most recent Alpine Fault earthquake (AD 1717) are smaller in extent than surfaces dating from the penultimate earthquake (AD 1610-1620). Second, using all of the available forest cohort age data from throughout Westland, we examine the spatial distribution of earthquake-induced cohorts along c. 280 km of the Fault to look for regional scale variation in earthquake effects on vegetation. STUDY AREA Our study covers the Westland region, from Rahu Saddle in the north to the Paringa River in the south. This area is west of the Southern Alps and the landscape processes in the region are dominated by movements of the Alpine Fault, glacial activity, and climate (Whitehouse 1988; Berryman et al. 2001). Westland is characterised by high rates of uplift, precipitation, and erosion (Koons 1990; Tonkin & Basher 1990). Upland areas are often steep and highly dissected by water erosion and mass movement (Tonkin & Basher 1990), whereas the lowlands are dominated by moraine deposits separated by outwash surfaces, floodplains, and blocks of hill country (Mew & Palmer 1989). Precipitation levels are extremely high; mean annual rainfall rises from 2500 mm at the coast to 12 000 mm in the Southern Alps (McSaveney et al. 1978). Much of Westland' s forests are dominated by long-lived, light-demanding conifer species. On lowland terraces and moraine surfaces, Dacrydium cupressinum forms the canopy, with Prumnopitys ferruginea and Weinmannia racemosa forming a scattered, patchy subcanopy (Norton et al. 1988). Floodplains are dominated by Dacrycarpus dacrydioides on silty deposits, and Podocarpus hallii and Prumnopitys taxifolia on coarse alluvium (Wardle 1974). Stands of the angiosperms Metrosideros umbellata and W. racemosa and conifers Libocedrus bidwillii and Podocarpus hallii form forests in upland areas and along steep valley sides. Species of Nothofagus are the major canopy trees on all landforms north of the Taramakau River and south of the Paringa River, with the area in between known as the "beech gap" (Wardle 1984). METHODS Floodplain scale variation in earthquake effects We aged trees in 13 stands scattered across the Whataroa floodplain (43°16'S, 170°22'E) in the summer of 2000/2001. Most of the forest in the upper reaches of the Whataroa floodplain (close to the Alpine Fault and Southern Alps) has been cleared for agriculture or logged. We


Cullen et al.—Variation in earthquake effects on forests

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identified remnant patches of unlogged forest from aerial photographs, by consulting local residents, and by driving across the floodplain. We sampled all of the remnant patches of unlogged forest that we could find in the upper reaches of the floodplain (nine stands), and we sampled four stands of trees in the more extensive unlogged forest in the lower reaches of the floodplain (closer to the Tasman Sea). In all but one stand, increment cores were taken from the largest 10-12 trees in each stand at a height of c. 1 m above the ground. The remaining stand was an extensive fragment of Prumnopitys taxifolia and Podocarpus totara forest (56 ha). We sampled this stand by coring c. 10 trees in each of 15 sample plots located systematically throughout the stand (a total of 148 trees). We cored a total of 267 trees on the Whataroa floodplain. We also cored trees in an extensive Dacrycarpus dacrydioides stand in the lower reaches of the Wanganui River (immediately north of the Whataroa River) in 1997. We established a 240 x 20 m transect in this stand orientated at right angles to the river and extracted cores at a height of c. 1 m above the ground from all trees along this transect that were > 10 cm diameter at 1.4 m height. We cored a total of 352 trees in this stand. The cores collected from trees growing on the Whataroa and Wanganui floodplains were mounted and sanded, and the number of rings counted using a binocular microscope. When cores did not intercept the pith but the arcs of the innermost rings were visible, we used the geometric model of Duncan (1989) and the mean growth rate of the innermost 20 rings to estimate the number of missing rings. Some cores taken from rotten or very large trees failed to reach the tree centre, and the number of missing rings was estimated from the length of the core, the diameter of the tree, and the mean growth rate of the innermost 20 visible rings (Duncan 1989). Some trees were cored twice, when the first core missed the pith by a wide margin, and we used the core with the smallest missing radius to estimate tree age. To examine the extent of aggradation surfaces of different ages that may have resulted from earthquakes, we examined the distribution of tree ages in the 13 stands on the Whataroa floodplain. Specifically, we compared tree age distributions on surfaces in the upper reaches of the Whataroa floodplain, close to the Alpine Fault and Southern Alps, with tree age distributions on surfaces in the lower reaches of the floodplain, closer to the coast. We also examined the distribution of tree ages on three other floodplains: Wanganui, for which we had collected tree age data from one large stand (see above), and Waiho and Ohinemaka, for which tree age data were available from three previously published studies (Wardle 1974; Duncan 1989, 1993). Regional scale variation in earthquake effects Wells et al. (1999) examined the age distribution of 59 cohorts of trees that had been initiated by major disturbance throughout Westland. Since that study, tree age data from an additional 67 cohorts are available: 51 cohorts in the Karangarua catchment (Wells et al. 2001), one cohort on the Waiho floodplain (Wardle 1974; Duncan 1989), one cohort on the lower Wanganui floodplain (this study), and 14 cohorts on the Whataroa floodplain (this study; in one of our 13 stands on the Whataroa floodplain, two distinct cohorts were evident). Cohorts of trees were identified, and their date of initiation was determined, from tree age data as described in Wells et al. (2001), including a colonisation delay of 28 or 20 years for conifer or angiosperm trees, respectively. In total, we have estimated dates for the initiation of 126 cohorts of trees along 280 km of the Alpine Fault in Westland. We examined the spatial extent of cohorts of similar age in Westland by grouping cohort age data by five subregions, listed in order from north to south: (1) central and northern Westland, which included widely scattered sites between Rahu Saddle and the Cropp River (22 cohorts), (2) the Whataroa and Wanganui floodplains (15 cohorts), (3) Saltwater and

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Journal of the Royal Society of New Zealand, Volume 33, 2003 Whataroa, Upper Floodplain

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Okarito Forests, which occupy old moraine surfaces (24 cohorts), (4) the Waiho floodplain and Karangarua catchment (56 cohorts), and (5) the Ohinemaka floodplain (9 cohorts). We constructed an overall cohort age distribution for all 126 cohorts, and a separate cohort age distribution for each subregion.


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Fig. 2 Age frequency distribution of cohort ages for 126 tree cohorts in Westland. Dashed lines indicate earthquake events.


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RESULTS Floodplain scale variation The upper reaches of the Whataroa floodplain (near the Southern Alps) are dominated by relatively young trees, most of which established after AD 1720, coincidental with the last rupture event on the Alpine Fault (Fig. 1). In contrast, the lower reaches of the floodplain (closer to the coast) are dominated by older trees with a pulse of establishment commencing around AD 1625. The same pattern occurs in Ohinemaka Forest, where relatively young trees (post AD 1717) dominate the upper reaches of the floodplain, while older trees dominate the lower reaches (Fig. 1). The age distributions of trees sampled in the lower reaches of three of the four floodplains (Ohinemaka, Wanganui, and Whataroa) are remarkably similar (Fig. 1). All show a major pulse of establishment commencing c. AD 1650, and a consistent but much smaller peak in establishment after the AD 1717 earthquake. The site in the lower reaches of the Waiho floodplain contained older trees, with the only peak in establishment around AD 1500-1600. Regional scale variation The cohort age distribution for all 126 cohorts suggests that, since AD 1300, there have been four distinct episodes of approximately synchronous cohort establishment in Westland during c. AD 1430-1500, 1610-1670, 1690-1750, and 1800-1850 (Fig. 2). Using 59 of the 126 cohorts included in the present study, Wells et al. (1999) identified three of these episodes and correlated them to disturbances triggered by Alpine Fault earthquakes dated to c. AD 1460, 1610-1620, and 1717. The fourth and most recent disturbance episode, evident in our expanded cohort age distribution, was most likely associated with a Fiordland earthquake (further south) recorded in AD 1826 (McNabb 1907; Wells et al. 2001). This AD 1826 disturbance episode has a limited spatial extent in Westland, with forest cohorts dating from the event evident in the south and extending only as far north as the Waiho and Karangarua catchments (Fig. 3). In contrast, the AD 1610-1620 and 1717 episodes are both regionally extensive, with cohorts dating from these events evident at both the northern and southern extremes of the study area. In the southernmost area (Ohinemaka Forest) cohort establishment occurred following both events, but is relatively low after AD 1610-1620, reflecting the small number of stands sampled (6 stands with 9 cohorts; Duncan 1991). Nevertheless, the timing of cohort initiation in Ohinemaka Forest is consistent with the timing of more obvious cohort initiation further north. Both the AD 1610-1620 and 1717 events are evident at most other locations in Westland, although cohort initiation following

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Fig. 3 Age frequency distributions of cohort ages (from Fig. 2) for five geographical subregions of Westland, arranged in order from north (top) to south (bottom). n is the number of cohorts from each subregion used to construct each distribution. Dashed lines indicate earthquake events.

the AD 1717 event is less obvious in the Saltwater and Okarito Forests. The earliest disturbance episode, dated c. AD 1460 is only evident as far north as the Saltwater and Okarito Forests, with little evidence of this event further north. DISCUSSION Our expanded dataset, comprising establishment dates for 126 cohorts of trees from throughout Westland, supports earlier findings from a subset of these data, in identifying three episodes of massive landscape disturbance and associated forest cohort initiation that were most likely triggered by the three most recent Alpine Fault earthquakes in c. AD 1460, 1610-1620, and 1717 (Wells et al. 1999, 2001). We also identify a fourth episode of disturbance and forest cohort initiation most likely triggered by a Fiordland earthquake in AD 1826. Cohorts of trees initiated by this latest event occur in the south of our study area, extending only as far north



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as the Karangarua River. Locations further north were probably too far from the epicentre of the earthquake for shaking to have caused sufficient damage to initiate subsequent tree establishment. In contrast, disturbances and forest cohort initiation associated with the two most recent Alpine Fault earthquakes were regionally extensive, suggesting that these were either much larger earthquakes or were centred in Westland compared with the 1826 Fiordland event. Consistent with this, Wells et al. (2001) estimated that only about 10% of the land area in parts of the Karangarua catchment was disturbed by erosion or sedimentation following the 1826 Fiordland earthquake, while the land area disturbed by the AD 1610-1620 and 1717 earthquakes was estimated at 40 and 49%, respectively. We have found no evidence in the patterns of cohort establishment for north or south biases in the effects of the AD 1610-1620 and 1717 earthquakes (cf. Yetton et al. 1998). Fault trenching shows that the Alpine Fault did not rupture north of Hochstetter Forest during the AD 1717 event (Yetton 1998; Wells et al. 1999). Tree-ring chronologies from c. 50 km north of the rupture limit, near Rahu Saddle, also do not show a sudden growth decline around AD 1717, which is a response evident in tree-ring chronologies from further south and is presumably due to damage from shaking and tree toppling associated with the earthquake in the rupture zone (Wells et al. 1999). In this study, however, there is evidence that the AD 1717 earthquake initiated cohort establishment in areas at least as far north as Rahu Saddle (Fig. 3, see also Urlich (2000)), which is at odds with the extent of earthquake effects inferred from the tree-ring chronologies. The apparent lack of a sudden decline in tree growth after AD 1717 in chronologies north of the rupture limit could result from sampling of trees on only stable substrates. More extensive sampling of trees on a range of landforms (e.g., Vittoz et al. 2001) is required to clarify the extent of earthquake-related effects on vegetation in this region. While the effects of both the AD 1610-1620 and 1717 earthquakes were regionally extensive, they appear to differ in the extent of their local impacts. This is most obvious on the Whataroa and Ohinemaka floodplains, where the AD 1610-1620 earthquake particularly appears to have caused a massive aggradation event as material from steep mountain hillsides was transported and deposited downstream. New surfaces available for tree colonisation must have formed over most of the floodplain, judging by the dominance of trees dating from this event in the lower reaches of both floodplains. This aggradation event also appears to have had a major effect on the lower reaches of the Wanganui floodplain. In contrast, the aggradation event caused by the AD 1717 earthquake was less extensive, being confined to the upper reaches of both the Ohinemaka and Whataroa floodplains. This variation in the scale or intensity of local effects could reflect differences in earthquake magnitude or strength of shaking. Yetton et al. (1998) suggested that the AD 1717 earthquake was of lower magnitude than the earthquake in AD 1610-1620. A smaller earthquake in AD 1717 would have caused less erosion in upland catchments (Keefer 1984; Hancox et al. 1997; Rodríguez et al. 1999) and, consequently, a smaller aggradation event, with initiation of new forest over a smaller area. However, the size of the aggradation event will also be determined by the amount of sediment available in upland catchments, which could be a function of the time since the last major aggradation event (P. Almond and P. Tonkin pers. comm.). Intervals between these earthquakes are c. 160 years before the AD 1610-1620 event, c. 100 years to AD 1717, and c. 280 years from 1717 to the present. Thus, more sediment may have been available for release during the AD 1610-1620 earthquake than in 1717, leading to a more extensive aggradation event across the floodplains. The 280 years that have elapsed since the AD 1717 earthquake suggests that even more material may be available in the next event.

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Trees that post-date the inferred c. AD 1460 earthquake also occur in the lower reaches of the floodplains, notably in the Waiho, suggesting that this was also a major aggradation event. Despite this, the c. AD 1460 event is evident in cohort age structures only in the south of our study area, with evidence of its effects largely absent from the northern and central region (but see Urlich 2000). This could reflect a more southerly location for the epicentre of this event, but could also be a consequence of relatively low sampling effort, the destruction of older trees during later events, or more extensive forest clearance by humans in the northern and central regions. ACKNOWLEDGMENTS We thank Rachel Smith, Doug Bray, Stephen Urlich, Cathy Allan, Hannah Buckley, and Daniel Ruth for help in the field, and Tim and Gavin Connell, Gerald Denehey, David Friend, Rodney Giles, Roger Johnson, the Nolan family, Philip Northcroft, Robert Vincent, and the Whataroa Golf Club for allowing us access to their land. This study was partly funded by the Foundation for Research, Science & Technology through the Institute for Geological and Nuclear Sciences NSOF programme, Tracking Landscape Change. We thank Kelvin Berryman, Stuart Read (GNS), Peter Almond, Phil Tonkin (Lincoln University), and Peter Wardle (Landcare Research) for helpful discussion and comments.

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