Forest dynamics in Westland, New Zealand: the importance of large, infrequent earthquake-induced disturbance

Abstract

Summary1 Large, infrequent disturbances can exert a dominant influence on the structure and dynamics of forested landscapes and in Westland, New Zealand there is evidence of at least three massive earthquakes within the last 650 years. We reconstructed the history of forest disturbance in two study areas, totalling 1412 ha, to quantify the role of such disturbance in structuring the conifer/hardwood forests. 2 We divided the study area into different land-surface types, aged trees on each land surface and identified cohorts of trees established in response to past disturbance. The type of disturbance (tree fall or erosion/sedimentation event) that initiated cohort establishment was determined from the type of land surface and other physical evidence. We also dated abrupt growth releases or suppressions in tree rings to aid disturbance history reconstruction. 3 Erosion and sedimentation events dominated the disturbance regime, affecting 86% of the study area in the last 650 years. Catchment-wide episodes of forest cohort-initiating disturbances were centred around 1820-30, 1710-20, 1610-20 and 1460  . Of the 51 cohorts identified in the study area, 47 were initiated during one of these episodes, when disturbance by erosion or sedimentation affected from 10-50% of the study area. Consequently, over 80% of the forested area currently comprises simple, first generation cohorts of trees established after catastrophic disturbance. Only 14% of the study area is more complex, all-aged forest. 4 Three disturbance episodes coincide with the three most recent Alpine fault earthquakes ( c . 1717, 1630 and 1460  ), while one coincides with earthquakes recorded to the south of the study catchment in 1826  . Age structures from throughout Westland show that extensive, similar-aged, post-earthquake cohorts of trees are a feature of the region, suggesting that infrequent, massive earthquakes are the dominant coarse-scale disturbance agent, triggering episodes of major erosion and sedimentation and leaving a strong imprint on the forest structure.

Patterns of tree community composition along a coastal dune chronosequence in lowland temperate rain forest in New Zealand

“…The presence of tree ferns on young soils and waterlogging on old soils also appear to be important in structuring the podocarp community along the sequences (Coomes et al 2005;Gaxiola et al 2010). However, the trend toward increasing conifer dominance differs from disturbance-related successional patterns observed in the region, whereby large earthquakes along the Alpine Fault promote initial dominance by conifers, followed by slow progression to a mixed conifer-angiosperm forest (Ogden and Stewart 1995;Stewart et al 1998;Wells et al 2001). This suggests factors other than pedogenesis-driven changes in nutrient status might be more important in determining forest community composition in New Zealand temperate rain forests.…”

“…Large scale disturbances such as landslides, flooding, and volcanic eruptions cause major soil disturbance and favor extensive even-aged cohorts of either angiosperms or conifers with high basal area and low seedling density (Rose et al 1992;Stewart and Rose 1990;Stewart et al 1998;Wells et al 2001). Without further major disturbance, the initial cohort collapses through smaller scale (gap phase) canopy disturbances and is replaced by a forest with a lower basal area but greater compositional and structural diversity (Kershaw and McGlone 1995;Ogden and Stewart 1995;Stewart and Rose 1990;Veblen and Stewart 1982).…”

“…Disturbance is a key factor shaping forest communities in lowland conifer-angiosperm forests of New Zealand (Duncan 1993;Ogden 1985;Veblen and Stewart 1982;Wells et al 2001). Large scale disturbances such as landslides, flooding, and volcanic eruptions cause major soil disturbance and favor extensive even-aged cohorts of either angiosperms or conifers with high basal area and low seedling density (Rose et al 1992;Stewart and Rose 1990;Stewart et al 1998;Wells et al 2001).…”

New Zealand forest dynamics: a review of past and present vegetation responses to disturbance, and development of conceptual forest models

“…Cowan & McGlone 1991) and in the age structures of extant tree populations (e.g. Wells et al 1998;Wells et al 2001). Earthquake triggered activity is not limited to the Alpine Fault; for example, the M w 8.2 Wairarapa Earthquake (1855) caused 2.5 m of uplift in the Rimutaka Range and triggered landslides up to 400 × 200 m in extent (Robbins 1958).…”

Late Holocene landscape change history related to the Alpine Fault determined from drowned forests in Lake Poerua, Westland, New Zealand

“…Wells et al, 1999Wells et al, , 2001Sutherland et al, 2007), and if so, what mechanism linked the rupture of the fault with a rise in relative lake level capable of drowning living forests. Precise dating also allowed for an assessment of the rapidity of landscape change and the time to re-stabilise the landscape following large to great Alpine Fault earthquakes.…”