Michael Marden

Root strength, growth, and rates of decay: root reinforcement changes of two tree species and their contribution to slope stability

Information on live root-wood strength, rates of root decay and root growth of both radiata pine (Pinus radiata D. Don) and kanuka (Kunzea ericoides (A. Rich.) Joy Thomps. var. ericoides) are combined to form a generalized conceptual model of changes in nett root reinforcement. The model provides an initial opportunity to rank the plant species having specific below-ground rooting habits that can be used to control erosion, and when linked with extreme flood probability can be used to indicate the risk of a storm likely to cause slope instability in the period between clear-felling and regrowth. Erosion-susceptible slopes planted 1 year after clearfelling in radiata pine at 1250 stems ha-1 regain root site-occupancy in 4.7 years, an interval during which there is an 80% chance of experiencing an extreme flood. Similarly for radiata planted at 800 and 400 stems ha-1, root site-occupancy is regained in 5.6 and 7.5 years, and the probability of occurrence of an extreme event within these periods is 85 and 90%, respectively. For erosion-susceptible slopes on which kanuka has become established, the probability of a significant event within the 2.8 years prior to root site-occupancy is 60%. Slopes felled of radiata pine are potentially more vulnerable to the stresses promoting slope instability, at least in the earlier years.

Tectonic and paleoclimatic significance of Quaternary river terraces of the Waipaoa River, east coast, North Island, New Zealand

Abstract Remnants of four aggradational terraces in the lower 45 km of the main branch of the Waipaoa River have been correlated with cold/cool climate episodes of the Otiran glaciation. The youngest of the aggradation levels—the Waipaoa‐1 terrace—has the c. 14.7 kaRerewhakaaituTephra as the oldest part of the coverbed sequence, indicating cessation of aggradation about 16 ka BP. This terrace is broadly correlated with Ohakean‐aged terraces in other parts of the North Island. The second most recent episode of aggradation—the Waipaoa‐2 terrace—is slightly older than the c. 28 ka Mangaone Tephra, and is broadly correlated with the Rata terrace. The third most recent aggradation episode— the Waipaoa‐3 terrace—is slightly older than the c. 55–57 ka Rotoehu Tephra (age estimate based on stratigraphic relationships in this study), indicating cessation of aggradation at c. 65 ka BP, and correlative with the Porewa terrace. The fourth, and oldest, aggradation episode we identify in the present landscape—the Waipaoa‐4 terrace—has poor age constraints, but is probably related to the cool period of late oxygen isotope stage 5 at c. 90 ka BP or the glacial period of oxygen isotope stage 6 at c. 140 ka BP. Tectonic deformation in the middle reaches of the Waipaoa catchment is deduced from the elevation difference of pairs of aggradation terraces, and takes the form of broad regional uplift in the range of 0.5–1.1 mm/yr. Uplift is probably driven by subduction processes in the middle part of the catchment and by a combination of deep‐seated subduction processes and local deformation associated with active faults and folds in the lower valley area. Downcutting rates of up to 7 mm/yr occur in upper reaches of the river. In the middle reaches of the valley, where there are both uplift and downcutting data, we find that downcutting is about four times faster than tectonic uplift. Thus, climate fluctuations are interpreted to be the primary control on formation of fluvial terrace landscapes in the region.

Dating the culmination of river aggradation at the end of the last glaciation using distal tephra compositions, eastern North Island, New Zealand

An extensive terrace (Waipaoa-1) that can be traced for about 29 km in the Waipaoa valley, eastern North Island, New Zealand, is underlain by at least 10 m of coarse, aggradational, river gravels. Terrace cover beds contain tephras erupted from central North Island volcanoes and these provide minimum ages for the underlying gravels. Tephra or tephric layers occurring in the lower cover beds were investigated at five sites using a combination of stratigraphy, mineralogy, and the major element composition of glass shards together with discriminant function analysis (DFA). The basal tephra is identified as the ca. 14,700 14C years old (ca. 17,700 cal. years B.P.) Rerewhakaaitu Tephra, erupted from Okataina Volcanic Centre. Using the stratigraphic relationship of Rerewhakaaitu Tephra, the end of aggradation is dated at ca. 15,000 14C years (ca. 18,000 cal. years). Correlation with aggradational terraces elsewhere in North Island and northern South Island indicates that aggradation ended at the same time over a wide area and confirms a climatic origin for the terraces. Subsequent downcutting was apparently rapid because Rerewhakaaitu Tephra also occurs at the base of cover beds on a ca. 15 m lower terrace. The downcutting represents a major change in river dynamics and is most likely the response to climatic change and the resultant upper catchment landscape stability.

A comparison of earthflow movement mechanisms on forested and grassed slopes, Raukumara Peninsula, North Island, New Zealand

Abstract Surface movement rates on forested earthflows are 2–3 orders of magnitude less than those on grassed earthflows. Subsurface deformation results largely from extension flow on grassed earthflows and from compression flow on forested earthflows. A rafting mechanism in which blocks of roots from individual trees interact with those of neighbouring trees to retard surface movement is used to explain deformation profiles of forested earthflows. The rheology of earthflow materials is changed by the presence of tree roots.

Sediment sources and delivery following plantation harvesting in a weathered volcanic terrain, Coromandel Peninsula, North Island, New Zealand

Sediment generation and vegetation recovery was measured over a 2-year post-harvest period in a 36-ha catchment of exotic forest located in andesitic terrain, Whangapoua Forest (36.46 ◦ S, 175.36 ◦ E), Coromandel, New Zealand. Slopewash, soil scraping (on-slope removal of the regolith by the repeated dragging of logs), and storm-initiated landsliding were identified as the principal sediment-generating processes. Slopewash and vegetation recovery rates were measured using field-based plots located on sites of shallow- and deep-disturbance and a regression relationship was established between sedimentation rate (accumulation (g)/day.mm rain.m 2 ) and per cent vegetation cover for both plot types. At the basin scale, slopewash was calculated using the plot-based rates times the total area of deep- and shallow-disturbance sites as identified from a ground-based, transect survey and using sequential aerial photography. Sediment production, by soil scraping and landsliding, was determined by multiplying mean scar depth by the total affected area. In the first post-harvest year deep-disturbance sites generated 92% of total slopewash produced from both disturbance classes combined, and in year 2, slopewash halved. Half of the first post-harvest years slopewash-derived sediment was generated within the first 7 months following the completion of harvesting and before the application of desiccant. Thereafter, on deep-disturbance sites, slopewash rates declined further as sites became hardened against the generation of new sediment (i.e. sites became sediment limited). In contrast, during both the initial post-harvest recovery period and the post-desiccation period, the decline in sediment production on shallow-disturbance sites was more a consequence of site recolonisation. Sediment generated and redistributed by scalping and by landsliding occurred at the time of the respective events and coincided with the early part of the first post-harvest year. Collectively, soil scraping, slopewash, and landslides generated 1864 t (52 t/ha) of sediment, 88% of which remained on-slope. Of the sediment delivered to streams (228 t), landslides contributed 72%, soil scraping 26%, and slopewash 2%. For this harvested basin a single, storm- initiated, landsliding event was the most important hillslope process responsible for the generation of sediment and its delivery to streams, and slopewash was the least important.

Hillslope response to climate-modulated river incision in the Waipaoa catchment, East Coast North Island, New Zealand

Quantifying how hillslopes respond to river incision and climate change is fundamental to understanding the evolution of uplifting landscapes during glacial-interglacial cycles. We investigated the interplay among uplift, river incision, and hillslope response in the nonglacial Waipaoa River catchment located in the exhumed inner forearc of an active subduction margin on the East Coast of the North Island of New Zealand. New high-resolution topographic data sets (light detection and ranging [lidar] and photogrammetry) combined with field mapping and tephrochronology indicate that hillslopes adjusted to rapid latest Pleistocene and Holocene river incision through the initiation and reactivation of deep-seated landslides. In the erodible marine sedimentary rocks of the Waipaoa sedimentary system, postincision deep-seated landslides can occupy over 30% of the surface area. The ages of tephra cover beds identified by electron microprobe analysis on 80 tephra samples from 173 soil test pits and 64 soil auger sites show that 4000–5000 yr after the initiation of river incision, widespread hillslope adjustment started between the deposition of the ca. 14,000 cal. yr B.P. Waiohau Tephra and the ca. 9420 cal. yr B.P. Rotoma Tephra. Tephrochronology and geomorphic mapping analysis indicate that river incision and deep-seated landslide slope adjustment were synchronous between main-stem rivers and headwater tributaries. Hillslope response in the catchment can include the entire slope, measured from river to ridgeline, and, in some cases, the interfluves between incising subcatchments have been dramatically modified through ridgeline retreat and/or lowering. Using the results of our landform tephrochronology and geomorphic mapping, we derive a conceptual time series of hillslope response to uplift and climate change–induced river incision over the last glacial-interglacial cycle.

Quantifying temporal variations in landslide‐driven sediment production by reconstructing paleolandscapes using tephrochronology and lidar: Waipaoa River, New Zealand

Hillslope response to climate-driven fluvial incision controls sediment export and relief generation in most mountainous settings. Following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (∼18 ka), the Waipaoa River (New Zealand) rapidly incised up to 120 meters, leaving perched, low-relief hillslopes unadjusted to that base level fall. In the Mangataikapua—a 16.5 km2 tributary principally composed of weak melange—pervasive post-LGM landslides responded to >50 m of fluvial incision by sculpting and denuding >99% of the catchment. By reconstructing LGM and younger paleosurfaces from tephra identified by electron microprobe analysis (EMPA) and lidar-derived surface roughness, we estimate the volume, timing, and distribution of hillslope destabilization in the Mangataikapua and the relative contribution of landslide-prone terrain to post-LGM landscape evolution. We calculate volume change between four paleosurfaces constrained by tephra age (Rerewhakaaitu, 17.5 ka; Rotoma, 9.4 ka; Whakatane, 5.5 ka; and Waimihia, 3.4 ka). From the paleosurface reconstructions, we calculate the total post-LGM hillslope sediment contribution from the Mangataikapua catchment to be 0.5 ± 0.06 (s.d.) km3, which equates to a subcatchment averaged erosion rate of ∼1.6 mm yr−1. This is double the previous hillslope volume when normalized by study area, demonstrating that landslide-prone catchments disproportionately contribute to the terrestrial post-LGM sediment budget. Finally, we observe particularly rapid post-Waimihia erosion rates, likely impacted by human settlement.

Holocene rupture of the Repongaere fault, Gisborne: Implications for Raukumara Peninsula deformation and impact on the Waipaoa Sedimentary System

Abstract The Repongaere Fault is one of a series of active normal faults within the Raukumara Peninsula, eastern North Island, New Zealand. These faults appear to form in response to rapid uplift of the Raukumara Range and related extensional strain. However, the activity of these normal faults is poorly constrained. This paper presents new mapping of the active surface trace of the Repongaere Fault, c. 18 km northwest of Gisborne, and the results of two paleoseismic trenches. These results are then used to assess the seismic hazard posed by this fault and impacts on the Waipaoa Sedimentary System in which the fault is situated. Active traces can be mapped for c. 4.5 km, but we infer the surface rupture length to be at least 9 km. Tephras within the trenches constrain the timing of the most recent surface rupture event to have occurred during deposition of the Waimihia Tephra (c. 3400 cal. yr BP), and at least one event in the period c. 13 800–5470 cal. yr BP, with single‐event displacements of ≥0.4–1. 1 m. From these data a mean dip‐slip rate of c. 0.1 mm/yr and a maximum recurrence interval of 4490–6900 yr, can be calculated. If the Repongaere Fault is representative of other Raukumara Peninsula normal faults, then this relatively low rate of activity supports the interpretation that these faults are not contributing significantly to the deformation of the Raukumara Peninsula. The low rate of activity is also consistent with the very localised evidence for landscape impacts, a calculated moderate Mw of 6.3–6.7, and the faults location within the lower part of the Waipaoa River catchment. Together, these observations suggest that Repongaere Fault earthquakes have minimal, localised impact on the Waipaoa Sedimentary System.

Observations of "coarse" root development in young trees of nine exotic species from a New Zealand plot trial

BackgroundForests and wide-spaced trees are used widely in New Zealand to control erosion from shallow landslides. Species that offer similar or better levels of protection to those currently used are sought to meet future needs. Determining what plants to use and when they become effective is important for developing guidelines and policy for land management. This study aimed to obtain data on above- and below-ground plant growth for young exotic tree species considered potential candidates for future ‘erosion control forests’.MethodsThe above- and below-ground growth of nine exotic tree species was assessed annually for 3 years from planting in a randomised block field trial. Whole trees were excavated and destructively sampled and several below-ground metrics (total root length of all roots > 1 mm in diameter, lateral root spread, total root biomass) assessed.ResultsDifferences between species for most metrics at the time of planting carried through to Year 3. The best performing species across most metrics was alder, followed by blackwood, cherry, and cypress. Allometric models relating total root length and below-ground biomass to root collar diameter were established.ConclusionTop performers with regard to root metrics were alder, cherry, and cypress followed by blackwood, radiata, and redwood. Root information contributes to improving our understanding of how and when, and at what planting density, plants become effective for controlling erosion in New Zealand.

Revegetation of steeplands in France and New Zealand: geomorphic and policy responses

BackgroundEfforts to address erosion and land degradation in steeplands of many countries have largely relied on revegetation. The policy responses to this issue are many and varied as have been their successes. Revegetation efforts tend to occur when it is realised that deforestation, mountain land erosion, and flooding of rivers are linked.MethodsUsing the Southern Prealps region in France and the East Coast North Island region of New Zealand as ‘study sites’, past and current revegetation efforts to address steepland degradation were compared.ResultsBoth areas have similarities in geology, geomorphology and types of erosion processes (shallow landsliding and gullying). Landscape responses to large-scale erosion and subsequent reforestation have been similar between France and New Zealand though major reforestation occurred in France more than a century before that in New Zealand. Attempts to control sediment production in headwater regions reinforces the view that conditions controlling the evolution of channel response (through time and space) to a change in sediment supply are complex. While there is a consistent sequence of responses in channels and on hillslopes to reforestation efforts and the direction of changes may be anticipated, the magnitude and timing of those responses are not.ConclusionThe key lesson for future management and policy development arising from these studies is that erosion-control efforts that are aimed at producing basin-scale impacts will require targeting of areas where the proposed land use change or intervention will have the most beneficial influence on reducing sediment supply to river channels.