landslide impact on organic carbon cycling in a temperate montane forest

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms 36, 1670–1679 (2011)Copyright © 2011 John Wiley & Sons, Ltd.Published online 22 July 2011 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/esp.2191

Landslide impact on organic carbon cycling in atemperate montane forestRobert G. Hilton,1* Patrick Meunier,2 Niels Hovius,3 Peter J. Bellingham4 and Albert Galy31 Department of Geography, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, UK2 Laboratoire de Géologie, École Normale Supérieure, 24 Rue Lhomond, 75231, Paris Cedex 5, France3 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK4 Landcare Research, P O Box 40, Lincoln, 7640, New Zealand

Received 15 October 2010; Revised 2 June 2011; Accepted 6 June 2011

*Correspondence to: R. G. Hilton, Department of Geography, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, UK. Email: [email protected]

ABSTRACT: In humid, forested mountain belts, bedrock landslides can harvest organic carbon from above ground biomass andsoil (OCmodern) while acting to refresh the landscape surface and turnover forest ecosystems. Here the impact of landslides on organiccarbon cycling in 13 river catchments spanning the length of the western Southern Alps, New Zealand is assessed over four decades.Spatial and temporal landslide maps are combined with the observed distribution and measured variability of hillslope OCmodern

stocks. On average, it is estimated that landslides mobilized 7.6�2.9 tC km-2yr-1 of OCmodern, ~30% of which was delivered to riverchannels. Comparison with published estimates of OCmodern export in river suspended load suggests additional erosion of OCmodern

by small, shallow landslides or overland flow in catchments. The exported OCmodern may contribute to geological carbon sequestra-tion if buried in sedimentary deposits. Landslides may have also contributed to carbon sequestration over shorter timescales(<100years). 5.4�3.0 tC km-2yr-1 of the eroded OCmodern was retained on hillslopes, representing a net-carbon sink followingre-vegetation of scar surfaces. In addition, it was found that landslides caused rapid turnover of the landscape, with rates of 0.3%of the surface area per decade. High rates of net ecosystem productivity were measured in this forest of 94�11 tC km-2yr-1, whichis consistent with rapid landscape turnover suppressing ecosystem retrogression. Landslide-OCmodern yields and rates of turnovervary between river catchments and appear to be controlled by gradients in climate (precipitation) and geomorphology (rock exhu-mation rate, topographic slope). Copyright © 2011 John Wiley & Sons, Ltd.

KEYWORDS: landslides; organic carbon; montane forest; landscape turnover; New Zealand

Introduction

Bedrock landslides play an important role in the geomorphicevolution of humid active mountain belts. In settings wherefluvial incision into bedrock keeps pace with tectonic uplift,and erosion proceeds faster than weathering, they are thedominant mechanism of denudation, responsible for supply-ing most of the clastic sediment exported from mountains byrivers (Burbank et al., 1996; Hovius et al., 1997, 2000; Korupet al., 2004). At the same time, landslides can clear tracts offorest biomass and soil from hillslopes (Garwood et al.,1979; Restrepo and Alvarez, 2006; Restrepo et al., 2009), mo-bilizing modern organic carbon (OCmodern), recently fixed fromatmospheric-CO2 during photosynthesis. In doing so, land-slides contribute to the cycle of organic carbon in montane for-ests, and potentially a wider erosion-related transfer, burial andlong-term sequestration of OCmodern (Stallard, 1998; Tate et al.,2000; Hilton et al., 2008a). In addition, landsliding can setthe disturbance regime of montane forest, potentially influenc-ing ecosystem productivity and nutrient status (Restrepo et al.,2009).While the areal coverage of landslides (km2) and the rate of

landscape-turnover (km2yr-1) have been assessed in several

mountain environments (Guariguata, 1990; Hovius et al.,1997; Restrepo and Alvarez, 2006; Blodgett and Isacks, 2007)the magnitude of the concomitant erosion of OCmodern hasnot been so well described. Some previous studies have quan-tified the OCmodern transfer by landsliding using inventories oflandslides and hillslope carbon-stocks derived from remotesensing and measurements of forest biomass. They have fo-cused on the impact of individual storms on montane forestsof Taiwan and California and strongly suggest that landslid-ing can erode significant amounts of OCmodern (Hilton et al.,2008a; Madej, 2010; West et al., 2011). However, there havebeen few attempts to assess the sustained erosion of OCmodern

by landslides over several decades, and its spatial variabilitywithin a mountain belt. Here, we evaluate this in the westernSouthern Alps, New Zealand.

The western Southern Alps are largely undisturbed by an-thropogenic upland activity (Reif and Allen, 1988; Bellinghamand Richardson, 2006). The spatial and temporal patterns oflandslides in this region are well known on a decadal timescale(Hovius et al., 1997). To estimate the erosion of OCmodern bylandslides we adapt published methods (Hilton et al., 2008a;Madej, 2010; West et al., 2011), combining maps of landslidearea in 13 catchments, which represent about 40years of mass

1671LANDSLIDE IMPACT ON CARBON CYCLING IN MONTANE FOREST

wasting (Hovius et al., 1997), with measurements of carbonstocks in biomass and soil. This approach allows us to assessthe significance of landslide-mobilization of OCmodern forcatchment-scale carbon budgets, and to identify opportunitiesfor further investigation. Our study does not detail the erosionof fossil organic carbon from sedimentary bedrock bylandslides. This is likely to closely track the clastic sediment yieldfrom the mountain landscape (Hilton et al., 2011), a parameterassessed in detail elsewhere (Hovius et al., 1997).In this paper, we aim to demonstrate that landslide OCmodern

yields are significant for catchment-scale carbon budgets. Thefate of the eroded OCmodern is poorly constrained. However,our analysis reveals a large proportion is probably not deliveredto river channels, instead accumulating on hillslopes overtimescales of ~100years. Moreover, landsliding has given riseto strong gradients in the rate of surface turnover, which mayinfluence ecosystem productivity and nutrient status. Landslideyields of OCmodern, and the rate of turnover are driven by thecombination of climatic, tectonic and geomorphic factorswhich govern the altitudinal limit of vegetation growth andthe spatio-temporal pattern, areal coverage and size distribu-tion of landslides.

Study Region

The Southern Alps of New Zealand mark the oblique compres-sional boundary between the Australian and Pacific plates(Walcott, 1978). Rock uplift rates reach ~7 mm yr-1 (Bull andCooper, 1986) and have produced 2 to 4 km of subaerial relief.This acts as a barrier to north-west winds from the Tasman Sea(Whitehouse, 1988), resulting in annual precipitation up to

0 20km

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MoerakiParinga

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KarangaruaCook Fox

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Figure 1. The western Southern Alps, New Zealand (grey box on insVegetation Index from Landsat 7 ETM (see main text) where values>0�200 m interval shown in white, and delimits the transition to barewhite, over 40m digital elevation model. Landslides mapped from airas black polygons. This figure is available in colour online at wileyon

Copyright © 2011 John Wiley & Sons, Ltd.

Nve-1

1

Tasma

Waiho

Wai

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et map)corresprock, snphotogrlinelib

>10 m yr-1 (Griffiths and McSaveney, 1983). Sustained highrates of mass wasting (Hovius et al., 1997; Korup et al., 2004,2005) and fluvial erosion (Hicks et al., 2004) have aided exhu-mation of high-grade meta-sedimentary rocks along the rangebounding Alpine Fault (Tippett and Kamp, 1993). These com-prise the Alpine Schist, which grades to the south-east from ol-igoclase zone amphibolite facies toward greenschist facies(Suggate and Grindley, 1972). The Alpine Fault is thought torupture in large earthquakes (Mw~7) every 100–280 years,with the last event likely to have been 1717AD (Wells et al.,1999).

The landscape is dominated by steep slopes in meta-sedimentary bedrock, with an average slope of 30–35�

(Korup et al., 2005; Clarke and Burbank, 2010). Despite this to-pographic steepness, the humid climate sustains dense temper-ate montane rainforest on hillsides. The area is dominated byevergreen angiosperms (Metrosideros umbellata, Weinmanniaracemosa, Quintinia acutifolia, Griselinia littoralis), conifers(principally Podocarpus hallii) and Dacrydium cupressinumand Dacrycarpus dacrydioides at low altitudes (<400 m; Reifand Allen, 1988). Forest is present below ~800 m (Bellinghamand Richardson, 2006). Higher up, shrubland and alpineherbfields (Wardle, 2008) give way to bare rock, snow and iceabove ~1250 m (Figure 1(a)). Thus, the distribution of standingbiomass is controlled at the first order by elevation. Beneath theforest cover, soils are most developed on alluvium and collu-vial accumulations, while thin, discontinuous regolith (<1m)is typical elsewhere (Basher, 1986; Tonkin and Basher, 2001;Richardson et al., 2004).

This study focuses on 13 river catchments that span ~140 kmalong-strike of the Alpine Fault (Figure 1). They range in sizefrom 59 km2 to 456 km2 covering a combined area of 2434

ormalised differencegetation index

Elevation (masl)0

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tangitaonaWhataroa

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Landslides (1965 & 1985)

sl

. (a) Distribution of vegetation cover revealed by Normalized Differenceond to biomass and soil. 1250 masl contour shown as a black line, withow and ice. (b) The 13 river catchments used in this study, outlined inaphs taken 1964–1965 and 1984–1985 (Hovius et al., 1997) are shownrary.com/journal/espl

Earth Surf. Process. Landforms, Vol. 36, 1670–1679 (2011)

1672 R. G. HILTON ET AL.

km2. Over this area, apparent rock exhumation rates broadlydecrease perpendicular to the Alpine Fault, toward the drainagedivide, and along strike from the north-east toward the south-west (Tippett and Kamp, 1993). The highest rates, >8 mm yr-1,are found near the fault between the Waitaha and the Cookcatchments (Figure 1(b)), with rates decreasing to the south-west to<4 mm yr-1 at the range front in the Moeraki catchment(Tippett and Kamp, 1993). The highest precipitation totals arelocated ~5–10km down-dip of the fault within the mountain-ous topography (Figure 2) and vary in a spatial pattern similarto that of rock exhumation. Average precipitation decreasesfrom ~10 m yr-1 at Franz Josef (Waiho catchment) to ~6 myr-1 at Haast (Moeraki catchment) (Figure 2). These patternsdo not appear to have a significant impact on forest structure

Distance to Alpine fault (km)

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Figure 2. Annual precipitation (m yr-1) as a function of distance to theAlpine Fault (increasing values toward the south-east). Black line indi-cates the river long-profile of the Whatarora and shaded backgroundshows the catchment relief (elevation, masl) projected perpendicularto the Alpine Fault. Precipitation values are from Henderson andThompson (1999) for northern (circles), central (squares) and southern(diamonds) transects with the nearest town and river catchment in thisstudy (Franz Josef/Waiho; Figure 1(b)) shown in parenthesis.

Table I. Biomass of montane forests, live and coarse woody debris (CWD)Measured in 20m�20m plots in 1995–1997, shown in ascending order of bicoefficients (r) between total hillside biomass and other variables and P valuesarcsine–square-root transformed, aspect values transformed as A′=cos(45 –

Plot identifier Total hillside biomass (t km-2) CWD (%) Elevation (m) As

70_1 3,500 6 32070_5 4,010 15 74070_2 8,660 37 42068_1 11,910 32 61067_1 16,750 21 42067_2 17,620 21 50064_2 19,580 67 70069_1 23,930 40 46069_2 25,440 18 58064_1 26,040 15 58068_2 27,390 91 71063_1 31,570 11 51067_3 36,350 55 61063_3 37,340 38 70063_2 41,780 21 63069_4 42,090 73 78062_2 44,170 15 59070_4 45,860 92 69062_3 59,270 46 75069_3 66,480 7 70062_1 73,830 98 47070_3 106,700 89 560r 0.49 0.24P 0.08 >0.5


at the mountain range scale (Reif and Allen, 1988) nor on thealtitudinal range of vegetation (Figure 1(a)).

Methods

To assess how much OCmodern has been eroded by landslidesin montane forests, spatial and temporal landslide inventoriesmust be combined with a quantification of the carbon stockpresent on hillslopes prior to landsliding (West et al., 2011).We have used previously published landslide inventories(Hovius et al., 1997). The spatial and temporal coverage ofthese inventories, and the number of documented processevents preclude precise calculation of hillslope carbon stocksfor all individual landslides in our data set. Instead, fielddata from forest plots have been used to constrain best esti-mates of the C-stock present on pre-failure hillslopes basedon first-order spatial trends in OCmodern stocks with elevation(Figure 1(a)).

Hillslope stocks of OCmodern

In the western Southern Alps, the mass of standing vegetationand coarse woody debris were measured on 22 forest plots(each of area 400m2) in 1995–1997 in the Kokatahi Valley,Hokitika River catchment (Table I). While these plots are~30km north-east of the area of landslide mapping, the bed-rock (schist) and precipitation in the Kokatahi River valleyare identical to that of the study area (Bellingham andRichardson, 2006) and the forest compositionally similar (Reifand Allen, 1988). Greater heterogeneity of vegetation and bio-mass occurs within catchments, as a result of widespread pri-mary and secondary successions, than between catchments

combined, in the Kokatahi Valley, inland from Hokitika, New Zealand.omass. CWD=percentage of total hillside biomass as CWD. Correlationdemonstrate no significant statistical link (where P>0.05) (CWD valuesaspect) +1))

pect (degrees) Slope (degrees) Easting (NZ Grid) Northing (NZ Grid)

250 33 2363698 5806153280 35 2364465 5806299230 34 2363818 5806188100 45 2364637 5803778205 40 2364625 5804952210 32 2364733 5805018230 15 2366582 5805347330 37 2363488 5805388325 43 2363460 5805230100 41 2366787 5805122225 33 2364512 5803728255 9 2366750 5804270196 30 2364854 5805125250 20 2367164 5804267265 8 2366962 5804273355 19 2363385 5804977250 36 2366282 5803193200 26 2364290 5806258220 30 2366450 5803172350 47 2363450 5805180250 24 2366129 5803246230 38 2364072 58061890.03 �0.05>0.5 >0.5



throughout the region (Reif and Allen, 1988). The Kokatahidataset therefore provides relevant constraint on biomass den-sity and its likely variability at the catchment scale. The forestplots are located on randomly oriented transects with a range inelevation from 320 m to just below the treeline at 780 m, slopeangle from 8� to 47� and aspect from 100� to 355� (Table I),therefore spanning the full range of prevailing geomorphicconditions (Bellingham and Richardson, 2006). The biomassin live vegetation and coarse woody debris (above groundbiomass) was determined using allometric equations andwood density values (live and dead) for individual species(Coomes et al., 2002; Richardson et al., 2009). The observedrange in mean total above ground biomass density, 35 000�11000tkm-2 (�2 standard errors of the mean), is not correlatedwith any of the physical site characteristics (Table I). Instead,the variability in measured above-ground biomass and coarsewoody debris represents natural heterogeneity within a rivercatchment. To convert biomass into OCmodern stock we usethe standard method, taking the average of measured OCmodern

concentrations in above ground biomass (Coomes et al., 2002;Phillips et al., 1998). In forests of the South Island, NewZealand, this is identical to that used in forests worldwide(50%) (Schlesinger, 1990; Hart et al., 2003). Additional con-straints on the distribution and density of vegetation comefrom calculations of the Normalized Vegetation Index (NDVI)using Landsat 7 ETM + satellite imagery. NDVI was calcu-lated using the red visible (0.63–0.69 mm) and near-infrared(0.76–0.90 mm) bands in ArcInfo. Previous studies haverevealed no significant correlation between above ground car-bon stocks and NDVI in the western Southern Alps (Coomeset al., 2002), yet this approach allows us to constrain the al-titudinal limit of significant vegetation cover with NDVI>0and transition to bare rock, snow and ice, which occurs at1250�200m (Figure 1(a)).From these observations we have constructed a generalized

model of the OCmodern stock on hillslopes in the western South-ern Alps (Figure 3(a)) which reflects: (i) the first-order patterns invegetation cover with elevation (Figure 1(a)); and (ii) heteroge-neity within river catchments (Table I). The primary input ofOCmodern to soils is supply of organic matter from above groundbiomass, we assume that soil OCmodern tracks the latter, with aproportion constrained by soil OCmodern concentrations mea-sured in the Hokitika River catchment (Basher, 1986). There,organic-rich materials (OCmodern concentration =16�5%) are

Elevation

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Figure 3. Distribution of biomass density and landslide area with elevationevation of 22 forest plots in the Kokatahi Valley (Table I). Whiskers show twictribution (see Methods) is shown as a solid black line. Shaded area accoudistribution of mapped landslides in 1964–1965 (’65) and 1984–1985 (’85)correspond to the landslide area connected to river channels.


restricted to the upper 5–10cm of the soil profile. To accountfor variability throughout the landscape we therefore assumea mixture of organic matter and mineral soil with a density of1500�250 kg m-3 to a depth of 7.5�2.5cm. This equates toa soil OC stock of 18 000�9000 tC km-2. There may be signif-icant differences in OC concentrations between sites affectedby landsliding, but we assume that at the scale of large(>5000m2) landslides in our inventory (Hovius et al., 1997),the large variability of concentrations estimated here is likelyto encompass the range in OCmodern stocks in the upper soilhorizons (Basher, 1986).

Rates of landsliding

Bedrock landslide scars were mapped by Hovius et al. (1997)from aerial photographs. They used reflectivity contrasts be-tween vegetated and non-vegetated areas to delimit those af-fected by mass movements. The mapping resolution wasabout 5�10-3 km2, or 70 m. Landslides from images taken in1964–1965 and 1984–1985 were used in this study as theyprovide the most complete coverage across the 13 major rivercatchments along the Alpine Fault (Figure 1(b)). Field work byHovius et al. (1997) established that landslides remain detect-able on aerial photographs for about 20 years after incidence.This was confirmed during fieldwork in 2003–2004, when itwas found that landslide scars first visible on 1984–1985images had been substantially revegetated unless they hadremained active in the intervening years. Therefore we as-sume that the two landslide inventories used in this study to-gether cover about 40 years of mass wasting in the westernSouthern Alps. It should be noted that landslide rates calcu-lated from the landslide database may be a slight underesti-mate. This is because some small landslide scars below theresolution of mapping (5�10-3 km2) may have been over-looked. Their omission represents an underestimation of thesurface area affected by landsliding and the corresponding pro-cess rate (Stark and Hovius, 2001). Thus, the rates of landscapeturnover and OCmodern yields estimated in this paper are explic-itly for bedrock landslides larger than 5�10-3 km2.

For efficient export, mass wasting must deliver sediment di-rect to the established channel network, or to a position fromwhich other erosion processes can transfer it into a channel.We have used a digital elevation model with a 40 m resolution

(masl)

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Connected

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Landslide area (km2)

. (a) Triangle is the mean above ground biomass density (t km-2) and el-e the standard error on the mean. The best estimate of the biomass dis-nts for variability in pre-failure biomass. (b) Dark grey bars show thefrom Hovius et al., (1997) as total disturbed area (km2). Light grey bars



to calculate the channel network using flow direction and flowaccumulation algorithms in ArcInfo, (confirming channel loca-tion on Landsat 7 ETM + imagines). Connected landslides aredefined as those which reached into a channel. Use of a100m wide buffer zone around the channel network resultedin an increase of the contributing area of connected land-slides by 6% and 10% of the total connected area in the1964–1965 and 1984–1985 images, respectively. We use thisas an estimate of the associated error in the connectivityclassification.

OCmodern mobilization by landslides

The biomass and soil stock on hillslopes have been modelledbased on their first-order distribution with elevation (Figure 3(a)).To calculate the erosion rate of OCmodern by landsliding, the ar-eal coverage and connectivity of landslides have been quanti-fied in altitudinal bins for each catchment (Figure 3b). Toaccount for the potential variability of pre-failure OCmodern

stock, the 2s range of the biomass and soil stocks and the rangeof altitudes at which the transition to bare rock occurs are used(Figure 3(a)). As such, a minimum and maximum bound foreach altitudinal bin is calculated, which represent an averageerror of 38% on the OCmodern erosion rate by landsliding.

able II. Estimates of landslide-driven OCmodern yields, the connectiv-y of landslide-mobilized OCmodern to the fluvial channels (%) andndscape turnover time by landsliding (τ, yr, defined in the main text)r river catchments in the western Southern Alps, New Zealand

atchment

DrainageArea(km2)

Landslide OCmodern

yield (tC km-2;yr-1)

Channel-connected

OCmodern yield(%) τ (yr)

oeraki 60 2.0�0.7 15 15,000aringa 232 3.1�1.1 15 8,900ahitahi 152 5.1�2.0 34 4,400akawhio 117 3.2�1.3 5 6,900arangarua 369 2.2�0.8 46 11,000

Results and Discussion

Landslides in steep uplands disturb surface area, mobilizing soiland above-ground biomass (Garwood et al., 1979; Restrepoand Alvarez, 2006), and erode into bedrock, producing clasticsediment (Hovius et al., 1997; Korup et al., 2005). Thus land-slides can play a role in the organic carbon cycle of montaneforests by: (i) harvesting OCmodern from biomass and soil; (ii) de-livering OCmodern to river channels; and (iii) influencing the av-erage landscape age and the turnover time of the ecosystem.We have assessed the significance of these impacts in multiplecatchments, and provide best estimates of their magnitude andspatial variability over four decades across a mountain belt.

ook 130 4.0�1.6 5 5,300ox 92 8.7�3.0 76 3,200aiho 191 10.0�3.4 39 2,600aitangitaona 59 16.7�6.1 7 1,500hataroa 458 13.4�5.2 24 1,700oerua 74 4.6�2.5 7 3,200anganui 349 10.0�4.0 35 2,100aitaha 151 9.5�3.4 23 2,200

Erosion of OCmodern by landslides

The harvesting of above-ground biomass and soil by land-slides represents a transfer of recently fixed atmosphericCO2 from forest. All mapped landslides (≥5�10-3; km2) are es-timated to have eroded 0.7�0.3�106 tC, or 300�110 tC km-2

Tasman Sea

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Landslide-OCmodern yield:tC km-2 yr-1

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Figure 4. Overview of the landslide-driven erosion of OCmodern from the w(tC km-2 yr-1). Shading reflects the average landscape turnover rate (% swileyonlinelibrary.com/journal/espl


over the whole area (2434 km2). The average annual yield was7.6�2.9 tC km-2; yr-1, with catchments ranging from 2.0�0.7tC km-2;yr-1 to 16.7�6.1 tC km-2; yr-1 in the Moeraki and Wait-angitaona catchments, respectively (Figure 4; Table II). TheseOCmodern yields represent significant catchment-scale transfersof carbon (Stallard, 1998), but they are generously sustainedby regrowth of the montane forest, with an annual net primaryproductivity of ~1100 tC km-2;yr-1; (Whitehead et al., 2002).

The spatial variability in landslide OCmodern yields is likelyto depend upon the multiple factors which lead to hillslope fail-ure (Carson and Kirkby, 1972; Densmore and Hovius, 2000;Lin et al., 2008; Meunier et al., 2008). The highest landslideOCmodern yields are found in the Waitangitaona and Whataroariver catchments, and they decrease toward the south-west(Figure 4; Table II). This mirrors the climatic and geomorphicgradients in precipitation and rock exhumation, which influ-ence saturation (Iverson and Reid, 1992), bedrock fracturing(Molnar et al., 2007; Clarke and Burbank, 2010) and topo-graphic slope (Korup et al., 2005) which prime hillslopes forfailure in the central north-east section of the study area.

A broad, positive relationship exists between the erosionalyields of OCmodern and sediment by landslides (Figure 5). Varia-bilty derives from the landslide area to volume scaling assumedin our analysis (Hovius et al., 1997; Guzzetti et al., 2009;

Titlafo

C

MPMMKCFWWWPWW

Landscape turnover time: % century-1

Alpine Fault

0-2 2-4 4-7

estern Southern Alps. Size of circle reflects the landslide OCmodern yieldurface area per century). This figure is available in colour online at



Larsen et al., 2010) and the observed size distribution of land-slides in each catchment. For example, small, shallow land-slides deliver clastic sediment with a higher organic carbonconcentration as OCmodern from soil and vegetation is notstrongly diluted by clastic sediment (Hilton et al., 2008b). Ourinventory suggests that such landslides are more important inthe disturbance of surface area in the Wanganui catchment,where the ratio of OCmodern to clastic sediment is ~0.1%. Incomparison, it is 0.01% in the Poreua catchment where largerlandslides disturb more surface area in the dataset (Figure 5).The landslide-OCmodern yields estimated in the western

Southern Alps are comparable with those estimated in thePacific Northwest of the USA, and Taiwan for landslides trig-gered by individual storms (Madej, 2010; West et al., 2011).For example, an event with a return time of ~10 years triggeredlandslides that were estimated to have mobilized 28 tC km-2 inRedwood Creek (area 714 km2), or 2.8 tC km-2yr-1 prorated forthe return time of the storm event (Madej, 2010). Landslidestriggered by tropical cyclone Morakot in Taiwan mobilized anestimated 377�87 tC km-2 from above-ground biomass in amountain catchment (area 3320 km2) with similar effects else-where on the mountain island (West et al., 2011). This excep-tional typhoon caused up to 2965 mm of rainfall in 4 days.For such an extreme event, the authors state that it is difficultto estimate its likely recurrence interval. While we do notknow the precise timing of landslides in our inventory, anevent of this magnitude did not occur over the mapping pe-riod (Hovius et al., 1997).To quantify the potential role of high magnitude, low fre-

quency events for the erosion of OCmodern on centennial timescales, we briefly investigate a hypothetical scenario followinga large earthquake (Mw>7) on the Alpine Fault in the westernSouthern Alps. The Alpine Fault is thought to rupture at~200 yr intervals (Bull and Cooper 1986; Wells et al.,1999) and more frequent, smaller earthquakes occur on sec-ondary faults (Korup et al., 2004). These events can trigger largeamounts of landsliding (Allen et al., 1999). Meunier et al.(2007) have reported that large earthquakes can result in4–8% of hillslope area being disturbed by landslides in theepicentre areas (Keefer, 1994; Parker et al., 2011). This wouldmobilize 900–1800 tC km-2 of OCmodern, or ~2–4�106 tC, in asingle event. With a return time of ~200 yr (Wells et al., 1999),earthquake mobilization of OCmodern in the western SouthernAlps may equate to 5–9 tC km-2yr-1. Our analysis suggests thatthese extreme events may play an important role in the

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Figure 5. Landslide yields of clastic sediment (t km-2 yr-1; Hovius et al., 199ern Southern Alps. Whiskers show the error estimates based on the measuredscaling. Dashed lines demonstrate a constant proportion of OCmodern in clas


long-term (>100yr) landslide-driven erosion of OCmodern,while also producing very large, transient yields. The estimatesherein are based on the altitudinal distribution of mapped land-slides (Figure 3(b)). While this assumption may not hold forearthquakes (Densmore and Hovius, 2000; Meunier et al.,2008) the calculations provide impetus for further investigationin other mountain belts.

For comparison, we briefly consider the fossil organic carbon(OCfossil) eroded from (meta) sedimentary rocks by bedrocklandslides (cf. Hilton et al., 2008b). OCfossil represents atmo-spheric CO2 sequestered in the geological past and it is impor-tant to trace its input in the modern environment (Blair et al.,2003; Hilton et al., 2010) because OCfossil weathering releasesC from the lithosphere (Petsch et al., 2000). If OCfossil is uni-formly distributed in the rock mass and landsliding dominatessediment production and mobilization, as is the case in moun-tain landscapes such as the western Southern Alps, then themobilization of OCfossil will track the clastic sediment yieldand the mean OCfossil content of bedrocks (Hilton et al.,2011). For the estimated landslide sediment yield of 18 025 tkm-2 yr-1 (Hovius et al., 1997) and a measured OCfossil concen-tration of 0.14% (Hilton et al., 2008b) the landslide yield ofOCfossil from the western Southern Alps is likely to be ~25 tCkm-2yr-1. This analysis confirms bedrock landslides as a potentsource of OCfossil (Leithold et al., 2006; Hilton et al., 2008b;Gomez et al., 2010) but further discussion regarding the fateof OCfossil is outside the scope of this article.

Onward transport of landslide-mobilized OCmodern

Landslides do not necessarily deliver clastic sediment andentrained OCmodern, directly to the stream channel network(Korup, 2005; Lin et al., 2008; Meunier et al., 2008; Hoviuset al., 2011). This is important to consider since OCmodern deliv-ered to the river thalweg can be exported from the mountainbelt by rivers and contribute to geological sequestration of at-mospheric CO2 when buried in sedimentary deposits (Galyet al., 2007; Hilton et al. 2008a). According to our criteria (seemethods), the hillslope–channel connectivity of the mappedlandslides was on average 26�3% (margin reflects inclusion of100 m buffer zone). The distribution of connectivity with altitude(Figure 3(b)) means that a slightly higher proportion, 29�3%of the landslide-eroded OCmodern is likely to have been deliv-ered direct to the channel network. This represents a delivery of

30,000 40,000 50,000

ic sediment yield -2 yr-1)

OC = 0.01%

7) and OCmodern (tC km-2 yr-1; this study) for catchments from the west-variability of the hillslope carbon stores and landslide area to volume

tic sediment, corresponding to a wt% (0.01 and 0.1) of OCmodern.



2.2�0.7 tC km-2yr-1 of OCmodern on average to river channels.In notable contrast, during Typhoon Morakot in Taiwan,hillslope–channel connectivity was close to 100% (Westet al., 2011). This is probably a consequence of the high rainfallintensity during this extreme event in which rivers reached veryhigh stage, undermining hillslopes.The fate of this landslide mobilized OCmodern remains poorly

understood. It is tempting therefore to compare the landslideyields with the export of particulate organic carbon (POC) byrivers from this mountain belt. We have previously estimatedthat rivers export ~40 tC km-2yr-1 of POC from OCmodern

sources, ~0.3 wt% of the total clastic sediment mass (Hiltonet al., 2008b). In contrast, the mapped landslides deliveredbetween 0.2�0.1 tC km-2yr-1 and 6.7�2.1 tC km-2yr-1 ofOCmodern to river channels in the Makawhio and Fox catch-ments, respectively (Table II). The discrepancy is also apparentwhen we consider the mass of OCmodern (tC), mass of sediment(t) and the corresponding average concentration of OCmodern

in sediment (in wt%) delivered by the landslide process. Assum-ing a thorough mixture of clastic sediment, soil and vegetation,landslide sediment delivers an average OCmodern concentrationof between 0.01wt% and 0.1wt% in the catchments (Figure 5).However, direct comparison between the estimates of

OCmodern erosion by landslides and its riverine transfer in thismountain belt should draw caution for several reasons. First,the estimates are made over different timescales. For example,several of the published riverine yields are based on a long-term, geological denudation rate (Hilton et al., 2008b). Overshorter time scales natural variability may give rise to signifi-cantly different yield estimates. Second, landslides can mobi-lize OCmodern with a large range of grain sizes (Wohl et al.,2009; West et al., 2011) whereas riverine POC is commonly es-timated from measurements of the fine (<sand) fraction of thesuspended load (Kao and Liu, 1996; Gomez et al., 2003; Hiltonet al., 2008a, 2008b; Hatten et al., 2010). It is not known howmechanical erosion of landslide-derived coarse woody debrismay contribute to the fine river load, either during mass-wastingor fluvial transport, nor the degree of sorting which may occuronce in the channel network (Hilton et al., 2008a; Wohlet al., 2009). Nevertheless, our observations suggest that addi-tional erosive processes supply OCmodern to mountain rivers,which confirms our current state of knowledge (Hilton et al.,2008a; Hatten et al., 2010). For example, bedrock landslidesare rare in the Alsea River catchment of the Oregon Coast Range,USA, yet small shallow landslides and overland flow are thoughtto drive a fluvial particulate export of 3.5 tC km-2 yr-1 ofOCmodern (Hatten et al., 2010).Notwithstanding these findings, landslides probably play an

additional role in the long-term OC budget of mountain areas.Landslides are a principal source of clastic sediment in thewestern Southern Alps (Hovius et al., 1997) and other mountainbelts (Hovius et al., 2000; Blodgett and Isacks, 2007). Highrates of landsliding result in high suspended sediment concen-trations in rivers (Fuller et al., 2003; Dadson et al., 2004) whichcan promote rapid sediment accumulation in sedimentarybasins (Gomez et al., 2004). Any OCmodern associated with thismaterial will probably experience an enhanced preservationrate in geological basin fills (Canfield, 1994; Burdige, 2005),contributing to a long-term atmospheric CO2 sequestration(Derry and France-Lanord, 1996; France-Lanord and Derry,1997; Galy et al., 2007; Hilton et al., 2008a).

Contribution of landslides to biosphere structure

Our analysis suggests that 71�3% of the OCmodern mobilizedby landslides in the temperate montane forests of the western


Southern Alps was not delivered within 100m of a streamchannel during an episode of mass wasting (Figure 3(b)). Thismay have implications for the accumulation of biomass andcarbon storage within the modern forest. Over the study period,0.53�0.29�106 tC, or 5.4�3.0 tC km-2 yr-1, of OCmodern wereeroded by landslides but not delivered to river channels. Thereare several lines of evidence to suggest that this acts to force anet accumulation of OCmodern on hillslopes over ~100 yeartimescales. First, dead OCmodern may remain in the terrestrialenvironment for 400 years or more (Adams, 1980; Wellset al., 1999), even with very high physical erosion rates in thissetting (Whitehouse, 1988). In fact, dead coarse woody debriscomprises an important component of the carbon and nutrientstore on hillslopes (Table I; Allen and Rose, 1983; Richardsonet al., 2009). Second, field observations suggest that land-slide scars, if not chronically active (Lin et al., 2008), are re-colonized by vegetation within several decades, with canopiesreaching 1.8m in height within ~30years (Bellingham et al.,2001). For example, forest plot 70_1 (Table I) was cleared bya landslide in 1972, leaving negligible live biomass. In1996, 1750 tC km-2 were present. At such rates of accumula-tion (73 tC km-2 yr-1) the plot would reach the average livebiomass stock of 8500 tC km-2 measured in the Kokatahi Valleyin ~120 years. The retention of 5.4�3.0 tC km-2 yr-1 OCmodern

on hillslopes appears therefore as a net carbon sink by the re-colonization of biomass on landslide scars.

Landsliding can also influence the carbon cycle in montaneforests by setting the time available for biomass and soils to ma-ture, i.e. the turnover time of the landscape. On average, ourlandslide inventory suggests that the hillslopes of the westernSouthern Alps are turned over at a rate of 0.3% per decade,which equates to an average landscape age (τ) of 3000 yr as-suming random spatial occurrence of landslides. This variesconsiderably between catchments (Table II), from 0.07% perdecade (τ=15 000 yr) in the Moeraki catchment to 0.7% perdecade (τ=1 500 yr) in theWaitangitaona catchment (Figure 4).). The turnover rates do not account for rare episodes of wide-spread mass wasting (West et al., 2011; Meunier et al., 2007;Hovius et al., 2011), yet are up to 20 times faster than thosequantified in Mexico and Central America (Restrepo andAlvarez, 2006).

Landscape turnover may be important for two reasons. First,it produces young sections of forest for which the rate of bio-mass growth is maximal (Zaehle et al., 2006; Restrepo et al.,2009). These sections of hillslope sequester atmosphericCO2 through net ecosystem productivity (NEP), whereas ma-ture forest may not accumulate further biomass. In addition,landscape turnover changes the nutrient balance of the ecosys-tem. In the absence of disturbance in the ambient, superhumidclimate of the western Southern Alps, soils can be podzolizedand gleyed in as little as 1000 years (Tonkin and Basher,2001) and limited by phosphorus in<10 000 years (Richardsonet al., 2004; Porder et al., 2007; Turner et al., 2007). Locationswhere landscape turnover has not been sufficiently recent maysuffer ecosystem retrogression as net primary productivity, de-composition, and rates of nutrient cycling decline (Wardleet al., 2004; Peltzer et al., 2010).

The impact of landslide-induced turnover on productivity inmountain forests remains to be quantified at the catchmentscale (Restrepo et al., 2009). Here, we have measured NEP ineach of the forest plots of the Kokatahi Valley (Table I) basedon radial growth increment on live stems >2.5 cm dbh follow-ing re-survey in 2002 (mean census interval=6.6 yr). Net bio-mass accumulation was 188�21 t km-2 yr-1 (� 2 standarderror of the mean) across the 22 plots. This represents a NEPof 94�11 tC km-2 yr-1. This rate of carbon sequestration is highin comparison with other regional and global datasets. For



example, Dixon et al. (1994) estimated that 0.26�0.09�109

tC yr-1 accumulated in biomass at intermediate (temperate) lati-tudes across the globe. Prorated over an associated forestedarea of 1038�104km2, this is equivalent to NEP of ~30 tCkm-2 yr-1. Evidence from long-term plots in the tropics suggestsbiomass accumulation of 77�44 t km-2 yr-1, a NEP of 37�21tC km-2 yr-1, in lowland forest of Amazonia (Phillips et al.,1998). The measured NEP in the temperate montane forest ofthe western Southern Alps are ~3 times the estimates from ma-ture old-growth forest. The rapid turnover of soil and biomassby landslides in this mountain belt (Figure 4) is consistent withhypotheses which suggest this may promote NEP in young soilsand forest stands (Wardle et al., 2004; Zaehle et al., 2006; Por-der et al., 2007; Restrepo et al., 2009). To assess quantitativelythe impact of landsliding on the C-balance in montant for-ested ecosystems, we recognize that a coupled geomorphicand ecological approach is required in future studies.

Conclusions

To assess the impact of landslides on organic carbon cycling ina montane forest, a spatial and temporal inventory of landslideshas been combined with measurements of carbon stock inabove-ground biomass and soil in the western Southern Alps,New Zealand. Over four decades, landslides are estimated tohave mobilized 7.6�2.9 tC km-2 yr-1 of OCmodern across 13river catchments, a significant natural transfer of carbon at thecatchment scale. The spatial distribution of erosion of OCmodern

by landslides appears to be governed by the geormorphic andclimatic setting, where gradients in precipitation, rock exhuma-tion and hillslope angle combine to control variability in therate of surface area disturbance by landslides. High magnitude,low frequency events which trigger widespread mass wastingare not accounted for here, and have the potential to contributesignificantly to the longer-term erosion of OCmodern.The connectivity of hillslopes and channels means that, on

average, landslides deliver 2.2�0.7 tC km-2yr-1 of OCmodern

to the fluvial network in this setting. The onward fate of this ma-terial is not well constrained, but our analyses suggest that riv-erine transfer of OCmodern is supplemented by additionalprocesses, such as overland flow or small, shallow landslides.Most of the landslide-mobilized OCmodern (71�3%) remainson hillslopes. Here it may contribute to net-accumulationof carbon on 100 year timescales as landslide scars are re-colonized and coarse woody debris is retained in the land-scape. Landslides also cause high rates of turnover, withthe mapped landslides refreshing 0.3% of the area per de-cade. High rates of net ecosystem productivity have beenmeasured in this forest (94�11 tC km-2 yr-1), which wouldbe consistent with the high rates of landscape turnover promot-ing biomass growth in young sections of forest and suppressingecosystem retrogression. The impact of landslides for contem-porary carbon transfers at the multi-catchment scale thereforeawaits further investigation.

Acknowledgements— Live stem biomass data derive from the NationalVegetation Survey databank (http://nvs.landcareresearch.co.nz/). PJBreceived funding from the New Zealand Ministry of Science and Inno-vation (ecosystem resilience outcome-based investment). Fieldwork byRGH was funded by the Cambridge Commonwealth Trust. C. P. Stark isthanked for assistance with the landslide inventory. A. J. West and D. G.Milledge provided helpful discussions during manuscript preparation.We thank M. Evans for handling the manuscript and B. Gomez andan anonymous referee for thorough reviews that improved themanuscript.


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landslide impact on organic carbon cycling in a temperate montane forest

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