New Zealand Journal of Forestry Science 42 (2012) 29-38 Published On-Line: 08/02/2012
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New Zealand Journal of Forestry Science 42 (2012) 29-38 www.scionresearch.com/nzjfs published on-line: 08/02/2012 Estimating carbon stocks in stands of Podocarpus cunninghamii in the eastern South Island high country of New Zealand Alwyn Williams*,† and David A. Norton Rural Ecology Research Group, School of Forestry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand †Current address: Centre for Environmental and Climate Research, Lund University, Lund 223-50, Sweden (Received for publication 4 February 2011; accepted in revised form 19 January 2012) *corresponding author: [email protected] Abstract The Land-Use, Land-Use Change and Forestry (LULUCF) sector of the Kyoto Protocol requires New Zealand to monitor changes in the country’s carbon stocks, including those within indigenous forests. Podocarpus cunninghamii Colenso was, in pre-human times, a dominant tree species within the forests of the South Island high country. Anthropogenic disturbance, primarily fire, has converted most of these forests to grassland. Despite this mass deforestation, remnant stands of Podocarpus cunninghamii still exist, and may represent important point sinks of carbon. This study provides first estimates of existing above- and below-ground carbon stocks in high country stands of Podocarpus cunninghamii and presents a preliminary model of Podocarpus cunninghamii carbon sequestration rate. Carbon stocks within high country stands of Podocarpus cunninghamii range from 7.3 t ha-1 in the drylands to 130.1 t ha-1 in the wetter areas. Estimates based on tree ring widths indicate a high country-wide Podocarpus cunninghamii carbon sequestration rate of 0.1 – 0.5 t ha-1 yr -1 for 250 – 1000 stems ha-1, respectively. Keywords: Afforestation; carbon sequestration; carbon stocks; deforestation; high country; indigenous forest; Kyoto Protocol; New Zealand; South Island; totara. Introduction Prior to human arrival, the eastern South Island high country was forested (McGlone, 1989; McWethy et al., As part of the global effort to confront climate change 2010). Outside of the areas dominated by Nothofagus, numerous countries have committed to reducing Podocarpus cunninghamii Colenso (mountain or Hall’s their carbon emissions (United Nations Framework totara; also known as Podocarpus hallii Kirk) forest was Convention on Climate Change UNFCC, 1998). The the main forest type. Human activity, primarily burning Land-Use, Land-Use Change and Forestry (LULUCF) (McWethy et al., 2010), has converted most of the pre- sector of the Kyoto Protocol requires signatory human cover of Podocarpus cunninghamii forest to countries, including New Zealand, to monitor changes grassland, however remnant forest stands still remain. in their carbon stocks (gains and losses), including both These remnants provide vital habitat for the high exotic plantation and indigenous forests. As a result, country’s indigenous biodiversity and offer valuable there is increasing interest in the quantity of carbon insight into the region’s pre-human flora (Norton, 2006; stored within New Zealand’s indigenous forests, Walker et al., 2009). encompassing both new (restored and regenerating) and old growth forests (Carswell et al., 2008; On a per-hectare basis, living carbon stocks within Coomes et al., 2002). forests are substantially greater than grasslands © 2012 New Zealand Forest Research Institute Limited, trading as Scion ISSN 0048-0134 (print) ISSN 1179-5395 (on-line) 30 Williams & Norton: New Zealand Journal of Forestry Science 42 (2012) 29-38 (Tate et al., 1997), consequently remnant high country River and State Highway 8 (44° 29’ S, 169° 47’ E). The stands of Podocarpus cunninghamii may represent Podocarpus cunninghamii dominated communities significant point sinks of carbon in comparison to occur within shallow but steep rocky gullies. Where adjacent induced grasslands; however, little is known Podocarpus cunninghamii forms a canopy it is dense of their carbon stocks. Collation of such information and other vascular plant species are rare; however the is necessary not only for inventories of existing shrub community can be quite diverse within areas of carbon stocks, but also to appreciate the potential of thin canopy or light gaps. Species such as Coprosma restoration of this forest type to increase carbon stocks. propinqua A.Cunn., Corokia cotoneaster Raoul, However, current estimates are limited, being based Aristotelia fruticosa Hook.f. and the invasive Rosa on coarse-scale extrapolations (Coomes, et al., 2002; rubiginosa L. are common. Scattered shrubs, often Tate, et al., 1997) or limited to a single stand Discaria toumatou Raoul, are present in the pasture (Kirschbaum et al., 2009). This paper provides some surrounding the stands and appear to have a positive initial estimates of carbon stocks for Podocarpus association with the appearance of Podocarpus cunninghamii growing as the dominant species in cunninghamii seedlings. Natural regeneration of forest stands in the eastern South Island high country. Podocarpus cunninghamii and other native shrubs Estimates were calculated based on living biomass is evidence of the gaps between the stands slowly and tree growth rate data for six sites across a west- beginning to in-fill. This is in spite of ongoing sheep east rainfall gradient. grazing on these faces. Mount Ben Ohau Methods This site is located in Ruataniwha Conservation Sites Park on the west facing slope of Mount Ben Ohau overlooking Lake Ohau (44° 15’ S, 169° 53’ E). Six South Island Podocarpus cunninghamii sites were The site is characterised by numerous rock fields, included in this study. These sites were chosen because: broken only by patches of grassland, shrub land or Podocarpus cunninghamii woodland. Despite the (1) they spanned the high country rainfall gradient that patchy nature of the vegetation, this site represents exists east of the Main Divide, and therefore provided the largest population of Podocarpus cunninghamii samples from both very wet and very dry environments; in the Mackenzie Ecological Region (Espie et al., and 1984). Where enough trees are present to form a canopy, Podocarpus cunninghamii is the only canopy (2) Podocarpus cunninghamii was the dominant tree tree. Canopy forming species such as Griselinia species at each site. littoralis Raoul and Sophora microphylla Aiton are only represented by a few scattered individuals. The A brief description of each site now follows; see shrub component is diverse and often extremely Tables 1 and 2 for data on each site’s climate and dense. Common species include Discaria toumatou, Podocarpus cunninghamii stand characteristics, Coprosma propinqua, Corokia cotoneaster and respectively. Aristotelia fruticosa. Given the openness of the vegetation, a diverse community of grassland plants Birdwood Station is also present, including the native Aciphylla aurea W.R.B.Oliv. and the invasive exotic Hieracium pilosella Part of the Diadem Range, this site lies on Birdwood L.. Station, near Omarama, and overlooks both the Ahuriri TABLE 1: Environmental data for each site. All figures, excluding rainfall, are annual means extracted from the Land Environments of New Zealand (LENZ) database, (Landcare Research, 2010). Rainfall figures are from Pascoe (1983) for Upper Hapuku and from Ryan (1987) for all other sites. Site Annual Soil water Vapour Mean Minimum Solar rainfall deficit pressure deficit temperature temperature radiation (mm) (mm) (kPa) (°C) (°C) (MJ m-2 d-1) Upper Hapuku 2000 0.00 0.50 8.09 -1.53 14.60 Upper Lawrence 1000 – 1500 0.00 0.44 8.00 -1.95 13.99 Mount Dobson 1000 – 1500 0.00 0.38 8.08 -2.30 13.40 Mount Cook 1500 – 2000 0.00 0.39 8.03 -2.30 13.85 Mount Ben Ohau 800 – 1000 30.13 0.43 8.20 -2.45 13.89 Birdwood Station <600 98.73 0.43 8.15 -3.07 13.79 © 2012 New Zealand Forest Research Institute Limited, trading as Scion ISSN 0048-0134 (print) ISSN 1179-5395 (on-line) Williams & Norton: New Zealand Journal of Forestry Science 42 (2012) 29-38 31 TABLE 2: Stand characteristics of Podocarpus cunninghamii at the six study sites. Site Altitude Plot size Stem density Basal area Average height Mean stand age (m) (ha) (stems ha-1) (m2 ha-1) (m) (years ± 1 s.e.) Upper Hapuku 850 0.20 250 35.29 9.90 156 ± 13 Upper Lawrence 850 0.20 250 44.65 12.25 287 ± 28 Mount Cook 800 0.40 125 6.17 6.97 109 ± 14 Mount Dobson 850 0.21 238 14.13 6.05 134 ± 11 Mount Ben Ohau 750 0.12 416 10.02 4.04 59 ± 3 Birdwood Station 700 0.05 1000 6.27 3.68 52 ± 2 Mount Dobson divaricata A.Cunn.. The sheer size and abundance of Podocarpus cunninghamii trees at this site, together The site is located approximately 10 km east of Lake with the intact broadleaf canopy, suggests that while Tekapo within the Te Kahui Kaupeka Conservation Park the historical forest area has suffered a considerable (formerly known as the Mount Dobson Conservation reduction, the remaining stands are relatively intact Area) at the southern end of the Two Thumb Range and representative of old growth forest. (43° 58’ S, 170° 41’ E). The stand is present on a steep gully face with rock outcrops at the top of the site. The Upper Hapuku canopy is dominated by Podocarpus cunninghamii, Griselinia littoralis and Pseudopanax crassifolius The site studied is located within a series of steep (Sol. ex A.Cunn.) C.Koch, with emerging Podocarpus valleys cloaked in Podocarpus cunninghamii cunninghamii also present. Trees and shrubs such as dominated forest in the Mount Manakau Scenic Pseudopanax colensoi var. ternatus Wardle, Coprosma Reserve (42° 14’ S, 173° 40’ E). Extensive linariifolia Hook.f. and Coprosma propinqua all occur stands of large Podocarpus cunninghamii trees frequently. The site lies within subalpine grassland and (15 – 20 m tall) dominate the vegetation. Griselinia scrub, and extensive cutting of wilding conifers has littoralis, Pseudopanax crassifolius and Coprosma taken place on adjacent land.