Carbon and nitrogen distribution and accumulation in a New Zealand ...

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Carbon and nitrogen distribution and accumulation in a New Zealand scrubland ecosystem Neal A. Scott, Joseph D. White, Jackie A. Townsend, David Whitehead, John R. Leathwick, Graeme M.J. Hall, Michael Marden, Graeme N.D. Rogers, Alex J. Watson, and Patrick T. Whaley

Abstract: Reversion of agricultural land to native woody vegetation can sequester carbon (C), influencing regional and global C budgets. We examined whole-ecosystem differences in C and nitrogen (N) storage and distribution, and sapwood – leaf area relationships in a scrubland vegetation chronosequence in New Zealand dominated by manuka (Leptospermum scoparium J.R. et G. Forst) and kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson). At 25 years, manuka dominated, and vegetation C was 6.5 kg C·m–2. In the 55-year-old stand, stem density was similar for the two species, and vegetation C storage was 15.1 kg C·m–2, similar to the 35-year-old stand (p = 0.9). Foliar biomass comprised 3–5% of vegetation C stock but contained 26%–37% of vegetation N. Root biomass was 10–15% of total and varied little with stand age. The sapwood – leaf area relationship differed significantly for the two species (p < 0.05). Mineral soil C and N (to 0.30 m) did not vary with stand age, but forest floor C and N were highest in the 55year-old stand (2 kg C·m–2; p < 0.01). Soil and forest floor C/N ratios were significantly higher in the 35-year-old stand (p < 0.04), possibly because of high interspecific competition for N. While the sampling intensity was too limited to allow spatial extrapolation, our results suggest that carbon accumulation in this scrubland is rapid and similar to plantation forests, suggesting that land abandonment could significantly impact New Zealand’s C budget. Résumé : La recolonisation des terres agricoles par la végétation ligneuse indigène permet d’immobiliser du carbone (C) et d’influencer ainsi le bilan régional et global de ce dernier. Les auteurs ont examiné les différences écosystémiques dans le stockage et la répartition du carbone et de l’azote (N) et la relation entre l’aubier et la surface foliaire, dans une chronoséquence végétale de scrub en Nouvelle-Zélande, dominée par le leptospermum manuka (Leptospermum scoparium J.R. et G. Forst) et le kunzea kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson). À l’âge de 25 ans, c’est le leptospermum manuka qui dominait et la teneur de la régénération en C était de 6,5 kg C·m–2. Dans le peuplement âgé de 55 ans, la densité des tiges était semblable chez les deux espèces et le stockage du C était de 15,1 kg C·m–2, c’est-à-dire, similaire à celui d’un peuplement âgé de 35 ans (p = 0,9). La biomasse foliaire comprenait de 3 à 5% du stock du C et jusqu’à 26 à 37% du N végétal. La biomasse racinaire représentait de 10 à 15% du total et variait peu avec l’âge du peuplement. Le rapport aubier-surface foliaire différait significativement chez les deux espèces (p < 0,05). Le C et le N du sol minéral (jusqu’à 0,30 m de profondeur) n’ont pas varié avec l’âge du peuplement, mais ceux du parterre forestier étaient les plus élevés dans le peuplement âgé de 55 ans (2 kg C·m–2; p < 0,01). Les rapports C/N du sol et du parterre forestier étaient significativement plus élevés dans le peuplement âgé de 35 ans (p < 0,04), probablement à cause d’une concurrence interspécifique pour le N. Malgré le fait que l’intensité de l’échantillonnage était trop limitée pour permettre une extrapolation spatiale, les résultats suggèrent que l’accumulation du carbone dans ce type de scrub est rapide et semblable à celle des forêts plantées, suggérant par là que l’abandon des terres agricoles peut avoir un impact significatif dans le bilan du C de la Nouvelle-Zélande. [Traduit par la Rédaction]

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Received July 20, 1999. Accepted February 25, 2000. N.A. Scott1 and J.A. Townsend. Landcare Research, Private Bag 11052, Palmerston North, New Zealand. J.D. White. Landcare Research, Private Bag 11052, Palmerston North, New Zealand and Department of Biology, Baylor University, Waco, TX, U.S.A. D. Whitehead, G.M.J. Hall, G.N.D. Rogers, and A.J. Watson. Landcare Research, Lincoln, Canterbury, New Zealand. J.R. Leathwick, and P.T. Whaley. Landcare Research, Hamilton, New Zealand. M. Marden. Landcare Research, Gisborne, New Zealand. 1

Corresponding author. e-mail: scottn@landcare.cri.nz

Can. J. For. Res. 30: 1246–1255 (2000)

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Introduction Regenerating forests may play a major role in the global carbon (C) cycle (e.g., Buschbacher et al. 1988; Houghton 1996). Current Intergovernmental Panel for Climate Change (IPCC) methodology for calculating greenhouse gas emission inventories (IPCC 1996) indicates that anthropogenicinduced changes in C stored in indigenous forests and shrublands could be included in national greenhouse gas inventories. In New Zealand, current calculations of CO2 emissions (7.5 Mt CO2-C/year in 1995 without C removals by sinks; Ministry for the Environment 1997) only consider C losses due to fire and harvest in indigenous forests and shrublands, yet both contain significant amounts of C (1835 © 2000 NRC Canada


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and 76 Mt C, respectively) (Tate et al. 1997). However, these ecosystems are unlikely to be “carbon neutral.” Work examining changes in forest biomass on the South Island using historical plot data suggests small C losses through time in some forest types, possibly because of the impact of introduced browsing animals (Hall and Hollinger 1997). In contrast, abandonment of marginal agricultural land and regrowth of shrublands could accumulate C. Further land abandonment is likely as agricultural use becomes marginal, increasing the potential C sequestration potential of native vegetation. Scrublands in New Zealand are generally characterized by seral vegetation arising from disturbance (e.g., abandonment of agricultural land, forest burning). In contrast to mixed shrublands (woody and herbaceous species), scrublands are usually dominated by one major (usually woody) plant form (Newsome 1987), often with a closed canopy. Overall, shrub- and scrub-lands are the primary successional vegetation type that establish following pasture abandonment, and their area is about 28% of New Zealand’s land area (Newsome 1987). Scrublands dominated by manuka (Leptospermum scoparium J.R. et G. Forst) and kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson) are a common, widespread secondary-successional vegetation community in New Zealand (Wardle 1991). These scrublands occur commonly around forest margins and (or) on sites cleared for agriculture but subsequently abandoned (Newsome 1987). Although estimates of the current extent of such vegetation vary markedly, Newsome (1987) estimated the extent of secondary vegetation dominated by manuka, kanuka, and (or) Pteridium esculentum (G. Forst) Cockayne, to be over 6000 km2 in the early 1980s. If mixed shrubland area is included where these two species are an important component, the area increases to about 14 000 km2 (Newsome 1987). In spite of their importance, no information exists on C stocks and accumulation rates for these scrublands. Manuka and kanuka differ in their longevity, habit, and environmental preferences. Manuka is generally of shorter stature (about 5 m maximum height) and can be distinguished from kanuka by its flakier bark, generally solitary flowers, and persistent, woody seed capsules (e.g., Allan 1961). Manuka is more tolerant of harsh environmental conditions (Wardle 1991), as evidenced by its widespread occurrence on sites prone to salt and wind (Esler and Astridge 1974), infertile soils (e.g., Esler and Rumball 1975; Leathwick and Rogers 1996), and elevated water tables (e.g., Leathwick 1987; Williams et al. 1990). Kanuka, which is the longer lived of the two species, grows much taller (10–15 m in older stands). It occurs infrequently on waterlogged soils but replaces manuka on dry sites provided soils are sufficiently fertile (Wardle 1991). While scrublands dominated by manuka and kanuka are seral communities, some may persist for hundreds of years (and multiple generations) depending on the rate of primary forest re-establishment (Newsome 1987). Because of their large spatial extent, these scrublands could be an important C sink. To test the hypothesis that C accumulation in these scrublands is rapid, we selected a stand chronosequence on similar soils with similar land-use history to quantify C stocks in different age stands. In addition, we examined species- and age-related differences in N distribution, sapwood area, and leaf area, as these factors in-

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fluence C accumulation rates. We also compared biomass estimates based on species-specific allometric equations to those based on tree volume estimates to test whether speciesspecific equations give more accurate biomass estimates for this scrubland than more generic tree-volume calculations.

Methods Site description We selected an age sequence of manuka–kanuka stands located in Tongariro National Park (TNP) in the central part of the North Island, New Zealand (39°05′ S, 175°45′ E). Historically, this area would have been covered by mature primary forests. However, much of the forest was burned initially around 600 years ago (McGlone 1983). Since then, and especially in the last 200 years, fires (primarily anthropogenic) were frequent in this area until the late 1940s, when active fire suppression and prevention began (Rogers 1994). The area is now covered by a mosaic of scrub vegetation of varying age dominated by manuka and kanuka. Topographic and soil variation are minimal, as the stands are located in a broad interfluvial area with soils homogenized as a result of historical volcanism. The stands (3–60 ha) comprising our chronosequence were created as a result of regrowth following burning and were approximately 25, 35, and 55 years old. Stand age was determined by examining marginal trees in adjacent unburned areas for signs of fire scars or growth release (Leathwick and Rogers 1996). These stands are located at an elevation of about 800 m, approaching the elevational limit of these species in this area. However, these species can be found at higher elevations; kanuka can be found up to 900 m in elevation, and manuka can occur up to 1250 m elevation. Mean annual temperature is 11.1°C, and mean annual precipitation is about 1610 mm. Soils in the area are derived from a series of ryolitic and andesitic volcanic eruptions, and are classified as Podzolic Orthic Pumice soils of the Rangipo series (Hewitt 1993) roughly similar to Vitrands in the U.S. Department of Agriculture (USDA) soil taxonomy system (Soil Survey Staff 1990). In general, these soils tend to be high in fertility (Hewitt 1993), and do not generally exhibit phosphorus limitation as do many New Zealand soils. In each stand (one stand of each age), we located five variablearea plots (Batchelor and Craib 1985) at 50-m intervals along a transect. Circular, variable-size plots were used to keep the number of stems sampled in the different stands similar (as close to 30 as possible) and ranged from 7.07 m2 in the 25-year-old stand to 12.56 m2 in the 35- and 55-year-old stands. Because of the irregular shape of the stands, we were unable to use straight transects while keeping all plots well within the stand, so transect shapes were modified to insure that all plots were at least 50 m from the edge and well within the stand of interest. In February 1996, we recorded the species, height, and diameter at breast height (DBH) for each individual stem in each plot. This included canopy-dominant individuals (dead and alive) as well as all understory species. We did not measure the age or wood density of the dead stems. Plot biomass was estimated using allometric functions developed for the site (see next section). Besides our site in TNP, we examined tree allometry for kanuka in a different area with contrasting climate and soils. Four sites were located in the east coast region of the North Island near Gisborne (38°30′ S, 178°04′ E) and comprised a stand chronosequence with mean stand ages of 3, 6, 16, and 32 years. Forests in this area were clear-felled in the latter part of the 19th century for agriculture, but increased erosion and declining productivity led to abandonment of significant areas after 70–100 years of grazing (Watson et al. 1995). Over 60 000 ha of land were identified as abandoned agricultural land as of 1995 (Dymond et al. 1996). Mean annual precipitation in this area is about 1409 mm, and mean annual temperature © 2000 NRC Canada

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1248 is about 14°C (New Zealand Meteorological Service 1983). Soils are derived primarily from Tertiary sedimentary material, and are classified as Orthic Recent Soils (Hewitt 1993), or as Inceptisols in the USDA Soil Classification system (Soil Survey Staff 1990).

Allometric functions for manuka and kanuka We developed allometric equations for the two major species at our TNP site by destructively harvesting 48 individual trees covering the size range present in the three stands. Individual trees were selected at random; cut at the base; and then divided up into stem, coarse branch (>5 mm), and canopy material (fine branches and foliage). Each component was weighed in the field on a spring scale. Individual disks were cut from the stem at four positions (base, DBH, lowest canopy branch (point of attachment), and top) for moisture and nitrogen determinations. Coarse branches and canopy material were subsampled, and all disks and subsamples were subsequently weighed on more precise scales for moisture determination. Canopy material (fine branches and foliage) was dried at 60°C, and then leaves were separated from the fine twigs to estimate the proportion of the canopy mass composed of twigs and foliage. Individual leaf area for these species is about 2–3 mm2 (smallest in kanuka), making quantitative separation of leaves difficult. Individual components were then dried at 60°C, weighed, and subsamples ground with a cyclone mill (0.5 mm) (UDY Corporation, Fort Collins, Colo.) for C and N analyses (LECO FP-2000 CNS Analyser, LECO Corp., St Joseph, Mich.). Individual tree root biomass was measured by excavating the roots of a subsample (15 trees) of the trees harvested for aboveground biomass determination. These trees represented a range of size, age, and species and were collected from each of the three age-classes. For the larger trees, we excavated an area of 12.6 m2 around the base of the tree. For smaller trees, only an area of 3.1 m2 was excavated. Any root extending outside this area was omitted from the individual tree biomass. Once the root mass was freed from the soil, we washed away the soil with water, exposing the roots. In most cases, there were large numbers of fine roots associated with the coarse root mass, but most of them were not attached to the tree and likely came from neighboring trees or from understorey species. These roots were not included in the individual tree biomass but were included in the total root biomass from the excavated area (one estimate of total root biomass). All roots were dried at 60°C, weighed, and subsampled for N analyses. We also estimated stand-scale fine-root (2.0 mm were included in the root biomass, as the water removed many of the finer roots (