Species-specific basic stem-wood densities for twelve indigenous forest and shrubland species of known age, New Zealand

Background: Tree carbon estimates for New Zealand indigenous tree and shrub species are largely based on mean basic stem-wood densities derived from a limited number of trees, often of unspecified age and from a limited number of sites throughout New Zealand. Yet stem-wood density values feed directly into New Zealand’s international and national greenhouse gas accounting. We augment existing published basic stem-wood density data with new agespecific values for 12 indigenous forest and shrubland species, including rarely obtained values for trees <6-years old, across 21 widely-distributed sites between latitudes 35° and 46° S, and explore relationships commonly used to estimate carbon stocks. Methods: The volume of 478 whole stem-wood discs collected at breast height (BH) was determined by water displacement, oven dried, and weighed. Regression analyses were used to determine possible relationships between basic stem-wood density, and tree height, root collar diameter (RCD), and diameter at breast height (DBH). Unbalanced ANOVA was used to determine inter-species differences in basic stem-wood density in 5-yearly age groups (i.e. 0–5 years, 6–10 years etc.) (P<0.05). As specific taxa of Kunzea ericoides (Myrtaceae) has only been identified at some study sites we combine the data from each site, and use the term Kunzea spp. We compare our ageand species-specific results with existing published data where age is specified versus non-age-specific values. Results: Kunzea spp. and Leptospermum scoparium exhibited positive correlations between basic stem-wood density and tree height, RCD, and DBH. No relationships were established for Melicytus ramiflorus, Coprosma grandiflora, Weinmannia racemosa ≥6-years old, or for Podocarpus totara, Agathis australis, Vitex lucens, and Alectryon excelsus <6-years old. Dacrydium cupressinum and Prumnopitys ferruginea <6-years old exhibited a significant positive relationship with DBH only, while for Dacrycarpus dacrydioides, each correlation was negative. Irrespective of age, basic stem-wood density is not different between the hardwood species L. scoparium and Kunzea spp. but is significantly greater (P=0.001) than that of the remaining, and predominantly softwood species of equivalent age. For Kunzea spp., L. scoparium, Coprosma grandiflora, Weinmannia racemosa, and Melicytus ramiflorus ≥6-years old there was no evidence that basic stem-wood density increased with tree age, and values were within the range of published and unpublished data. For naturally reverting stands of Kunzea spp. located between latitudes 35° to 46° S, basic stem-wood density values tended to increase with decreased elevation and increased temperature. Conclusions: Increasing basic wood density values in Kunzea spp. with decreased elevation and increased temperature suggest that where local data are available its use would improve the accuracy of biomass estimates both locally and nationally. Furthermore, refining biomass estimates for existing communities of mixed softwood species, stands of regenerating shrubland, and new plantings of indigenous species will require additional basic stem-wood density values for scaling from stem wood volume to total stand biomass. New Zealand Journal of Forestry Science Marden et al. New Zealand Journal of Forestry Science (2021) 51:1 https://doi.org/10.33494/nzjfs512021x121x E-ISSN: 1179-5395 published on-line: 15/02/2021 © The Author(s). 2021 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Research Article Open Access


Introduction
The variability in basic stem-wood density and age are critical factors influencing estimates of wood biomass and carbon storage capability (Chave et al. 2004, Dale 2013. Stem-wood density values feed directly into New Zealand's international greenhouse gas accounting of forest carbon stocks, and for internal schemes such as the Emissions Trading Scheme (ETS) (Ministry for Primary Industries 2017), and the 1 Billion Trees Programme (1BT) (Ministry for Primary Industries 2018).
Previously, New Zealand studies have estimated the biomass of indigenous forest stands for tree carbon stocks and sequestration using diameters and height measurements of individual trees in forest inventory plots (Carswell et al. 2012, Scott et al. 2000, Trotter et al. 2005, Beets et al. 2014, Schwendenmann & Mitchell 2014, Dale 2013, Holdaway et al. 2014. When basic wood density values are available for only a limited number of species and locations, wood volume is converted to carbon stocks using generic (as opposed to speciesspecific) functions based on the basic density of stemwood (oven-dry mass/ 'green' volume). Where speciesspecific and/or regional basic stem-wood density values are unavailable, congeneric values are used instead, or in their absence, the mean of all published values e.g. Beets et al. (2012 and unpublished data 1 ).
While most early studies in New Zealand collected basic stem-wood density data from sites of wellestablished indigenous shrubs and trees, age-specific and species-specific stem-wood density data for the early growth period of many species remain elusive. The absence of taxon-specific stem-wood density and age-class distribution data of a wide variety of species over a range of geographic sites introduces uncertainty in the accuracy of New Zealand's national carbon budget calculations (Scott et al. 2000, Chave et al. 2004, Holdaway 2014. The use of taxon-specific stem-wood density to scale tree volume, as yield or growth, to stem biomass, and from stem biomass to total biomass will improve the accuracy of species-specific allometric equations for estimating tree carbon storage, and avoid potential bias to national carbon budgets. Furthermore, basic stem-wood density values for a few widespread indigenous species (Entrican et al. 1951, Hall et al. unpublished data 2 ), and for specific species with a more restricted geographic range (Wardle 1991), can vary depending on geographic location, though no relationships have been verified with respect to climate or site factors (Hall et al. unpublished data 2 ). Clifton (1990) suggests that basic stem-wood density varies according to the age of the tree, the location of the wood within a tree (outer-wood/inner-wood, base or top of a tree), and while densities have been determined for some of New Zealand's historically important merchantable wood species (Hinds & Reid 1957, Beets et al. 2012, the age of the trees and variations in basic stem-wood density were not determined, the sample size was generally small, the methods uncertain, and the location vague. Stand basic stem-wood densities will also change with time, influenced by climatic variability and site-specific physical factors, including soil type, slope, aspect, elevation and rainfall regime, all of which can affect growth rates, plant survival, and carbon sequestration rates. Furthermore, as the area of indigenous species plantings and their diversity increases with age, age-specific and speciesspecific stem-wood density data, will be relevant for Afforestation/Reforestation reporting, for updating the national carbon inventory system (Land Use and Carbon Accounting System -LUCAS), and policy, to reduce net greenhouse gas emissions as required under the Kyoto Protocol (Ministry of the Environment 2010), and for comparison with pre-calculated forest carbon stocks (includes stem, bark, branch, leaves, litter, woody debris, stumps and roots expressed in units of tonnes of CO 2 ha -1 ), by age, for given forest types in the Emissions Trading Scheme (Ministry for Primary Industries 2017).
We augment existing published basic stem-wood density data with new age-specific values for 12 of New Zealand's indigenous forest and shrubland species from 21 widely distributed sites located between latitudes 35° to 46° S. We explore relationships between basic stem-wood density and tree parameters commonly used to estimate stem carbon stocks, and applicable to future efforts to reduce the uncertainty of carbon stock estimates for forest and shrubland communities where basic stem-wood density values for different age classes of many species is currently missing.

Study sites
Basic wood density data was collected from 14 sites located in the North Island and from 7 sites in the South Island of New Zealand with a latitudinal range between 35° and 46° S (Fig. 1). Details of species, elevation, and substrate characteristics are summarised in Table 1, and presented in more detail in Appendix Table A1.

Species nomenclature
Since this study began, there has been a taxonomic revision of the New Zealand Kunzea ericoides (Myrtaceae) complex in New Zealand (de Lange 2014). Ten Kunzea species endemic to New Zealand are now recognised, seven of which are new. Where we have some confidence in the identification of new taxa these are presented in Table 1 and Appendix Table A1. As specific taxa have not been identified for all sites we have not attempted to analyse for possible inter-specific variations in basic stem-wood density for this genus but instead we combine data for all sites where present and use the generic term Kunzea spp.

Wood sampling and density
There are many methods of sampling wood and determining wood density (Chave 2005, Williamson & Wiemann 2010. In this study, wood density is defined as the ratio of the oven-dry mass of a stem-wood disc sampled at a standard height divided by the mass of water displaced by its green volume to give wood specific gravity (WSG). WSG is described as basic wood density or stem-wood density throughout the text.
Discs cut from the stem account for the change in density from pith to bark (Williamson & Wiemann 2010, Beets et al. 2012. Basic stem density measurements of discs were sourced from trees located in areas of naturally regenerating Kunzea spp. (sites 2-7, 9, 12-21), regenerating Leptospermum scoparium (sites 1, 2, 9, 14, 16, and 21), a lowland shrub community (site 11), a species growth trial of indigenous softwood and hardwood species (site 8), and from an area of low-density plantings of L. scoparium (site 10). As the purpose of the research undertaken at each site differed, 256 of the basic stem-wood density measurements were of discs with the bark intact (Cornelissen et al. 2003) (e.g. sites 2, 4-11 & 21) and 222 measurements were of discs with the bark removed (e.g. sites 1, 3, 12-20). All discs were sampled at breast height (BH) (1.4 m above ground-level). The fresh volume of each wood disc was determined by water displacement, then oven dried at 105°C (Cornelissen et al. 2003) and weighed. For multiple-stemmed trees, a disc was cut from each stem, and the density averaged for the tree. Tree age in naturally regenerating stands was based on ring counts of the single oldest stem. The age of the species established in the plant growth trial (site 8) was based on the known date that seedlings were 'pricked-out' into seed trays in the nursery. For the site established in L. scoparium for honey production (site 10), the year in which 1-year-old, nursery-raised seedlings were planted was known.
For Melicytus ramiflorus and Coprosma grandiflora (site 11), discs were collected in the field at BH and transported in a sealed container to avoid moisture loss. In the laboratory, discs were soaked before the volume was determined by water displacement. Discs were dried at 80°C until dry and weighed (Cornelissen et al. 2003). Tree height was based on the tallest single stem. Tree age was based on ring counts of a disc cut from a representative stem of the tree.
For Kunzea spp. and L. scoparium collected from sites 2, 9, and 21, discs were collected at BH and frozen at -20°C. The discs were thawed at room temperature and soaked in water for 2 days before their volume was assessed. As L. scoparium and Kunzea spp. tend to split during drying making ring counting and measuring difficult, the discs were partially dried at 35°C, the rings counted, and then dried at 80°C and weighed.

Statistical analyses
Linear regression analysis best fitted the data and was used within each tree species to determine the possible relationship between basic stem-wood density and tree height, root collar diameter (RCD), diameter at breast height (DBH), and tree age.
Unbalanced ANOVA with least significant differences (LSD) was used to determine differences in basic stemwood density between species and for Kunzea spp. to assess if densities differed between 17 sites located throughout New Zealand.
Density values were grouped into 5-yearly age classes (e.g. 0-5-years, 6-10 years etc.). Only data sets within a species, and within an age class with three or more replicates (irrespective of the geographical position) were used in the analysis. The average basic stem-wood densities for younger (<6-years old) and older (≥6-years old) trees are compared with published values. For the FIGURE 1: Location of 21 New Zealand indigenous forest, shrubland, and experimental trial sites where discs were collected for analysis of basic stem-wood density.  Hinds & Reid 1957, Harris 1986, and Clifton 1990, tree age is rarely specified, and variations in basic stem-wood density values derived from merchantable-sized trees after removal of the bark is not given. For comparative purposes we use these few available published values (Appendix Table A2) together with a larger data set of mean age-specific/non-age-specific wood density values (bark removed) collected from Carbon Monitoring System (LUCAS) plots (20m x 20 m) across a wide range of well-established and pre-defined natural forest and shrubland types (Table A2)  We did not attempt to analyse for the influence of bark thickness on basic stem wood density values (i.e. inclusive versus exclusive of bark), as for the age-range (3-to 105-years old) of the shrubland species presented in this paper, all values were expected to fall well within the range of the published data. In the absence of reliable basic stem-wood density values for individual stems, often determined for only a small sample size of trees with widely varying, or of unknown age, and variability in basic stem-wood density values, the values in this paper are presented as means (Appendix Tables A3-A5).
All statistical analyses were undertaken using Genstat (VSN International, Hemel Hempstead, UK) and were considered significant if P<0.05.

Basic stem-wood density-allometric relationships
For >6-year-old regenerating Kunzea spp. basic stemwood density was significantly, positively correlated with tree height, as was also the case for L. scoparium (Table 2). Of the plot-based species <6-years old, the correlation for basic stem-wood density with tree height was strongest (and positive) for Prumnopitys ferruginea (Table 2) but was only just statistically significant, probably due to the small sample size (n=7). Interestingly, Dacrycarpus dacrydioides exhibited a significant negative correlation with about 30% of the variation in basic stem-wood density explained by tree height. There were no other significant relationships between basic stem-wood density and tree height for the remaining plot-based or regenerating shrubland species. Basic stem-wood density and RCD were positively correlated for regenerating L. scoparium and Kunzea spp. >6-years old (Table 2). Root collar diameter and density values were negatively correlated for plotbased Dacrycarpus dacrydioides (Table 2). There were no significant correlations between basic stem-wood density and RCD for the remaining plot-based and regenerating shrubland species <6-years old.
Basic stem-wood density and DBH were positively correlated for regenerating L. scoparium, Kunzea spp., plot-based Dacrydium cupressinum and Prumnopitys ferruginea (Table 2) with DBH explaining 17-73% of the variation in density. Basic stem-wood and DBH were negatively correlated for Dacrycarpus dacrydioides (Table 2). There were no significant correlations between basic stem-wood density and DBH for the remaining plot-based and regenerating species.
Basic stem-wood density was not correlated with tree age for low-density plantings of L. scoparium (site 10) between ages 4-and 6-years and increased with increasing tree age (data not shown). Conversely, for naturally reverting stands of L. scoparium, Kunzea spp., Coprosma grandiflora, Melicytus ramiflorus and Weinmannia racemosa, basic stem-wood density values of ≥6-years-old trees were not significant.

Comparisons of mean basic wood densities by ageclass
Basic stem-wood density of L. scoparium was greater than for the remainder of the plot-based species trialled for trees <6-years of age ( Fig. 2a). Basic stem-wood density was as follows for the various species in this age group: L. scoparium > Alectryon excelsus > Dacrycarpus dacrydioides = Podocarpus totara = Prumnopitys ferruginea = Dacrydium cupressinum > Agathis australis = Vitex lucens.
Irrespective of age, the basic stem-wood density values for both Kunzea spp. and L. scoparium were not significantly different from each other but were significantly greater than that for all other species for which age-specific data was available.

Comparisons of basic stem-wood density values with published data
Basic stem-wood densities for ≥6-year-old specimen trees of L. scoparium, Kunzea spp., Melicytus ramiflorus, Coprosma grandiflora, and Weinmannia racemosa derived from natural stands indicative of advanced succession toward indigenous forest, fall within the range of these published values (Fig. 3a).
Conversely, the mean basic stem-wood density values for trees <6-years old were either bordered on the lower limit of published means of older trees or significantly lower than published values (Fig. 3b). FIGURE 3: Comparison of: a) age-specific mean basic stem-wood density values for Kunzea spp. and Leptospermum scoparium ≥6-years old with densities sourced from published and unpublished literature. Density data for trees of known age was analysed separate to that for trees where age was not specified (see Table A2); and b) comparison of mean basic wood densities for trees <6-years old (grey bars) with mean densities of ≥6-yearold trees (dots) as sourced from published and unpublished literature (see Table A2). For Melicytus ramiflorus, Coprosma grandiflora, and Weinmannia racemosa, age-specific mean basic stem-wood density values (white bars in Fig. 3a) are compared with mean densities (dots) sourced from published and unpublished literature where age was not specified. Sample numbers shown at base of each bar. Error bars represent the standard error of the mean.

Geographic distribution in Kunzea spp. and L. scoparium basic stem-wood density
While there is considerable variation in mean basic stem-wood values within naturally regenerating stands of Kunzea spp. and L. scoparium, there is no supporting evidence that their density is significantly different between locations within either the North or South Island of New Zealand, between these islands, or between latitudes 35° to 46°S (Fig. 4). For all remaining species there was insufficient basic stem-wood density data to support a similar statistical analysis.

Discussion
Basic wood density is one of the largest sources of variation in estimates of biomass and in the calculation of carbon sequestration (Holdaway et al. 2014), yet these estimates are essential for New Zealand's international and national reporting of GHG budgets. To date, allometric functions have largely been based on limited stem-wood density data, and where species-specific and/or regional basic stem-wood density values are unavailable, congeneric values have been used instead, or, in their absence, the mean of all published values have been used (Peltzer & Payton unpublished data 3 , Beets et al. unpublished data 1 ). However, given that the earliest of the published values of basic stem wood density for merchantable timber trees were likely determined following the removal of the bark, a comparison with the means of all age-specific stem-wood densities, whether determined with the bark intact or after the removal of bark, might be considered invalid. Nonetheless, as has been shown in this paper, the basic stem-wood densities of ≥6-year-old trees comprising natural stands indicative of advanced succession toward indigenous forest fall well-within the range of the earlier published values. Furthermore, given the dearth of available data for many of the dominant and larger tree components of New Zealand's indigenous forests, the diversity of species, and the difficulty of accessing them in remote locations, where species-specific wood density values obtained for indigenous species harvested for timber exist, they serve as valuable reference points.  Table 1 and presented in greater detail in Table A1. Error bars represent the standard error of the mean. Bars with different letters were significantly different (P<0.05).
At the younger end of the age spectrum, for species typically associated with the early phase of shrubland regeneration, tall statured shrubland classes, and mixed species forests, insufficient basic wood density data together with simple field measurements are a limitation to the development of appropriate allometric functions for improving estimates of biomass and carbon stocks. Furthermore, the use of different methods in the measurement of basic stem wood density (over bark versus under bark) has necessitated the development of equations that account for related variations in basic wood density in the calculation of tree biomass and changes in carbon stocks over time (Hall et al. unpublished data 2 ). However, until additional basic stem-wood density data can be collected for a sufficiently diverse range of specimen trees comprising a wide range of indigenous shrubland, forest types, and ages, the continued use of the mean of all available basic stem-wood density values will likely give the best estimate of stem carbon stocks.
Although the basic stem-wood densities of Kunzea spp. and L. scoparium (both widely distributed shrubland species and a dominant component of regenerating forest on extensive areas of marginal hill country), are not significantly different from each other, they are both significantly higher than those of most of New Zealand's oldest indigenous forest and other shrubland species typically falling between 400 and 600 kg m 3 (Allen et al. 1992). Therefore, using functions based on the stemwood density of either Kunzea spp. or L. scoparium to scale tree volume, as yield or growth, to stem biomass, and from stem biomass to total biomass for different mixed-species indigenous forest communities is likely to overestimate total biomass.
For Kunzea spp., while there is variation in intraspecific mean basic stem-wood density values at different sites, there is no evidence from our data that stem-wood density is significantly different between the 17 locations where this species occurs as naturally regenerating shrubland. Trends of increasing wood density values with decreased elevation (Lassen & Okkonen 1969) and increased temperature (Filipescu et al. 2014) have been reported for New Zealand-grown Douglas-fir (Kimberley et al. 2017), and for P. radiata basic wood density values show a gradual decrease from sea level to higher elevations, and from north to south (Clifton 1990, Palmer et al. 2013. For Kunzea spp., however, while the results support a correlation between decreasing basic wood densities from sea level to higher elevations, there remains little evidence in support of wood densities decreasing north to south. Other environmental influences, including intolerance to salt (Esler & Astridge 1974), soil fertility (Cown & McConchie 1981), soil moisture retention and stress (Smale 1994), variations in genetics (de Lange 2014) and rainfall distribution, are also likely to affect growth strategies (Wardle 1969), tree form, and ultimately basic stem-wood density of many of New Zealand's indigenous shrubland and forest species. A site-by-site analysis of these factors was considered beyond the scope of this paper.
Mean basic stem-wood densities of trees <6-years old were either significantly lower, or at the lower end of published values (Fig. 3b), but that within ca. ≥6 years after establishment, basic stem-wood density values approach that of older trees, and differs little thereafter (Fig. 3a). We therefore concur with Beets et al.
(unpublished data 1 ) on the strength of this relationship. Differences in basic stem-wood density values between trees <6-years old and older are therefore likely to be primarily a function of their age. Deng et al. (2014) found that stem-wood density of Pinus massoniana stems was significantly influenced by tree age, relative heights, and social class, while Beets et al. (2012) confirmed that stem-wood density at each relative height in older trees (age unspecified) was significantly higher than that of younger trees. Iida (2012) found that low stem-wood density was linked to the propensity of some species to select for vertical growth (tall and thin stemmed with narrow and shallow canopies) and may therefore underlie the interspecific trade-off between effective height gain and a persistent life in the understorey (Kohyama 1987(Kohyama , 1993Kohyama & Hotta 1990). Furthermore, relationships between stem-wood density and tree height may be related to differences in stand density. For example, L. scoparium <6-years old in densely-stocked, naturally reverting stands are tall and thin-stemmed and contrast markedly with the shorter and thicker-stemmed trees that develop when planted at low densities (Marden et al. 2020). Perhaps, as has been shown in studies across a range of conifer species (Watt et al. 2011), the basic stem-wood density of L. scoparium would be expected to be lower in wider-spaced (planted) stands than in fully stocked stands that have reverted naturally. Unfortunately, insufficient wood density data for L. scoparium <6-years old from naturally reverting stands precluded such an analysis.
To reduce net greenhouse gas emissions, as required under the Kyoto Protocol (Ministry of the Environment 2010), a number of government-funded schemes (e.g. Afforestation Grant Schemes (Ministry for Primary Industries 2015a) and the Permanent Forest Sink Initiative (Ministry for Primary Industries 2015b) have been introduced to facilitate natural regeneration of shrubland, and the planting of new areas of forest (exotic and indigenous). Together with the recently announced government goal to plant one billion trees over the next 10 years (1 BT Programme) (Ministry for Primary Industries 2018), ca 1.45 million ha of steep, erosionprone pastoral hill country considered marginal for long-term agriculture will be targeted for transitioning to a permanent indigenous shrubland or forest (Trotter et al. 2005). In such high-risk areas woody indigenous shrubland largely comprising Kunzea spp. and L. scoparium has in the past played a significant role in mitigating erosion (Marden & Rowan 1993;Ministry for Primary Industries 2015a, 2015b. Together with increasing interest in high UMF (unique mānuka factor) values associated with honey produced by L. scoparium, the establishment of low-density plantings averaging ca 825 to 1100 stems ha -1 (McPherson & Newstrom-Lloyd 2018) is seen as an alternative and viable land management option for erosion prone steeplands (Ministry for Primary Industries 2015c).
Using linear regression analyses based on mean wood density values measured for Leptospermum scoparium <6-years old, new plantings at the recommended planting density, would by year 5 amass a forest carbon stock of 6.1 t CO 2 ha −1 (excluding coarse woody debris and fine litter on the forest floor) (Marden & Lambie 2016). Alternatively, a mixed planting of successional broadleaved and conifer species would within the same time frame potentially amass a carbon stock of ~3.8 t CO 2 ha -1 (Marden et al. 2018), while plantings consisting of a mix of early colonising seral species would amass a forest carbon stock of 8.8 t CO 2 ha -1 (unpublished). Thus, the establishment of early colonising seral species on marginal land would amass an additional ~1 t CO 2 ha −1 over and above the 7.8 t CO 2 ha −1 estimated for the 5-year period from the date of planting (Ministry for Primary Industries 2017). Conversely, the planting of mixed indigenous broadleaved and coniferous species at the same density would amass ~4 t CO 2 ha -1 less, and plantings of Leptospermum scoparium ~1.7 t CO 2 ha −1 less. By implication, to achieve a similar level of carbon stock for new plantings of broadleaved and conifer species within this time frame would require an increase in planting density to ~2000 stems ha -1 and for areas planted and managed for mānuka honey production, a planting density of 1200-1300 stems ha -1 would be required.
These estimates of carbon stocks are however based on only a few studies of indigenous species that comprise the many shrubland and forest communities present within New Zealand. With the pending conversion of extensive areas of former pastoral land to indigenous shrubland and forest through passive reversion, and by planting, therein lies an opportunity to validate and/or improve the accuracy of current estimates of biomass and carbon stocks during their early growth period, and for a wider range of species, by developing further allometric functions based on species-specific, basic stem-wood density values.

Conclusions
This study presents an analysis of a significant database of previously unpublished basic wood-density values collected for a range of New Zealand's indigenous shrubland and forest species of varying age, and from sites located throughout both North and South Islands. The findings indicate that for the most geographically widespread shrubland species, Kunzea spp., differences in local site factors may affect tree parameters including basic wood density to a greater extent than wide differences in latitude within the normal growing range of the species. The data do however support trends showing that basic wood density values increase with decreased elevation, and increased temperature and where local data are available its use would improve the accuracy of biomass estimates both locally and nationally. Insufficient site-specific information precludes further comment on other factors (e.g. soil fertility, plant spacing) that likely contribute to variability in basic stem-wood density values.
For each of the species <6-years old for which basic stem-wood densities were collected, their mean values were significantly lower, or at the lower end of published values for trees ≥6-years old after which basic stemwood density values remain unchanged.
Age-specific basic stem-wood density data is scarce for shrubland communities dominated by mixed softwood species that comprise 90% of the national live tree biomass stock. Furthermore, as their stem-wood density is considerably lower than for hardwood species, additional stem-wood density data are needed for use in combination with species-abundance information from LUCAS plots to update allometric functions applicable to areas of naturally reverting shrubland and to areas of former pastoral land pending their conversion to indigenous shrubland.
As shown for the few indigenous species for which biomass and/or wood density data has been collected, at a planting density of 1000 stems ha -1 , early colonising seral species would within 5-years amass a higher carbon stock of 8.8 t CO 2 ha −1 than would plantings of Leptospermum scoparium ~6.1 t CO 2 ha −1 or a mixedspecies planting of indigenous broadleaved and coniferous species ~3.8 t CO 2 ha -1 .
To account for the variability in densities between outer-wood (and bark) and inner-wood with tree height, estimates of the mean density of whole stems will require the collection of stem-wood data from discs at intervals along the stem, as opposed to just breast height or by coring.

Competing interests
The authors declare that they have no competing interests.

Authors' contributions
MM was the primary author. SL compiled the data into spreadsheets and completed the statistical analyses. LB contributed data. All authors read and approved the manuscript.
trial was located and to other landowners for allowing access to their respective properties at the time these studies were undertaken. We thank interns Claire Butty (France), Sandra Viel (Germany), Kaisa Valkonen (Finland), and Landcare Research colleague's Dr Chris Phillips, Alex Watson, Richard Hemming and Scott Bartlam for assistance with data collection. Hawke's Bay Regional Council provided Stevie and Jack Smidt to assist with data collection at Lake Tutira. John Dando and Ted Pinkney assisted with the collection of discs and growthring counts. Graphics were drawn by Nic Faville. Anne Austin edited the script and GIS support was provided by Anne Sutherland of Landcare Research, NZ, Ltd. This paper was reviewed by Dr Mark Smale, thanks also to the anonymous external reviewers for their valuable comment. Over the years, research has been supported by funding from the Ministry of Business, Innovation and Employment, the Sustainable Land Use Research Initiative (SLURI) to Plant and Food, the Ministry for Primary Industries, and the Landcare Research Capability Fund.

Availability of data and materials
Please contact the corresponding author for data requests. Bier, H. & R.A.J. Britton (1999) (Table A4) stand on gently, southwest-facing slope 100-140m above sea level. Bedrock consists of greywacke argillites and sandstones (Geological Map of New Zealand, 1967). Soils are deeply weathered and classified as Altic Soils (Hewitt, 2010).
Site 2: Waitakere Range. Within the Waitakere Range (174° 35´ 14 42 E, 37° 00´ 10 17 S) stem-wood discs were collected from naturally reverting stands of well-established Kunzea spp. (Table A3) and L. scoparium (Table A4) of unknown age. At an elevation of ca. 40 m, slopes ranged between 0 and 35°. The geology comprises volcanic andesitic lava, conglomerates, and breccia of the Waitemata and Waitakere groups of early Miocene (late Otaian-middle Altonian) age. Soils comprise weathered volcanics consisting of yellow-brown granular clay grading to a compact yellow brown to brown subsoil (Hayward, 1983). The climate is relatively mild and moist with annual rainfall of ca 1250 mm increasing to over 2000 mm in the higher central parts at elevations of ca 460 m (New Zealand Meteorological Service, 1966).  (Table A3) stands of all ages 40-100 m above sea level. Bedrock consists of undifferentiated greywacke (Geological Map of New Zealand, 1967). Pumice Soils consisting of Tarawera and Whakatane Ash overly bedrock on rolling hill country (Hewitt, 2010).
Each site represents an even-canopied stand of naturally reverting Kunzea robusta (Table A3) at a different stage of development, the age of which was determined by the history of vegetation clearance, and verified by growth ring counts (Watson et al., 1994). The Tolaga Bay sites occur on slopes between 23° and 32°, have a NW (300 o ) to NE (60 o ) aspect, and are at elevations between ca 64 m and 160 m above sea level. The Waimata site is on a SW aspect at an elevation of 207 m. The underlying bedrock at these sites consists of Pliocene-age calcareous sandy siltstones with banded sandstones and thick tuffaceous horizons (Kingma, 1965). Soils are a stony colluvium varying from Orthic Recent Soils and their intergrades to Brown Soils (on well-drained sites) and Gley Soils (on poorly drained sites) typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010). The climate is warm temperate maritime, with moist summers and cool wet winters. Mean annual rainfall varies from about 700 mm at the coast to 2500 mm at higher elevations (New Zealand Meteorological Service, 1973). Lengthy periods of little or no rainfall are common during January to April (mid-summer to late autumn). This region has a history of extreme rainfall events (Kelliher et al., 1995), often associated with storms of tropical origin (e.g. Cyclone Bola in 1988).
Site 8: Gisborne. Five indigenous softwood (Agathis australis, Prumnopitys ferruginea, Podocarpus totara, Dacrycarpus dacrydioides, Dacrydium cupressinum) and two hardwood species (Alectryon excelsus and Vitex lucens) (Table A5) were established as a planting trial to establish their relative growth performance, above-and below-ground, over a 5-year period (Marden et al., 2018). The trial site was located on a low-lying (5 m above sea level), even-surfaced alluvial terrace adjacent to the Taraheru River, in Gisborne City (178° 00´ 16 02 E, 38° 38´ 44 82 S). The soil is a naturally fertile, free draining, Typic Sandy Brown Soil of the Te Hapara soil series (Hewitt, 2010) with no physical or chemical impediments. Temperatures over summer average 23° C and over winter 12° C and mean annual rainfall is ca 1200 mm (Hessell, 1980).  (Table A3) and L. scoparium (Table A4) were selected in Tongariro National Park near Turangi township in the central North Island (175° 47´ 11 53 E, 39° 09´ 19 20 S) at an elevation of 800 m, approaching the maximum elevation at which these species are found (Scott et al., 2000). Mean annual temperature is 11.1C°, and mean annual precipitation is ca 1610 mm. Soils derived from a series of rhyolitic and andesitic volcanic eruptions are classified as Podzolic Orthic Pumice soils of the Rangipo series (Hewitt, 2010).

Site 11: Wainuiomata and Cannons
Creek. This site consists of well-established indigenous hardwoods and lowland shrub communities dominated by mixed hardwood Coprosma grandiflora, Weinmannia racemosa, and Melicytus ramiflorus (Table A5) shrubs indicative of advanced succession progressing toward indigenous forest. Three plots were installed (174° 57´ 19 75 E, 41° 17´ 45 29 S) on slopes ranging between 17° and 28°, with a southwest aspect between 200° and 240°, and at an elevation of ca 117 m. The geology consists of complexly deformed alternating dark grey argillite and greywacke sandstone, rare limestone and minor spilitic lava of Triassic age (Kingma, 1967). Soils are a stony colluvium derived from greywacke bedrock and vary from Orthic Recent Soils and their intergrades to Brown Soils (on well-drained sites) and Gley Soils (on poorly-drained sites) typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).
Site 12: Long Gully. 6 km southwest of Wellington (174° 40´ 55 30 E, 41° 18´ 34 82 S). Regenerating wind shorn stands of Kunzea amathicola (Table A3) on south-facing slope 300-400 m above sea level. Bedrock consists of alternating argillite and greywacke sandstone with rare limestone and volcanics (Kingma, 1967). Soils are a stony colluvium derived from greywacke bedrock and vary from Orthic Recent Soils to Brown Soils and Gley Soils typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).  Kingma, 1967). Soils are a stony colluvium derived from greywacke bedrock and vary from Orthic Recent Soils to Brown Soils and Gley Soils typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).  (New Zealand Geological Survey, 1972). Soils are derived from greywacke bedrock and vary from Brown Soils to Orthic Recent and Gley Soils typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).
Site 15: Long Spur. 9 km south of Tururumuri near the southeast coast of Wairarapa (175° 32´ 09´ 01 E, 41° 27´ 22 12 S). Dense regenerating stands of Kunzea robusta (Table A3) on slopes on a range of aspects 40-200 m above sea level. Bedrock consists of graded bedded, fine-grained, sandstone and mudstone, minor conglomerates and volcanics (Kingma, 1967). Soils are a stony colluvium derived from greywacke bedrock and vary from Orthic Recent Soils to Brown Soils and Gley Soils typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).
Site 16: Peggioh. 10 km west of Ward (174° 01´ 13 67 E, 41° 51´ 31 57 S). Dense, regenerating Kunzea robusta (Table A3) and L. scoparium (Table A4) stands 200-300 m above sea level. on south-facing slopes. Bedrock consists of interbedded greywacke and argillite with minor volcanics, conglomerates, and rare limestone (New Zealand Geological Survey, 1972). Soils are a stony colluvium derived from greywacke bedrock and vary from Brown Soils to Orthic Recent and Gley Soils typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).  (Table A3) on north-facing slopes 420-540 m above sea level. Bedrock consists of interbedded greywacke and argillite with minor volcanics, conglomerates, and rare limestone (New Zealand Geological Survey, 1972). Soils are a stony colluvium derived from greywacke bedrock and vary from Orthic Recent Soils to Brown Soils and Gley Soils typical of slopes being eroded or has received sediment mainly as a result of slope processes (Hewitt, 2010).