Growth and productivity of New Zealand kauri (Agathis australis (D.Don) Lindl.) in planted forests
© Steward et al.; licensee Springer. 2014
Received: 27 November 2013
Accepted: 30 October 2014
Published: 18 November 2014
The establishment of even-aged planted stands of New Zealand kauri (Agathis australis (D.Don) Lindl.) for timber has been constrained by a lack of quantitative information on productivity and rotation length on which forest management and investment decisions could be made.
Stand-level models of height and basal area against time were developed (as well as a stand-volume function to calculate volume from height and basal area) based on planted stands that were up to 83-years old and represented planting sites both within and outside the current natural range of the species.
Planted kauri was shown to be slow to establish with little height growth for the first five years after planting. Similar trends were observed for basal area and whole-tree volume development. A Schumacher equation with local slope parameter and asymptote bounded at 45 m gave the best fit for height, while a von Bertalanffy-Richards equation in difference form with local slope parameter gave the best fit for basal area. For plantations with an average site index (20.4), height was predicted to be 22.3 m in height at age 60, with a basal area of 78.1 m2 ha-1. Whole-tree volume was predicted to be 702 m3 ha-1. Predicted volume mean annual increment was 11.7 m3 ha-1 yr-1for all stands at age 60. From age 20-60 years, stands with a higher site index had a volume mean annual increment of 18.6 m3 ha-1 yr-1. The best stand exceeded 20 m3 ha-1 yr-1.
This study indicates an opportunity to grow kauri in plantations on selected good-quality sites over rotations of 60-80 years or less.
Planting of kauri for timber has become increasingly popular, but has been constrained by a lack of quantitative information on growth and productivity (New Zealand Forest Research Institute ), and hence rotation length and return on investment (Herbert et al. ; Steward ).
Early assumptions were that kauri would have to be grown to similar dimensions as trees in old-growth forest (Hutchins ), over rotations commonly assumed to be in the hundreds of years (Laing and Blackwell ). This followed the perceived need to replicate and recover large quantities of durable heartwood. As a consequence, only a small number of kauri forests were planted with any sense of replacing the original resource.
Initial estimates of productivity of kauri were based on data from natural stands (Lloyd ) and pole-kauri volume tables (Ellis ). The productivity by age in natural stands was assumed to be low (2.8-8.8 m3 ha-1 yr-1) (Halkett ) and directly applicable to planted stands. A comprehensive survey of the performance of a range of New Zealand indigenous species' identified kauri as one of the most commonly planted softwoods (Pardy et al. ). Annual growth of planted kauri averaged 0.7 cm in diameter and 0.36 m in height from a wide range of sites with different stocking rates and management history. These data were used to predict a mean annual height increment for planted kauri of 0.44 m at 20 years, reducing to 0.26 m at 80 years. This rate was among the highest for the indigenous conifers surveyed. Ecroyd et al. () reported that in some kauri plantations average diameter growth exceeded 1.0 cm yr-1 for periods of up to 40 years. Height increments of 1.0 m yr-1 were recorded for individual trees. Productivity estimates were not developed from the data of either Pardy et al. () or Ecroyd et al. ().
Herbert et al. () developed a preliminary stand productivity and economic model for planted kauri, based on two 60-year old stands growing outside the species current natural range in the Taranaki region (Figure 1). Models, based on the Chapman-Richards growth function (Richards ; Chapman ), of mean top height, basal area, and whole-stem volume were produced for stands at an average 1375 stems ha-1. Volume mean annual increment (MAI) was estimated to be 13.4 m3 ha-1 yr-1 at age 60 years, increasing to 13.8 m3 ha-1 yr-1 at age 80 years. Chikumbo and Steward () developed a basal area model using data from thirteen planted stands. The dynamical modelling approach was based on a von Bertalanffy-Richards generalised growth function (Richards ; von Bertalanffy ) and led to the development of a state-space model that was asymptotically stable, and was valid for stand density within the range of 300-1,400 stems ha-1. Predicted basal-area values were similar to those of Herbert et al. () until age 60. These earlier models were based on a limited number of planted stands and were not tested against other kauri plantings. They were therefore not necessarily reflective of the overall performance of the species and could not be considered suitable for general use across New Zealand. More robust models were expected to provide the basis for further model development and validation as more data became available.
The objectives of this study were to:
compile the largest available data set of growth yet developed for New Zealand kauri in planted stands,
determine the productivity of kauri grown in plantations across a range of sites and conditions,
determine whether improved growth rates would result in shorter rotation lengths for commercial harvest.
Hypotheses were tested by developing and validating robust stand-level models for height, basal area and standing whole-tree volume. These models will facilitate early predictions from which investment and management decisions can be made for kauri forestry in New Zealand in the future.
This study concentrated on the development of planted kauri at the stand level, therefore the variables of height, basal area and volume were emphasised. The number of stands available, the variability in stand density, and periodicity of measurement did not allow for the development of models of diameter at breast height (DBH), mortality, or a thinning function. However, mortality and the relationship between stand density (stems ha-1) and mean diameter were characterised.
Initial stand density and site characteristics of all planted kauri stands
Initial stand density (stems ha-1)
Annual rainfall (mm)
Annual sunshine hrs
Daily mean temp (°C)
Data were obtained from permanent sample plots (PSP) (Ellis and Hayes ) established in the interior of the stands. In larger stands more than one PSP was established. Small stands typically had adjoining forest comprised of species of equal stature and similar growth rate to kauri, therefore all stems were used, including those that would otherwise be defined as edge-trees (Cancino ). In 1986, Pardy et al. () established growth plots to obtain data on the height and diameter growth of planted kauri. During the 1986 assessments, all measured trees were tagged. In later measurements, PSPs were overlaid to include previous growth plots. For each PSP, data were gathered on diameter of all stems at 1.4 m breast height; total tree height of selected stems; planting pattern; current and initial stand density; current stand age; survival/mortality; site characteristics (elevation, annual sunshine hours, rainfall and daily mean temperature). Not all kauri were measured for height, so unmeasured heights were estimated by fitting non-linear regression curves to the height and diameter data for each stand, at each measurement period. Predicted heights were calculated for each stand at each measurement period and estimated heights were entered onto the database.
where V = total stem volume m3; D = diameter at breast height (cm); H = total tree height (m)
For kauri in planted stands site indexa was defined as mean top height at age 50, and was calculated from the height model.
Three sigmoidal growth functions (in yield form) from which mean top height and basal area models were developed
y = a(1-e-bT) c
For the von Bertalanffy-Richards model, two general methods of fitting the different forms of each model were tested. Firstly, the SAS Version 9.1 NLMIXED procedure was used (Littell et al. ). In this approach, one of the parameters was specified as a local parameter which varies with each site. This parameter was assumed to be randomly distributed from a normal distribution. Various forms in which either the slope or asymptote were assumed local were tested, along with more complex versions in which both slope and asymptote varied as functions of a local parameter. When using NLMIXED, the dependent variable was height (or basal area), and the independent variable was age. Secondly, the difference form of each equation was created and fitted using the SAS NLIN procedure. Two forms of difference equation in which the slope or the asymptote parameter was eliminated were tested. In this method of fitting the model, the function was fitted using adjacent pairs of measurements. The mean number of measurement intervals was 3.3 (range 1-10). The dependent variable was the second measurement (of height or basal area) and the independent variable was the first measurement of each pair. The model forms tested for the Schumacher model were similar to those tested for the von Bertalanffy-Richards model. Early attempts at fitting the Schumacher model for height produced extreme estimates for the asymptote (a parameter) (e.g. >150 m). The known maximum height recorded for kauri is 60 m (Ecroyd ). In planted stands, a maximum mean top height of 29 m was recorded. Therefore a height of 45.0 m was considered an acceptable compromise between the extreme maximum and the measured heights found in comparatively young planted stands. For the Weibull models, only the simple yield and the nonlinear mixed models (NLMIXED) with local slope parameter were tested. Early analysis showed that this sigmoidal model was inferior to either the von Bertalanffy-Richards or Schumacher models and produced predictions that did not reflect the data.
Final fitted models were selected that had the smallest root mean square error (RMSE) and least biased residuals (Additional file 2). Predicted MTH, basal area or volume were calculated for each stand at each measurement period and subtracted from the actual measured value. The residuals were plotted by predicted values and interval length. The normality of residual distributions was a third criterion for model selection.
Two stand-level volume functions were fitted to the per hectare estimates of volume. Predicted basal area (G) and mean top height values for each site index were used in conjunction with the stand-level volume functions to provide predicted volumes. The volume function of Beekhuis () was tested but tended to over-predict volume from age 30. The generalised volume function (V = b × G a ×MTH c ) gave the best fit to the data.
To validate the models for planted kauri, the one-at-a-time cross-validation method was used. Cross-validation is a method for testing models where datasets are too small to divide into training and test sets, and can be used for estimating prediction error (Efron and Tibshirani ). The models were re-fitted to the data, leaving out one stand at a time. New parameter estimates were acquired and the models were refitted and root mean square error (RMSE) and bias were calculated.
The relationship between stand density and diameter for kauri (self-thinning function) (Reineke ; Yoda et al. ), was determined by establishing temporary plots in forests where kauri was the dominant species (numerically and/or basal area) and full site occupancy was assumed. Stem counts (stand density) for all species and their diameters were obtained. Additional data were obtained from Ahmed and Ogden () from a study of 25 kauri forests throughout the species natural range. The quadratic mean diameter and stem density were calculated for each site and the data were graphed on logarithmic scales and a regression equation fitted.
Mortality for all stands was assessed at each measurement. It was calculated as percentage loss and percentage loss per year (% yr-1). Mortality was calculated for three periods (1) planting to the first assessment, (2) first to last assessment, and (3) over the entire rotation.
Mean performance of kauri in planted stands at their last assessment
Stand density (stems ha-1)
Quadratic mean DBH (cm)
Mean top DBH (cm)
Mean top height (m)
Basal area (m2 ha-1)
Volume (m3 ha-1)
Site Index (at age 50 years) was calculated for all planted kauri stands. Maximum site index was 28.4 m, while the lowest was 15.8 m, and mean Site Index was 20.4 m. Site index values were compared (Pearson correlation) with site parameters. Kauri height growth as expressed by Site Index was not influenced by the site parameters of elevation (r = - 0.073, p-value 0.727), annual rainfall (r = -0.054, p-value 0.797), annual sunshine hours (r =0.052, p-value 0.807), daily mean temperature (r = -0.045, p-value 0.830) and latitude (r =0.146, p-value 0.486). Site index was negatively correlated with age (r = -0.642, p-value 0.000), younger stands <20 years-old tending to have a higher site index predicted than older stands.
where MTH = mean top height; T = age; 0.5 = starting height of seedlings; a = bounded asymptote parameter estimate; cc = shape parameter estimate; SI = Site Index (mean top height at age 50).
where G = predicted basal area; G i = basal area at initial measurement; T = age of prediction; T i = age of initial measurement; a = asymptote parameter estimate; c = shape parameter estimate. Parameter estimates (and their standard errors) for the polymorphic von Bertalanffy-Richards basal area model (R 2 = 0.95) for planted kauri stands were a 101.4 (s.e. 6.7) and c 5.697 (s.e. 0.642).
where V = volume; G = basal area; MTH = mean top height; a = asymptote parameter estimate; b = slope parameter; c = shape parameter estimate.
Parameter estimates (and their standard errors) for the volume model for planted kauri stands (R2 = 0.99) were a 0.956 (s.e. 0.089), b 0.703 (s.e. 0.03), and c 0.883 (s.e. 0.048).
Estimates of stand growth for planted kauri stands at given ages. Values to age 80 are modelled on actual performance
Basal Area (m2 ha-1)
Volume MAI (m3 ha-1 yr-1)
Volume PAI (m3 ha-1yr-1)
The planted stand models for mean top height and basal area were validated using the one-at-a-time cross-validation method
Discussion and conclusions
The models of height (Herbert et al.), basal area (Herbert et al., and Chikumbo and Steward), and volume (Herbert et al.) were the only models of growth and productivity available for New Zealand kauri in planted stands. Growth and productivity models have been developed for few other Agathis species. The exceptions are three species of kauri from Queensland, Australia and one grown in Indonesia. Volume regression equations and estimates have been developed for A. robusta (two provenances), and one mixed stand of A. atropurpurea (B.Hyland) and A. microstachya (J.F.Bailey & C.T. White) that were established in South Africa (Bredenkamp ). Site index for A. loranthifolia (Salisb.) was modelled using site elevation as an environmental factor but no relationship was found (Parthama, and Habagung ). Modelling approaches have been inconsistent and are species, site and characteristic specific.
The relationship between mean stand diameter and stand density has not previously been investigated for kauri. The relationship was strong and indicated the point at which mean stand diameter and basal area increment slows, and where self-thinning would likely occur unless a silvicultural thinning was undertaken. Using a simple visual assessment resulted in little deviation of stands assumed to be at or near full site occupancy. Six of the current planted stands had reached or were approaching the self-thinning line and had a current annual diameter increment of 0.38 cm yr-1 against a mean of 0.61 cm yr-1 MAI for all stands. The two stands used by Herbert et al. to model productivity had quadratic diameters that were marginally in excess of the predicted diameter (Equation 5) from the relationship between diameter and stand density.
The models developed in this study have shown growth and productivity of kauri in planted stands to be higher than previous estimates, and substantially higher than historical observations suggested possible (Matthews ; Laing and Blackwell ). Kauri is slow to establish with little height growth and volume production in the first 5-15 years after planting. Once established and growing actively, kauri were shown to have volume current increments of 17-18 m3 ha-1 yr-1. The development and application of appropriate management regimes, and a programme to select and breed kauri for production should allow for substantial improvements in early growth and productivity. The lack of knowledge in site selection, after-planting maintenance and silviculture indicates that the productivity estimates obtained to date are likely to be conservative.
Kauri height growth expressed by site index was not influenced by the site parameters for each stand, although there was a negative relationship between site index and age with younger stands having a higher predicted site index than older stands. This was most apparent for stands less than 20 years old. Historically, kauri grew on a much wider range of sites than where it is currently found. The species was widespread in New Zealand until the Pleistocene epoch (400,000-14,000 years BP), when glaciation caused retreat to the northern half of the North Island. Resin of kauri has been identified in fossilised material found in Tertiary lignite deposits in the Roxburgh and Mataura areas of the South Island (Evans, ). This suggests that locations where kauri have been planted outside what is considered its natural range are actually sites well within the species wider tolerances for soils and climate. What is considered the "natural range" of the species could therefore be reconsidered.
The models indicate a slow or extended establishment phase in kauri for young seedlings and saplings. This is most likely attributable to a number of factors. While the majority of the stands used in this study were established as woodlots, none of the expected management after planting was undertaken. The root development of kauri seedlings can be poor. Young kauri have a well-developed taproot, and it is possible that penetration and exploitation of free-draining soils is important for optimum growth (Morrison and Lloyd ). Therefore, slow development of roots after establishment may account for the slow establishment of planted seedlings (Bergin and Steward ). Nursery practice and the development of appropriate sized and aged seedlings are also likely to play a part in the early growth of kauri. It is common for kauri to be raised in PB3 planter bags (that contain the equivalent of 3 pints of potting mix) or similar containers, with seedlings up to one metre tall, or more. As the moisture and fertility requirements are supplied artificially to a seedling in a nursery situation it is easier to grow seedlings where a large top is out of proportion to the root system. Hence, seedlings may take some time after planting to re-establish an appropriate root structure able to support the top and initiate growth. A further important consideration was the lack of knowledge of the seed source for individual stands, the number of parent trees from which collections were made and the size of the parent stand. The productivity in some stands may therefore simply reflect poor seed collection techniques where only a narrow genetic base is represented. A `juvenile' ontogenetic phase of slower growth may also be the cause. These explanations must be tested in order to achieve early site capture and improve site productivity if kauri is to be planted for production.
The growth and productivity of one stand (Stand 16) was considerably in excess of all other stands. Diameter MAI did not fall below 1.7 cm yr-1 for the six years that it was assessed, and had been as high as 2.6 cm yr-1 for periodic mean annual increment. Height MAI was not below 0.9 m yr-1 during the measurement period. At age 14, the largest kauri had reached 30 cm DBH (2.14 cm yr-1). It is unknown whether this stand represents the absolute maximum growth for kauri, and whether the rate of growth in the stand will be maintained. Both these points will be the subject of further observation.
McConchie () suggested that timber properties of native species would be largely age-dependent and would be compromised by pursuing (excessively) short rotations. A detailed study of wood quality was undertaken from material recovered from 68-year old planted kauri (Steward and McKinley ). The stems selected for the study were the largest diameter trees, therefore the fastest growing element of the stand. The wood properties of logs largely comprised of sapwood were similar to those of old-growth heartwood and second-growth mixed sapwood/heartwood timber for stiffness and shrinkage, but slightly inferior for basic density, and were uniform across the width of the stem. A recent study of wood density of kauri in eleven planted stands that ranged in age from 14 to 69 years-old indicated an average density of 448 kg m3 for trees largely comprised of sapwood (G. Steward unpublished data). Wood density was not affected by age, diameter, growth rate, stand density, latitude, or any other site variable. Wood density in planted stands was also similar to that found in second-growth natural stands where individual kauri were up to 287 years old. This suggests that the observations of McConchie () do not apply to kauri as the growth rates observed did not negatively influence wood quality.
The models used here indicate that kauri planted and grown on suitable sites can produce useful volumes in rotations as short as 60 years. The diameter data from the current study of growth and productivity indicates that quadratic DBH at age 60 years would be 37.4 cm and 45.7 cm for mean top DBH for all planted stands combined. Best performing planted stands would have DBH ca. 55.0 cm at age 60. A previous wood quality study of kauri in planted stands (Steward and McKinley ) examined the variables of wood density, shrinkage and stiffness of sapwood boards milled from 68-year old stems. For these variables, observed values were found to be similar to or better than old-growth heartwood, and were uniform pith to bark. If it is assumed that wood quality across all sites is the same or similar to that found in the study of Steward and McKinley (Steward and McKinley ) then harvesting for timber from kauri grown in planted stands could occur at age 60, or earlier. A commercial harvest or thinning could occur as early as age 50, as mean top DBH was estimated to be 39.7 cm and quadratic mean DBH was estimated to be 31.7 cm at this age. These values are also well within the DBH range for logs tested from the two Taranaki studies of Herbert et al. These rotation lengths compare to 40-year rotations for Agathis dammara Warb. in Indonesia (Bruijnzeel et al. ), 35 40 years for Agathis sp. in South Africa (Bredenkamp ), 40-45 years for Araucaria cunninghamii Aiton ex D.Don in Queensland (Huth et al. ), 45-50 years (estimated) for Agathis macrophylla (Lindl.) Mast. in Vanuatu and Fiji (Keppel et al. ), and 40-45 years for A. lanceolata and A. moorei (Lindl.) Mast. in plantations in New Caledonia (Direction Du Développement Rural ).
The models developed here of height, basal area and volume are based on data for kauri in the monopodial form only. Diameter and height were found to be strongly correlated, with DBH being a good predictor of total tree height. Models of growth and productivity for kauri in stands where a mature form predominates (i.e. a large spreading crown) will need to be developed separately if kauri is grown over longer rotations to produce heartwood or to store carbon, and where diameters of 1.0 m or more might be required.
The kauri dieback disease Phytophthora taxon Agathis (PTA) (Beever et al. ; Gadgil ) that is affecting kauri in the northern distribution of the species poses a risk not only to the survival of the species, but also its potential productivity. The current mature kauri population represents only an estimated 0.5% of the original forest that existed before Maori burning and European logging (Steward and Beveridge ). It is typified by numerous small, disjunct populations spread throughout its natural range. If the economic potential of kauri can be unlocked then growing kauri in plantations in-situ and ex-situ is a potential means by which both conservation and production outcomes might be achieved. Selection and breeding programmes for production will rely on some knowledge of the species genetic diversity that will be useful in determining the extent of natural genetic variation, which will facilitate any future breeding, conservation and genetic management of kauri. Combined with information on natural resistance within the kauri population, to PTA and other diseases, this information will help build an informed management strategy to ensure the long-term existence of this species in the landscape.
Numerous historical and contemporary references indicate that kauri has a potential role in the development of New Zealand's economic well-being (Hutchins ; Herbert et al. ; New Zealand Forest Research Institute ). Planting of kauri in New Zealand will continue, and the rate is likely to increase, both within and outside the current natural range of the species. Careful management is likely to allow the production of a very desirable timber over much shorter rotations than were previously thought to be possible. Those wishing to plant kauri for future timber production will require more information about best-practice regimes and potential yield. Continued development of techniques and growth models is likely to accelerate the expansion of a unique national resource.
a Site index refers to the timber potential for a site for a particular species, usually at a fixed age somewhere near the expected rotation length for the species. In forestry, the usual method to develop site index is from stand height records, as good site quality is also often reflected in good height growth (Clutter et al. ).
GAS undertook the latest measurements of the kauri stands, compiled the databases, initial analysis of the data and drafted the manuscript. MOK assisted with analysis and development of the models, EGM assisted with the design of the study and development of the models. HSD assisted with the manuscript. All authors read and approved the manuscript.
We are indebted to those people and organisations that made their developing stands available for measurement and inclusion in this study. We would also like to acknowledge those individuals who made the early observations of individual stands. The authors thank the reviewers of this paper for their comments. This study was made possible through the support of the Future Forest Research Diverse Species Theme with funding from the Ministry of Science and Innovation Contract No C04X0805.
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