Above-ground dry matter, energy, and nutrient contents of trees in an age series of Pinus radiata plantations.

Eight sample stands ranging in age from 2 to 22 years were studied to characterise dry matter, energy, and nutrient contents of the above-ground portion of intensively managed Pinus radiata D. Don plantations on a good site. Site index averaged 36 m at age 20. Dry matter content was closely comparable to similar data from Australia. Net dry matter production averaged 14.4 tonnes/ha/annum over the 22-year period; gross production was 22-25 tonnes/ ha/annum during initial canopy closure and during canopy closure after heavy thinning. Stand nutrient contents were predominantly in the order N = K>Ca>Mg = P>Mn>Na>Zn. The high potassium content probably reflects the potassium-rich pumice soils on which the stands were growing. Compared with published results for a wide variety of pine species, these stands had high rates of dry matter accumulation and nutrient uptake. Nutrient content in relation to dry matter content was high for potassium, low for calcium, and intermediate for magnesium and manganese compared with other pines. Heavy thinning-to-waste with high pruning of remaining trees would have returned about 55% of the nutrients in the above-ground stand to the forest floor. The total amounts returned would have been similar to those resulting from harvesting of the 22-year-old stand. The total energy capture and the percentage of incoming radiation stored in woody tissue were high compared with published data for other forest types, even though the silvicultural practices employed were not designed to maximise energy capture. INTRODUCTION Information on nutrient and energy content of plantations is of increasing importance as a basis for understanding the ecological impact and economic limits of continuous forest production. Recent suggestions that forests can supply a renewable source of energy (Szego et al., 1972; Troughton, 1976) heighten the need for data. Temporary sample plots covering an age series of stands have been used as a traditional method of studying the volume growth of forests. More recently, following the lead of Ovington (1957a), this method has been used to study dry matter production and nutrient uptake. Use of temporary sample plots in this way assumes that all sites studied are uniform throughout the age range. Such plots can give reasonable estimates of standing crop but N.Z. J. For . Sci. 7(3): 445-68 (1977). 446 New Zealand Journal of Forestry Science Vol. 7 estimates of periodic annual increment are not reliable (Spurr, 1952). Thus the data from such studies may be used to indicate general trends only. In New Zealand, Orman and Will (I960), Will (1964; 1966; 1968), and Gadgil (1976) have provided a number of estimates of dry matter production and nutrient content of Pinus radiata!). Don stands. In Australia, Forrest and Ovington (1970) have studied dry matter of P. radiata plantations up to age 12 years, with additional data provided in papers by Stephens and Bond (1957), Waring (1969; 1974), Siemon (1973), and Williams (1976). Data for one plantation are also available from Italy (Giulimondi and Duranti, 1975). The dry matter production and concurrent nutrient uptake of New Zealand P. radiata are of particular interest as the more productive sites carry some of the fastest-growing temperate conifer forests in the world. The purposes of the study reported here were to: (1) Determine the dry matter, energy, and nutrient content of an age series of P. radiata plantations on a high quality site; (2) Compare the results with comparable data from Australian plantations; (3) Place P. radiata dry matter production and nutrition in the perspective of pine as a genus. MATERIALS AND METHODS Sample Stands A series of eight plots, all within 9 km of each other, was selected in Pinus radiata plantations in the north-eastern corner of Kaingaroa Forest (38° 18'S, 176° 44'E). The average annual rainfall (1941-1970) for this area is between 1425 mm (Kopuriki, immediately south-east) and 1549 mm (Kaingaroa Forest Headquarters, to the southwest). The latter station has a mean annual temperature of 10.7 °C, with a seasonal range from 5.4°C in July u> 15.8°C in February, and an average of 107 ground frosts per annum. The sampled stands ranged in age from 2 to 22 years, and were growing on 75 to 100 cm of Tarawera scoria and Kaharoa ash, overlying older and more consolidated volcanic ash showers. These are classified as Matahina gravels or Matahina hill soils, with a site index of about 36 m at age 20 years (C. J. Goulding, unpubl.). Sample plots and stands chosen reflected as closely as possible the then current management practice of planting 2500 stems/ha, with live-crown pruning of selected crop trees to a height of 6 m followed by thinning to 540 stems/ha at about age 8 years, when the trees were 12 m tall. It was impossible to find stands over 10 years old which had received this treatment; thus, the 17-year-old plot had been thinned to 860 stems/ha, and the 22-year-old plot had been thinned at age 9 years. The 8-year-old stand had a low stocking (Beekhuis, 1966) and additional data on the effects of thinning and pruning, collected later, are included in the Appendix. Field Procedures In each plantation a previously established sample plot of 0.1 acres (approximately 0.04 ha) was selected and all the trees were measured for height and diameter. In the 8-year-old stand, which had been marked for thinning, a note was kept of trees to be removed. Sample trees covering the size range within each plot were felled. Seven trees were taken in unthinned plots and five trees in thinned plots. Sampling extended from June to September 1971. The crown of each sample tree was divided into zones No. 3 Madgwick et al.—Dry Matter, Energy and Nutrients in P. radiata 447 representing the year in which the branches were initiated. For each zone all firstand second-order branches were separated according to the age of needles which they bore, and each subdivision was weighed. Large subsamples were then separated into needles and woody material prior to being weighed after drying at 60 °C. Ratios of dry weight of needles and woody material to fresh weight were used to obtain estimates of total dry weight of each component for each zone of the crown. All cones within the crown were combined before drying and weighing. Male strobili were present on only the 2 2-year-old stand at the time of sampling. Stems were divided into sections and total fresh weight was determined prior to sampling for moisture content. Within the live crown each section comprised an annual height increment; below the lowest live branch, sections consisted of 2-metre lengths. Discs were cut from the end of each segment and at breast height for detailed study. Before drying, the discs were separated into* wood and bark. Laboratory Procedures In order to reduce the numbers of samples to be analysed, material was combined to provide one representative sample for each category of material for each tree. The categories chosen were needles by age class, live branches, dead branches, cones, strobili, stem bark, and stem wood. Canopy material was combined according to the relative proportion of material in each crown layer. Stem material was combined according to the relative weight of each segment of the stem. Woody material was chipped before grinding in a stainless-steel Wiley Mill, and needle material was ground directly. After drying, samples were ground to pass through a 1-mm round-holed sieve. Nitrogen was determined by a semi-micro-Kjeldahl procedure using a selenium catalyst (Bremner, I960). The ammonium N in the diluted digest was determined colorimetrically by an automated adaptation of the indophenol blue method. Phosphorus and cations (Na, K, Ca, Mg, Zn, and Mn) were determined after dry-ashing at 480 °C, Test solutions were prepared from a dilute hydrochloric acid extract of the ash after preliminary removal of silica by a process involving dehydration. Phosphorus was determined colorimetrically by an automated adaptation of the vanadomolybdophosphoric acid yellow method described by Jackson (1958). Cations were determined by atomic absorption spectrophotometry. For Ca and Mg, strontium chloride was added to the solutions as a releasing agent (at a concentration of 1500 ppm of Sr+) to prevent interference from aluminium and phosphate. All samples were analysed in duplicate. Analyses were repeated where the difference between duplicates relative to their mean values exceeded 6% for N , 4 % for P, 8% for K and Mg, 10% for Ca, and 14% for Zn. The variations m precision between elements reflect the known variation in analytical procedures (Madgwick, 1970b). Energy content was determined for ground samples using an adiabatic calorimeter (Lieth, 1965) and expressed on a dry weight basis. When the quantity of sample material was limited, the highest priority was given to nitrogen determinations and lowest to caloric value. Calculations The nutrient and energy contents of each sample tree were obtained by multiplying the oven-dry weight by the relevant concentration. Regressions were calculated relating 448 New Zealand Journal of Forestry Science Vol. 7 logarithm component dry weight and nutrient content to logarithm height for 2and 4-year-old stands and logarithm height X (diameter) for older stands. Total dry weight and energy content of stand components were found by applying these regressions to the unsampled trees after correction for bias due to logarithmic transformation (Finney, 1941). This method is known to give reasonably close estimates of dry weight and overestimates of error terms (Madgwick and Satoo, 1976). In the present study 9 5 % confidence intervals in unthinned stands were about ± 20% and in thinned stands ± 40% of estimated totals, respectively. Separate estimates were made for the 8-year-old stand (a) in its unthinned condition and (b) assuming that the stand had been thinned and crop trees pruned to a height of 6 m. Net annual