Impact Of Management Practices On Long-Term Site Productivity Of Radiata Pine And Poplar Plantations

Emphasis on sustainability of forest production is increasing as world demand for wood expands and greater areas of natural forest are being placed under increasing environmental constraints. Forest managers need understanding wood production in relation to both the site and to management practices to achieve sustainable practices on specific sites. Impact of management practices on long-term site productivity of radiata pine and poplar plantations were studied in this paper. Three trials were established over the period 1986 to 1992 in different physiographic regions of New Zealand to determine (i) the effects of residue management and fertiliser treatments on radiata pine nutrition and above-ground productivity; (ii) treatment effects on forest floor litter quality; (iii) the relationship between forest floor litter quality and tree productivity; and (iv) implications for sustainable forest management. The results from these trials will be used to develop decision support tools for site specific management. Poplar trial was established in 1992 with one-year-old seedlings, which located at Hanyuan Forestry Farm, Baoying County, Jiangsu Province, China. This study was designed with the primary objective of assessing the growth pattern, biomass and nutrient content accumulation for poplar plantations with different clones, growing spaces and rotation lengths. This assessment enables us to estimate growth, biomass and nutrient removals, and the relationship between nutrient removal and maintenance of site productivity for short rotations envisioned for poplar plantations. The key results are as follows: 1 Impact of management practices on long-term site productivity of radiata pine plantation Site effects on nutrient availability: Foliar nutrient concentrations in unfertilised treatments reflected differences in nutrient availability among the three sites. Unfertilised tree foliar N at Woodhill dropped below the fertiliser intervention level (Will, 1985) from age 3-11 years and trees were considered N deficient, while trees at Tarawera were marginal to age 9, and satisfactory at Kinleith to age 6. Foliar P in unfertilised trees at all sites remained above the fertiliser intervention level. Foliar Mg concentrations were considered satisfactory at Woodhill and Tarawera, but were deficient in both fertilised and unfertilised trees at Kinleith. Levels of foliar B in unfertilised trees at Woodhill were generally remaining above the fertiliser intervention range, Tarawera dropping well below the range in years 4-6, and Kinleith marginally requiring B fertiliser.These results suggest critical ages for interpreting foliar N concentrations in unfertilised stands are between ages 3 and 6 years. Fertiliser effects on tree nutrition and growth: Overall, fertiliser additions increased tree growth on all sites, and corrected any decreases in growth caused by residue removal. Effects of urea fertiliser additions on foliar N was related to site N availability. Urea fertiliser treatments have reduced foliar concentrations of several nutrients, including P (all sites), Mg (Tarawera and Kinleith), B, Mn and Zn. Vector analysis of Woodhill foliage indicated urea-N was antagonistic to P and B uptake. The divergence in DBH among sites and treatments suggests the occurrence of three growth responses in this trial series, including: (a) differences in site productivity between Woodhill and Tarawera due to inherent climate and site quality effects; (b) increased productivity from fertiliser additions; and (c) reduced productivity due to reduced soil quality at Woodhill following forest floor removal. Trees at Kinleith are too young to make meaningful growth comparisons with the other trials. Divergent growth patterns due to whole-tree harvesting and forest floor removal, and recent observations of increased growth from double slash at Woodhill, would be justification for management recommendations for stem-only harvesting on Pinaki sand. Effects of residue management on long-term productivity: The effect of residue management on second rotation growth varied by site N availability. The relationship between forest floor C:N ratio and tree DBH at age 5 years indicated that litter quality only had an effect on site productivity when total soil N content was small, as observed on Woodhill Sandy Recent soils. The reduced relationship between forest floor C:N and tree growth with increasing ecosystem N content suggests the mineral soil N supply has the capacity to 'buffer'immobilisation by coarse woody debris and maintain adequate N availability. Residue management has the potential to markedly influence site micro-climate. At all sites in this trials series, residue retention has increased seedling mortality; however, the effects of slash on planting quality and micro-climate are not clear. In this trial, the early growth of radiata pine was generally not favoured by slash retention, with significantly less growth observed at Woodhill and Tarawera, where DBH following stem-only harvesting was significantly less that FF and WT treatments in years 3-5. It is not certain how increase in soil organic matter will benefit productivity in the long term. However, these results indicated the importance of organic matter as a store of nirogen on the poor site (Woodhill site), and the importance of forest floor materials in supplying second-rotation P. radiata with nitrogen and boron. Nitrogen loss from the system was positively related to organic matter removal. Productivity was reduced only by removal of the forest floor and was increased in all harvest treatments by the addition of urea. It is necessary to retain all harvest residues and add fertiliser nitrogen to maintain the nitrogen content of the ecosystem at pre-harvest levels. Applicability to developing indicators of sustainable forest management: Ages 3 to 6 years are during the period of maximum demand by the developing crop for soil nutrients, and dynamic changes in foliar nutrient concentrations in unfertilised trees at the three sites provided adequate information on soil nutrient availability. These results suggest use of plant-based indicators ofsustainable forest management, as foliar nutrient concentrations in unfertilised trees 3-6 years old appear to be a reliable indicator of site nutrient availability. We do not recommend tree productivity be used as an indicator of sustained soil quality. We are not able to recommend soil-based indicators at this time. 2 Impact of management practices on long-term site productivity of poplar plantation Minirotation management system: The above-ground biomass production, distribution model, nutrients exported and wood quality characteristics of eastern cottonwood were studied in minirotation management systems among the combinations of 3 densities and 3 rotations, The stand , with planting density of 10000 stem·ha-1 and 3 years rotation, could obtain the highest biomass production 10.52·ha-1·a-1 for clone I-69 and about 12.00 t·ha-1·a-1 for clone NL-80351. The economic biomass production for pulp-making was also the highest in the stand of 10000 plant ·ha-1 and 3-year-rotation, respectively 7.14 t·ha-1·a-1 for clone I-69 and 8.47 t·ha-1·a-1 . Nutrients exported from the minirotation management systems bear close correlation to the biomass production and distribution model. In the stand of 10000 stem·ha-1 and 3-year-rotation, the amount of nutrients exported ( N, P and K ) were 33.97 kg·ha-1·a-1 for clone I-69 and 38.50 kg·ha-1·a-1 for clone NL-80351. However, the amount of nutrients exported for producing one metric ton of economic biomass was slightly lower for clone NL-80351 than for clone I-69. Results from the stands of age 7 in this trial indicated that the optimum rotation was 3 years for the stand of 10000 stem·ha-1, 4 years for the stand 5000 stem·ha-1 and about 5 years for the stand of 2500 stem·ha-1 . Midirotation management system: Two phases of tree growth can be singled out when studying the effects of planting densities on DBH growth. The first phase (pre-canopy closure) ranged from 3 years to 5 years, which depended on the initial stand density. Variance analysis showed that there were significant difference (p≤0.05) among the three clones in DBH and height growth. At six years, the DBH averaged over planting densities was 18.2 cm, for NL-80351, 17.4 cm for I-69 and 17.0 cm for I-72 and the tree height averaged over planting densities was 18.08 m for I-69, 18.05 m for NL-80351 and 16.60 m for I-72. From the second through sixth years, annual growth of the P. deltoides clones I-69 and NL-80351 was about 5.0% and 9.0% greater than the euramerican hybrid clone I-72 in DBH and height respectively. Leaf area index (LAI) of three poplar clones increased in all four planting densities throughout the first 6 years of growth. Differences among three clones and four planting densities were significant throughout the first six years. The clonal LAIs averaged from age 1 to 6 over four densities were NL-80351 2.56 m2/m2 , I-69 2.45 m2/m2 and I-72 m2/m2 . LAI increased with increasing of planting density, and the LAI was 2.91 m2/m2 for the stand of 1111 stems/ha, 2.62 m2/m2 for 833 stems/ha, 2.16 m2/m2 for 625 stems/ha and 1.98 m2/m2 for 500 stems/ha (three clones averaged over six years). The highest values of LAI measured in this study exceeded 4 m2/m2, but there was no indication at age 6 years that LAI had reached a maximum at any planting density for any of the three poplar clones.Regression analysis showed that a polynomial function best described the relationship between LAI and current annual biomass increment. The maximum biomass increments around 17 tons/ha/a were associated with LAIs of 4.0 m2/m2. Up to a LAI of 4.0 m2/m2, the correlation between LAI and current annual biomass increment was strongly positive for three poplar clones (R2=0.52, n=36). However, once LAI exceeded 4.0 m2/m2, the current annual stand biomass increments tended to decrease due to the self-shading in the foliage. Ranking of the plantation biomass production by planting density was 1111 stems/ha > 833 stems/ha > 625 stems/ha > 500 stems/ha for three poplar clones and three rotation lengths, with the exception of clone NL-80351 at six years. For clone I-69 and clone I-72, the highest biomass was achieved in the stand of 1111stems/ha at age six, 78.381 t/ha and 71.826 t/ha respectively. However, for clone NL-80351, the highest occurred in the stand of 833 stems/ha with six year rotation, 75.769 t/ha. As to the three poplar clones, the biomass productivity of clone NL-80351 was slightly higher than that of clone I-69 and clone I-72 was the lowest. Ranking of the plantation biomass production by components was stemwood > branches > foliage > stembark. Production of the support components of the plantation (i.e., stemwood, stembark and branches) was more than 10-fold that of the productive component (i.e., foliage), and this distribution did not change significantly over the age span of this study. The pattern of accumulation of nutrients by the plantations was similar to biomass and the primary difference was in the distribution among components. Compared with the clone I-69, the nutrient accumulation increased by an average of 4.1% for clone NL-80351 and decreased by an average of 16.3% for clone I-72. Nutrient accumulation in the plantations was in the order of Ca > N > K > Mg > P, and averaged 171.6, 123.5, 110.5, 21.2 and 14.1 kg/ha/a from four to six years. Annual accumulation of Ca in the plantations was about 1.4 times of N, 1.6 times of K, 8.1 times of Mg and 12.2 times of P respectively. Accumulation of N, K and Mg was highest in the foliage. The maximum accumulation of Ca was in the branches. However, P accumulation in the plantation was changed over the planting density variation, the highest in the stemwood when the planting density was 833 stems/ha or more, otherwise the greatest in the foliage. The different nutrient accumulation patterns resulted from the variation in nutrient concentration and biomass productivity of plantations among components. Generally, the stem components accumulated nutrients exponentially through time, and nutrient accumulation in the crown components was liner. Management implications and maintenance of productivity: Mean annual yields of stem-only, and woody biomass (stem + branches) utilisation were about the same at 5-year rotation and 6-year rotation. However, yields for stem-only and woody biomass utilisation were 12.9% and 10.7% higher at 6-year rotation than at 4-year rotation respectively. Nutrient removals for stem-only, and woody biomass utilisation did not differ appreciably for the 5-year and 6-year rotations, which are about 10.0% and 9.3% higher than those of 4-year rotation, respectively. Nutrient removals were about 89.0% less for N, 67.0% for P, 60.0% for K, 93.0% for Ca and 75.0% for Mg with stem-only utilisation than with woody biomass utilisation, respectively.Changing utilisation standards markedly affected the export of nutrients from a site. Whole-tree utilisation yielded the most biomass and removed the most nutrients. More conservative utilisations remove specific portions of the standing crop and remove less nutrients. Removal of the entire stem provided about two thirds of the total biomass and removed 31.3% total N and 37.5% total nutrients respectively. Including the branches in the removal increased biomass yield to 92% of the total, and N and nutrients removal were about 60% and 68% of the total respectively. If foliage was included in removal, the biomass yield would increase only about 10%, but the removal of N would increase by 40%. 3 Difference in nutrient accumulation between radiata pine and poplars The rapid early development of poplars is also illustrated by comparison with radiate pine (Pinus radiata). Biomass of clone I-69 at five years is about 1.6 times that of radiata pine and accumulation of nutrients is greater in plantations of clone I-69 than in radiate pine plantations; e.g., accumulation of N, P, K, and Ca in poplars is about 1.5, 1.3, 1.9 and 5.6 times that of radiata pine at 5 years respectively, with the exception of Mg. The greatest differences between two species are for Ca and K. The higher values for Ca are associated primarily with the stembark and branches, where the quantity in poplar is about 15.3 and 7.4 times that in radiata pine, respectively.