Mechanisms Of Planting Density Effects On Productivity Of Chinese Fir Plantations

Chinese fir?Cunninghamia lanceolata?Lamb.?Hook.?is a commercially valuable timber specie that is widely planted in southern China.Recently,most plantations have been established on the same site after clear-cutting,resulting in a productivity decline.Serious concerns have been raised among forest managers and researchers about the long-term productivity of Chinese fir plantations.The crucial phase of any plantation project is the choice of tree spacing and planting density because the arrangement of the trees within a field determines growth patterns and productivity by manipulating below-ground resources?soil nutrients and water?,above-ground resources?such as light quality and intensity?and other environmental factors.However,appropriate planting density that ensures high productivity in Chinese fir is largely unexplored.Moreover,competition among the neighboring trees can directly influence the individual tree development because the level of competition severity on the trees sides facing each other and side facing less/no competition can affect growth pattern.The objectives of our study were to examine the impact of competition?planting density and spacing?on the 1)growth,biomass production,canopy dynamics and nutrient distribution,2)root development and its spatial distribution,and 3)impact of planting density on soil quality and soil bacterial diversity.Due to continuous increase in area under Chinese fir plantations and higher demand of Chinese fir timber,the study on this topic is of great importance.According to our research objectives,low?1450 tress hm^-22 with planting spacing of 2.36×2.36 m?,intermediate?2460 trees hm^-22 stems with 1.83×1.83 m?and high density?3950 trees hm^-22 with planting spacing of 1.44×1.44 m?Chinese fir plantation stands were selected in Xinkou Chinese fir forest plantations in Sanming city,China.Main results were as following:?1?Diameter of trees increased with decreasing planting density while tree height was independent of planting density.Biomass production of individual trees was significantly?p<0.05?less in high-density stand(32.35±2.98 kg·tree^-1)compared to low-density(54.72±4.96 kg·tree^-1)and intermediate-density stand(61.35±4.78 kg·tree^-1)while stand biomass production differed significantly in the order of intermediate(67.63±5.14 t·hm^-2)>high(57.08±3.13 t·hm^-2)>low(37.39±3.42t·hm^-2)stand density.?2?Analysis of spatial distribution of root systems revealed no overlap between roots of neighboring trees in the competition zone in low density stand,a subtle overlap in the intermediate density stand and larger overlap in the high density stand.Both average root length and root volume were significantly?p<0.05?lower in the high-density stand than stands with low and intermediate density.?3?Planting density had a significant effect on nutrient status and use efficiency of Chinese fir plantation.The contents of N,P,and K in different organs were as follows:leaf>branch>bark>root>dead branch>stem.As the planting density increases,the accumulation of nitrogen and phosphorus increased and reached a maximum in intermediate density stand,at that point further increase in planting density resulted in reduced nutrient accumulation and uptake.Accumulation of potassium was high in intermediate density stand compared to other stands.Total accumulation of N,P and K in the intermediate density stands was significantly higher(640.43 kg·hm^-2)than that of the low density stands,but not significantly different to high density stand(633.32 kg·hm^-2).In both whole tree and stem wood estimates,nitrogen and potassium use efficiency decreased as the plantation density increased while the phosphorus use efficiency was higher in intermediate density stand than other stands.?4?Planting density affected nitrogen?N?,phosphorous?P?,and magnesium?Mg?content in the soil,whereas potassium?K?and calcium?Ca?were not affected.Soil organic matter was significantly higher in the intermediate density stand than in the high-and low-density stands.Soil bulk density increased as the soil depth increased and soil pH differed significantly between soils of different stand densities but remained within an optimum range?4.1-4.5?for Chinese fir plantations.There was no difference in the number of soil bacterial community in 0-20 soil layer among all stand densities while in 20-40 cm layer soil bacterial community was highest in high density stand while lowest in the intermediate density stand.This indicates that the abundance of soil bacterial community at 0-20 cm is similar among all three different stand densities,but significantly higher than 20-40 cm.The common species in the 0-20 cm soil layer was more than in 20-40 cm.Soil bacterial richness decreased from low density to high density by up to 10%while in 20-40cm soil layer it was the opposite;i.e.,the highest in the high density stand.?5?Planting density was the primal factor responsible in canopy structure and a strong positive relationship was observed between leaf area index?LAI?and planting density?R²=0.872?as compare to LAI and leaf nitrogen?R²=0.510?,while soil fertility seems to play a secondary role for canopy dynamics.?6?It can be concluded that biomass production was significantly higher in stands with intermediate density than in stands with low or high density.The spatial distribution of the root system suggest lack of or minimal competition between trees in stands with low and intermediate density.Planting density was the crucial factor responsible in altering canopy growth while soil fertility seems to play a secondary role in canopy dynamics in stands varying in planting density.Overall planting density had a little effect on the diversity and dominance of soil bacteria in Chinese fir monoculture plantations.Better growth and biomass production in intermediate density stand could be attributed to better root structural development coupled with the minimal competition with understory vegetation and between trees;thus can be optimal for sustaining long-term productivity and may reduce the management cost in the early phase of the plantation.