Effect of spacing and plant density on the growth of poplar (Populus deltoides) trees under agro-forestry system.

摘要

Poplar (Populus deltoides) AY-48 was planted at spacings of 3.7x6.1 m, 3.7x9.1 m and 3.7x12.1 m to give densities of 455, 305 and 230 trees per hectare, respectively. Arable crops, wheat and fodder maize were sown alternately under an agroforestry system at the research garden of the Punjab Forestry Research Institute, Pakistan, for 8 years. Poplar trees put significantly greater diameter (dbh) growth under the spacing of 3.7x12.1 m. Increment in dbh was highest during 3rd year of planting and later on tapered down with the increase in age. There was no significant difference in height gain and clean bole formation under all the spacing and densities. Crown development was significantly better in lower density (230 trees/ha.). Diameter growth increase with the increase in spacing, however at certain point it levels off and there is non-linear relationship between diameter growth and spacing. Crown development has a linear relationship with tree spacing and increases with the increase in spacing. Wood production was higher under closer spacing of 3.7x6.1 m with a tree density of 455 trees/ha. While it was lower under wider spacing 3.7x12.1 m. It was observed that wood volume contribution per tree was 31.4% more in lower density of 230 trees/ha (3.7x12.1 m.) as compared to higher density of 455 trees/ha (3.7x6.1 m) while the difference was only 9.6% under 305 trees/ha. Density (3.7x9.1 m.), indicating competition amongst trees for below ground resources (water and nutrients) at closer spacing. At the spacing of 3.7x12.1 m. and density of 230 trees/ha. 72.2% trees were found in bigger dbh class of 30.6-35.5 cm while only 23.2 and 27.3% trees were able to touch this dbh class under higher density of 455% 305 trees/ha at a spacing of 3.7x12.1 m. and 3.7x9.1 m, respectively.