MSc

The focus of forest management in India has moved from timber production to
conservation and eco-restoration. Hence, plantations are the viable scope for meeting
our ever increasing wood demand.
Effective stand management practices are
indispensable for deriving optimal productivity from these plantations. Thinning is
generally recognised as an important silvicultural tool for creating qualitative and
quantitative improvement in plantation productivity. However, its temporal effect
varies with species and thinning regime. In this context, the present study was
undertaken by destructive sampling of trees and assessing the aboveground and
belowground biomass, soil parameters, understorey PAR and wood physical properties
in a 20-year-old Acacia mangium stand that was subjected to stand thinning 13 years
ago.
The results indicated that the impact of stand thinning on radial growth, mean
tree basal area and mean tree volume was still persisting even 13 years after thinning.
The highest response was from two-third thinning intensity (533 trees ha-1) followed
by half thinned stand (800 trees ha-1). The tree level biomass followed the same pattern
with the maximum aboveground (663.67 ±121.03 kg tree-1) and belowground (225.20
±20.62 kg tree-1) biomass production recorded from the two-third thinning intensity.
However on stand basis, the effect of thinning on volume and biomass did not show
significant variation across treatments, probably due to the confounding effect of the
higher number of trees in the low density stands. This trend was visible in stand carbon
sequestration potential also. Even though two-third thinning intensity had maximum
carbon sequestration at tree level (405.62 kg C tree-1), the stand level carbon
sequestration did not vary with thinning treatments.
Long-term effects of thinning had no influence on soil bulk density, pH, and
available phosphorus and exchangeable potassium. However, all the thinning regimes
registered significantly higher soil carbon stock compared to contiguous treeless
control plot.
Total nitrogen content also followed the same trend. Understorey
Photosynthetically Active Radiation (PAR) availability observed in this study revealed
reduced availability across all thinning regime except for the heavily thinned stand
suggesting the possibility for intercropping.
Thinning also had an impact on the root spread on a long term basis. The
maximum spread of 2.84 m was recorded in the two-third thinning treatment which
was significantly higher compared to other thinning intensities. Characterization of
spatial root distribution pattern using logarithmic spiral trenching method suggested
restricted root spread for the unthinned stand, while better root spread was observed
with increased intensity of thinning. The results indicate that influence on thinning on
wood physical properties are not significant across treatments. The wood density of
20-year-old A. mangium was 570.50 (kg m-3) albeit, was higher compared to mangium
wood of relatively younger ages. Mean moisture content (37.51±3.1 %) and shrinkage
values were relatively very low compared to the values reported from previous studies.
Furthermore, the incidence of the soft core which was relatively less indicates better
seasoning of the harvested wood. Understanding the long-term effects of stand
management activities on productivity will help in taking better management decisions.
On the whole, the study revealed that, since long term effects of thinning were
significant for most of the productivity traits, thinning intensity regimes have a strong
influence/bearing on stand management objectives.