MSc

A study on influence of planting density and pruning on thermal, radiative and moisture regimes and water use efficiency of Acacia mangium was conducted at Livestock Research Station, Thiruvizhamkunnu ( 11⁰ 12 N, 76⁰ 21 E) during the period 2014-15. The experiment was laid out in a factorial RBD with two factors (density and pruning) replicated thrice. Density treatments include 625, 1250 , 2500 and 5000 trees/ha, while pruning treatments included pruning up to 50 % height of trees and no pruning. Measurements on air temperature (AT), relative humidity (RH), photosynthetically active radiation (PAR) at 5 and 10 m above ground; soil moisture content (SMC) at 5, 10, 20 and 30 cm depth, soil temperature (ST) at 5, 10 and 20 cm depths, relative water content (RWC) and canopy air temperature difference (CATD) were taken on a weekly interval for one year. The weekly observations were then grouped into summer, pre-monsoon, monsoon and post monsoon season for analysis. Observations on DBH, tree height, LAI and PAR were taken once during the study period. Evapotranspiration, biomass and WUE were estimated using appropriate methodologies.
Diameter at breast height, tree height and individual tree biomass increased with decreasing density, while aboveground standing biomass on stand level and LAI followed a reverse trend. The unpruned stand had significantly higher DBH and biomass as compared to that of the pruned stand. Biometric characters had positive correlations with the AT, PAR at 5 and 10 m above the ground level, RWC, ST (5 , 10 and 20 cm) and SMC (20 and 30 cm depth), while it was negatively correlated with the evapotranspiration and SMC (5 and 10 cm depth). No significant correlation was found between CATD and RH (5 and 10 m above ground).
Air temperature (5 and 10 m above ground) and ST (at depths of 5, 10 and 20 cm) were negatively correlated to planting density and LAI. It was found that the largest differences with open area for these parameters were recorded at a density of 5000 trees/ha, throughout all seasons. It was found that there is an average difference of 2.62° C and 2.52° C between AT in the Acacia mangium stand compared to the open area at 5 m and 10 m above the ground level respectively.
Soil temperature (5, 10 and 20 cm depth) was negatively correlated with the SMC (5, 10, 20 and 30 cm depth). An average ST difference of 2.6° C, 2.6° C and 2.5 ° C was found between the Acacia mangium stand and open area at depth of 5, 10, 20 and 30 cm depth respectively. Pruning did not affect both the AT and ST except, ST at 5 cm depth during the post-monsoon season.CATD was positive and higher in high density treatments during the summer and pre-monsoon, while it remained negative and unaffected by density during the other seasons. CATD was negatively correlated to evapotranspiration, RWC, and SMC at various depths.
Influence of planting density on RH was evident only during the summer season. . It was found that there is an average difference of 7.51 % and 5.76 % in RH between the Acacia mangium stand and open area at 5 m and 10 m above the ground level respectively. The RWC, evapotranspiration and SMC (20 and 30 cm depth) were significantly higher in the low planting density, while reverse trend was noticed for SMC (5 cm and 10 cm depth). An average SMC difference of 4.50 %, 3.70 %, 3.32 % and 3.11 % was found between the Acacia mangium stand and open area at depth of 5, 10, 20 and 30 cm depth respectively. The RWC was significantly higher in the pruned stands during the pre-monsoon and post-monsoon season, while no differences were observed during the summer and monsoon season. Water use efficiency (WUE) of Acacia mangium stand was not significantly affected by the planting density and pruning treatments.
Radiation below the canopy was found negatively correlated to planting density and LAI, while it was not influenced by pruning. It was found that there is an average difference of 450 W/m2 and 466 W/m2 between the Acacia mangium stand and open area at 5 m and 10 m above the ground level respectively The average mid-day PAR transmittance from the unpruned stand (5000 trees/ha) to the pruned stand (625 trees/ha) ranged from 24.27 % to 54.72 % of incident PAR above the canopy. There was strong relationship between the PAR and LAI. The light extinction coefficient (K) for Acacia mangium stand was estimated to be 0.328.