PhD

Extensive field study was carried out to investigate the effect of fertilizer
treatment and intercrops on the growth and productivity of 4-year-old Swietenia
macrophylla King. The study was carried out in a S. macrophylla plantation that
was established at Mala, Thrissur during 2009 at a spacing of 2.5 x 2.5m. The
fertilizer cum intercropping trial was superimposed on the existing S. macrophylla
plantation in a split plot design with fertilizer levels as main plots and intercrops
as sub plots during two consecutive years (2014-2016). The various N, P, K
fertilizer combinations were viz. F1: 0:0:0; F2-50:25:25 (68:78:26 g per tree); F3-
100:50:50 (136:156:52 g per tree) and F4-150:75:75 (205:234:78 g per tree) kg
ha-1 year-1 N, P2O5 and K2O; equivalent to 0:0:0, 50:10.75:20.75, 100:21.5:41.5
and 150:32.25:62.25 kg ha-1 per year elemental N, P and K, respectively. The
fertilizers were applied to the mahagony trees at a basal ring of 50 cm radius just
after the pre-monsoon rains. The various intercrops selected were shade tolerant
ginger (Zingiber officinale Roscoe), wild turmeric (Curcuma aromatic Salisb) and
turmeric (Curcuma longa L.). There were total 16 combinations of treatments
with three replications (total 48 plots). The main plot size was 40 x 10 m and sub
plot 10 x 10 m.
The growth observation of the S. macrophylla tree after the fertilizer application
showed consistent increase with increasing fertilizer dosage. Tree growth in terms
of height, diameter, basal area and volume showed characteristic increase with
increase in fertilizer levels. For instance, the stand basal area increased from 8.69
(F1: unfertilized) to 14.87 m2 ha-1 (F4: heavily fertilized). Similarly the tree
volume also showed increase with fertilizer application. The difference in basal
area for the higher fertilizer regimes were on par (F3, 14.50 m2 ha-1 and F4, 14.87
m2 ha-1) suggesting F3 as the optimal fertilizer regime if basal area production is
the objective. However, the volume production was the highest under F4 regimes
(105.28 m3 ha-1) which was significantly different from F3 (93.31 m3 ha-1). Hence

tree management at F4 fertilizer regime would be ideal for optimal volume
production for S. macrophylla.
The biomass production results also showed positive response to applied
fertilizers. Total mean tree biomass production was in the order 52.24, 60.62,
64.32 and 83.62 kg per tree for F1, F2, F3 and F4 fertilizer dosage regime
respectively. The corresponding stand level biomass (per ha basis) was in the
order 83.59, 97.00, 102.91 and 133.80 Mg ha-1 for F1, F2, F3 and F4 fertilizer
dosage regime respectively. The highest fertilizer dosage plots showed almost 60
per cent increase in biomass production as compared to unfertilized control.
Among the biomass components, stemwood represented almost 50 % of the total
biomass production for all fertilizer regimes followed by roots which accounted
almost 18 % of total biomass production. Branchwood biomass represented
roughly 14-15 % of total biomass. Biomass accrual by the various components in
the decreasing order was: stemwood> roots> branch wood >leaves>twigs.
Mean tree and stand level carbon sequestration showed positive response to
fertilizer application for 6 year old S. macrophylla. The total mean tree carbon
stocks ranged from 29.1 kg (F1) to 46.66 (F4). The total carbon sequestration on
per hectare basis was 74.66, 57.24, 54.31, and 46.56 Mg ha-1 for fertilizer regimes
F4, F3, F2 and F1 respectively.Among the various biomass components,
stemwood accounted bulk of the biomass carbon which was roughly 50%
followed by roots (17 %).
Nutrient partitioning S. macrophylla suggests that in general, nitrogen and
potassium concentrations decreased in the order leaves > stem > branch > roots >
twigs for N, P and K. However, tissue phosphorus concentration followed the
order branches > leaves > roots > twigs > stem. Various components stored
considerable amount of nutrients in their biomass. The total N stock in the
standing biomass ranged from 0.428 (unfertilized control) to 0.716 Mg ha-1 (F4;
heavily fertilized). The stock of phosphorus in the biomass was 0.174 (unfertilized
control) to 0.223 Mg ha-1 (F4; heavily fertilized) while the corresponding stock

for potassium was 0.090 (unfertilized control) to 0.144 Mg ha-1 (F4; heavily
fertilized).
Root distribution studies using logarithmic spiral trench technique in 6-year-old S.
macrophylla showed increase in rooting intensity with fertilizer application for
total roots and root class <2.5 mm. Fine root (< 2.5 mm) represented
approximately 58 to 62 % of the total roots. Hence the increase in fine root count
in high fertilized plots suggest higher nutrient uptake and there by higher tree
growth for S. macrophylla. The present study showed the maximum foraging
zone for S. macrophylla was at rhizosphere volume of 2.17 m lateral distance and
40 cm soil depth. At the present stocking this leads to considerable overlapping of
the rhizosphere of S. macrophylla and intercrops and thereby limits the prospects
of intercropping. Hence the possible optimal spacing suggested for 6-year-old S.
macrophylla would be 5.5 m x 5.5 m for effective intercropping.
Effect of intercrop on the tree growth in all the fertilizer treatment plots suggested
non-significant response. Despite the overwhelming effect of fertilizer on tree
growth and yield, the presence of intercrop had only very modest influence on tree
growth. Interestingly some of the S. macrophylla tree growth variables were
marginally better in the intercropped plots suggesting possible complementary
interaction between the intercrop and trees for applied fertilizers. The intercrops
viz. ginger, wild turmeric and turmeric showed better growth in fertilized plots as
compared to unfertilized control. Also the biometric growth and rhizome
yieldswere higher in the treeless open control plots as compared to S. macrophylla
intercropped plots. Nevertheless the growth differences were lower in the heavily
fertilized plotsas compared to open control. The better growth and rhizome yields
during the second year for all the three intercrops was due to improvement in
understory light regimes consequent to uniform tree pruning.