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Indian J. of Agroforestry Vol. 20 No. 2 : 9-14 (2018)
Root distribution pattern of young Swietenia macrophylla King. stands in Central Kerala, India 1
2*
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Vikas Kumar , T.K. Kunhamu , V. Jamaludheen , A.V. Santhoshkumar and Asha K. Raj 1 Department of Agriculture, Vivekananda Global University, Jaipur–302 012, Rajasthan. 2 College of Forestry, Kerala Agricultural University, Thrissur-680 656, Kerala. * Corresponding author's E-mail:
[email protected]
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ABSTRACT: The major constraints to intercropping in woody ecosystems are belowground competitions for water and nutrients which are in short supply. The spatial spread of roots of the trees play major role in this context. The root distribution pattern was examined in a seven-years-old mahogany (Swietenia macrophylla King.) plantation established at 2.5 m × 2.5 m spacing by following logarithmic spiral trench technique. The root intensity steadily declined with increase in root diameter classes. The small roots (2.5 to5.0 mm). Significant variation in root intensity was observed for different soil depths and lateral distances sampled. The roots generally followed decline in intensity with increasing lateral distance from the tree as well as with increasing soil depth. The small root count at the proximal end of the trench (closest to the tree base; 0.39 m) was 2266 while the corresponding count at farthest lateral distance (2.99 m) was only 133. The shallow soil depth of 0-20 cm represented almost 41% of the total small root count. The soil zone lying within 2.17 m lateral distance from the base of the tree and at 50 cm soil depth accounted for almost 84% of the total roots for the S. macrophylla tree studied. This forms the zone of root activity for most of the field intercrops hence could lead to intensive competition with field crops. The observed root distribution for S. macrophylla at seven year age indicates that the probable safer spacing for effective intercropping would be nearest to 5.34 m between trees. Key words: Foraging zone, logarithmic spiral trench and root intensity. Received on: 25.04.2018 Accepted on: 14.09.2018
1. INTRODUCTION Swietenia macrophylla (big leaf mahogany) belonging to family Meliaceae is one of the valuable timber species due to its attractive colour and dimensional stability (Lamb, 1966). It has a widespread geographical range in the Neotropics, from Mexico through central America and across the southern Amazon of Bolivia and Brazil (Rodan et al., 1992). Mahogany was introduced to India from West Indies first to Royal Botanical Garden, Culcutta (India) in 1795 which was later introduced to Edacode, North Forest Division, Kerala in 1893 (Troup, 1921). In the recent times, mahogany has received wide acceptance among tree growers in Kerala due to its fast growth, remarkable wood qualities, durability, workability, better form and higher sawn out turn. It is cultivated in diverse fashion such as small wood lots, along farm boundaries and intimate multi-storey combination as in homegardens of Kerala. Belowground competition with the component crops for nutrients and water is the cardinal factor that limits productivity of agroforestry systems. Root production and its spatial distribution patterns among the component crops play major role in deciding the resource acquisition for their optimal productivity. Superficial root systems may reduce loss of nutrients by leaching and soil erosion while improving porosity, infiltration and aeration. However, competition with
associated food crops for the available nutrients and water in the topsoil is undesirable (Ball, 1985). Deep roots intercept leached nutrients and recycle them to the surface and may not cause competition or cultivation difficulties in tree-based cropping system. Hence, maintenance of differential zone of water and mineral absorption help to reduce interspecific competition especially when the resources are in short supply. However, our understanding on the belowground mechanisms of resource sharing especially in polyculture systems involving trees is very much limited. The methodological difficulties in assessing the root production and distribution often limit our search for such vital information. Such awareness may help to minimize the tree-crop interactions and thereby help attain optimal combined productivity. Most of the direct methods of root studies are destructive and laborious in nature. To tackle such obstacles, different methods have been developed for characterising the spatial distribution of root systems in soil (van Noordwijk et al., 2000).The logarithmic spiral trenching method is one such strategy which can give detailed information on overall spatial pattern of root distribution (Tomlinson et al., 1998). This nondestructive method involves partial excavation of the soil around the tree and counting the roots at various lateral distances from the tree at various soil depths. The present study aims to understand the root
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Root distribution pattern of young Swietenia macrophylla in Kerala
distribution of mahogany and explore the possibility of its integration with other crops in poly-culture scenario. 2. MATERIALS AND METHODS The study was conducted in a young mahogany plantation which was established during June 2009 in Vynthala, Chalakudy, Thrissur, Kerala (N 10015'33” latitude and E 76018'7” longitude and at an elevation of 39 m above sea level). The area experiences warm humid equable tropical climate with mean annual rainfall around 2400 mm most of which was contributed by the south-west monsoon. The top soil is rich in humus formed by decomposing litter and the soil is lateritic in origin (oxisol), well drained and loamy. The trees were planted at spacing 2.5 m × 2.5 m. Three trees with moderate growth were randomly selected for the root characterization studies. Care was taken not to disturb the root system of the adjoining trees. At five years of stand age the average height, girth at breast height (GBH), crown width and mean tree volume of the mahogany stand was 9.30 m, 26.01 cm, 3.97 m and 0.32 m3, respectively. Root distribution studies using logarithmic spiral trench technique Twelve trees were randomly selected for the root distribution study. The trees were selected in such a way that they were located in the corner locations of the stand so as to minimize the counting of cross feeding roots of adjacent trees. The root system of each selected trees were partially excavated using logarithmic spiral trenching technique (Huguet, 1973; Tomlinson et al., 1998). The crown radius of the selected tree was measured by projecting the crown edges to the ground. Root of the selected tree was partially excavated by trench whose dimensions were determined using the following formulae: x = 1.5 (d) r y = 1n d
[ ( )]/π
direction with ϴ taking value 00, 22.50 (π/8), 450 (π/4), 67.50 (3π/8), 900 (π/2), 112.50 (5π/8), 1350 (3π/4), 157.50 (7π/8) and 1800 (π) to obtain the seven coordinates of the inside trench: OA, OB, OC, OD, OE, OF and OG as shown in Figure 1. External side of the trench was fixed by stretching the co-ordinates for the internal side by 60 cm to give OA', OB', OC', OD', OE', OF', and OG' (Figure 1). The trench was dug to a depth of 60 cm and a breadth of 60 cm and care was taken so that the sides remain intact. Severed roots (living) on the internal and external trench walls were counted by placing a 60 cm × 50 cm quadrat (subdivided into 10 cm depth intervals). The quadrats were placed at successive points starting from the proximal end to distal end of the trench. Roots were classified into three categories viz., 5.0 mm diameter classes at the time of counting by placing the quadrats at fixed distances from the trunk. Root counts were converted into rooting intensity (number of roots per square meter) (Bohm, 1979). 3. RESULTS AND DISCUSSION Lateral root spread Table 1 shows the changes in root intensity for S. macrophylla for various root diameter categories viz. 2.5 to 5mm as function of lateral distances from the tree. In general, the root intensity declined with increasing lateral distance from the tree for all the three root diameter classes. The highest root intensity was observed in the proximal lateral distance (0.39 m) from the tree base for all the root size classes which was followed by a general reduction in the root count with increasing lateral distance from the base of the tree for all the root size classes including the total roots (sum of all the three root size classes). For example, the small root count at the proximal end of the trench (closest to the tree base; 0.39 m) was 2266.67 while the corresponding count at farthest lateral distance (2.99 m) was only 133 which imply that
z = xeyϴ where, d = tree diameter in m; r = the average of the crown radius at four cardinal points in m; x = distance of the starting point of the spiral from the tree in m; y = natural logarithm of the ratio of crown radius to the diameter of the tree divided by π; z = distance of any point on the spiral from the tree base in m, and ϴ = 00, 22.50, 450, 67.50, 900, 112.50, 1350, 157.50 and 1800. The trajectory of each trench (A) was laid down on the field using plastic ropes by calculating the distance 'x' on the north side from the tree which will be the origin and further extension is done in the spiral clockwise
F’
E’
G’
D’ E
C’
F
G
D
B’
C B
A’
A Fig. 1. Diagram to show the co-ordinates of the modified logarithmic spiral trench
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Indian J. of Agroforestry Vol. 20 No. 2 : 9-14 (2018)
small roots show drastic reduction with increasing lateral distribution from the tree. The root intensity also steadily declined with increase in root diameter classes. The total root intensity was highest for the 2.5 to 5.0 cm). Spatial distribution of roots suggested high rooting intensity towards the proximal distance of the tree (0.39 m and 1.01 m lateral distance). Again, consistent decline in root counts have observed with the lateral distances for all root size classes. At the farthest lateral distance (i.e. 2.99 m), rooting intensity was 5.24, 3.44 and 3.8% of the proximal root intensity (0.39 m) for the root classes2.5 to 5.0 mm root diameter, respectively. Rooting intensity at the soil zone in the vicinity of mid spacing between the trees was also found to be variable across size classes. For instance, rooting intensity at 1.34 m lateral distance was 50, 63 and 58% of the proximal rooting intensities (at 0.39 m) for the three root size classes. Consistent reduction in root count with increasing lateral distance from the tree is quiet logical though the rates of reduction may vary with tree species, age and management conditions (Kunhamu et al., 2010; Wasson et al., 2012; Bardgett et al., 2014; Zwicke et al., 2015). In the present study, the small root proportion at the distal end (2.99 m) was only 5.88% of the proximal end of the tree while the corresponding values for medium sized and large sized root were 2.73% and 13.08%, respectively. Higher per cent of coarse roots in the distal end suggest the potential of coarse roots to stretch the roots as compared to other
two categories for better anchorage and dimensional stability to the tree. The lateral root spread may considerably vary with tree species and management conditions. For instance, the lateral root spread for Bombax ceiba was 2.05 m while Acacia auriculiformis reported a wide root spread of 8.05 m (Das and Chaturvedi, 2003). Interestingly, Dendrocalamus strictus registered higher root spread of >10 m (Kumar and Takeuchi, 2009). Vertical distribution of roots Table 2 shows the vertical distribution pattern of various root size classes for mahogany. As observed for lateral growth trend, the downward growth of roots also showed gradual reduction with increasing soil depth for small roots and medium sized roots. However, our study revealed that there were reasonably higher root counts in all the soil depths up to 60 cm. Interestingly, coarse roots showed variable root count with an increase with increasing soil depth at shallow depths. Invariably the highest root concentration was in the superficial soil layer (0-10 cm) which was 1666 and 1240 numbers m-2,, respectively for small and medium roots, respectively. Despite such trends, reasonably good number of roots was found even at 50-60 soil depth for all the root size classes. For instance, at 50-60 cm soil depth, the small root retained almost 53% of the root count at 0-10 cm soil depth indicating that seven-years-old mahogany roots go deeper beyond 60 cm soil depth. Many tree species showed deeper rooting nature which in turn depend on the tree age and management conditions. For instance, the vertical root spread of four-year-old Gmelina arborea trees showed variable trends with coarse roots spread increased from 35 cm at 2 m × 2 m spacing to 75 cm at 2 m × 5 m spacing (Swamy et al., 2003).
Table 1. Root intensity as function of lateral distance from the tree for 7-years-old Swietenia macrophylla at Mala, Thrissur, Kerala Lateral distance (m) Root intensity (number m-2) in different root diameter classes (mm) 2.50 to 5.00
Total
0.39
2266.67
1460.00
113.33
3840
1.01
2293.33
1160.00
93.33
3546.66
1.34
1240.00
966.67
86.67
2293.34
1.92 2.17 2.55 2.99 Total
846.67 486.67 300.00 133.33 7566.67
660.00 320.00 106.67 40.00 4713.33
40.00 26.67 46.67 0.00 406.67
1546.67 833.34 453.34 173.33 12686.67
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Root distribution pattern of young Swietenia macrophylla in Kerala
Small root distribution Table 3 shows the lateral and vertical distribution of feeder small roots for seven-years-old S. macrophylla. The distribution of the root intensity suggested that small root (
