Root distribution pattern of the wild jack tree - Springer Link

Agroforestry Systems 35: 329-336, 1997. © 1997 Kluwer Academic Publishers. Printed in the Netherlands.

Short communication

Root distribution pattern of the wild jack tree (Artocarpus hirsutus Lamk.) as studied by 32p soil injection method V. JAMALUDHEEN 1, B. M. KUMAR l'*, P. A. WAHID 2 and N. V. KAMALAM 3 1College of Forestry and 3Radiotracer Laboratory, Kerala Agricultural University, Vellanikkara, Thrissur 680 654, India; 2 College of Agriculture, Padanakkad, Nileshwar 671 353, Kerala, India; * Author for correspondence Key words: multipurpose trees, 32p recovery, root competition Abstract. Root distribution of the wild jack tree (Artocarpus hirsutus) was determined by selective placement of 32p at various depths and lateral distances from the tree, in Kerala, India. In eight-and-a-half-year-oldtrees growing on a lateritic site, absorption of 32p from a lateral distance of 75 cm and 30 cm depth was much greater than from 150 and 225 cm lateral distance and 60 and 90 cm depth. Root activity declined with increasing depth and lateral distance. Most of the physiologically active roots were concentrated within a radius of 75 cm and 30 cm depth, although the tap root might reach even deeper. Possibly, surface accumulation of feeder roots may cause considerable overlap of the tree and crop root zones in intercropping situations. However, as the tree roots seldom extend beyond 2.25 m laterally from the trunk, the effect of overlapping root zones and the associated competitive effects may not be a serious problem for intercropping during the first few years (< 10 years after planting) of tree growth.

Introduction Wild jack (Artocarpus hirsutus Lamk.; Family: Moraceae) is a prominent multipurpose tree in homegardens and other mixed species systems of southern India. They occur either as scattered trees in the farm fields and/or as trees on farm boundaries (Kumar et al., 1994). Although the wild jack has not been considered a promising species for monocultural woodlots (Jamaludheen, 1994), this species shows impressive growth in polycultural systems. The tree also occurs naturally in the evergreen forests of Western Ghats (peninsular India). It tolerates shade, but thrives best under full sunlight (CSIR, 1985). Wild jack coppices well and produces root suckers. It yields small (compared to jack fruit, Artocarpus heterophyllus Lamk.) edible fruits and provides shade to sciophytic plants (Kumar et al., 1995). The wood is strong, but also has the advantage of being light in weight. Its calorific value is 5223 kcal kg -1 (CSIR, 1985). Despite being a major component of many agroforestry systems, it is a general concern for farmers that the wild jack tree will compete with the associated field/tree crops. Moreover, research reports abound on concentration of feeder roots of trees in the surface layers of soil (Jonsson et al., 1988; Sankar et al., 1988; Dhyani et al., 1990; Ruhigawa et al., 1992). We, there-

330 fore, hypothesised that root competition for nutrients between the tree and the associated field crops, is a cardinal production decreasing factor in integrated land use systems involving wild jack trees. Although root system morphology and distribution of physiologically active roots are determinants of the magnitude of below ground competition in mixed species systems, there are very few reports on these factors, apart from root competition, in polycultural systems involving the wild jack tree. A recent study in this direction was concerned with the evaluation of fractal branching models of roots in 18 multipurpose broad-leaved trees, including two Artocarpus species growing on an acid soil in Lampung, Indonesia (Van Noordwuk and Purnomosidhi, 1995). An attempt is made in the present study to characterise the distribution of active roots in wild jack, by using 32p, which will provide an insight into the problem of root competition between the tree and other components of mixed species systems.

Materials and methods

The study was conducted using a field experiment initiated in June 1985, involving wild jack tree and eight other fast growing multipurpose trees at the Livestock Research Station, Thiruvazhamkunnu, Kerala, India (see Jamaludheen and Kumar (1997) for a list of all species involved, and George et al. (1996) for a description of the climatological factors of the locality). The soil of the experimental site is of lateritic origin (Oxisol), with a pH of 6.7. The experiment involved trees planted at a distance of 2 x 2 m from each other in 20 x 20 m plots. As part of a biomass estimation process, trees in alternate-diagonal rows were thinned out during July 1993. Mean height and diameter (at breast height) of Artocarpus trees at the start of the 32p experiment (October 1993) were 5.5 m and 7.75 cm, respectively (tree age: eight years and six months).

Characterisation of Artocarpus root distribution To assess root distribution, the 32p soil injection technique was employed. This approach has proved to be an efficient method for obtaining root distribution data (Nye and Tinker, 1977; Vose, 1980) and has been used by many previous workers in plantation and other agricultural crops (IAEA, 1975; Wahid et al., 1989a; Wahid et al., 1989b). George et al. (1996) used it for characterising root competition in intercropping systems involving multipurpose trees. The method involved applying phosphorus-32 at specified lateral distances from the tree and at specified soil depths, and comparing the 32p activity recovered in the foliage. Twenty-seven experimental units involving single trees of uniform growth were selected for this purpose in October/November 1993.

331 Trees were selected in such a way that the minimum distance between two adjacent experimental units was 6 m (two planting spaces in between), which would preclude any significant root interference between them. Nine 32p treatments formed by combinations of three lateral distances (75, 150 and 225 cm) from the tree and three soil depths (30, 60 and 90 cm) were assigned to the 27 experimental units, in a randomised block design (replicated three times).

Soil injection of 32P F o r 32p soil injection, eight equi-distant holes were dug to the required depth,

according to the treatment, around each experimental tree along the circumference of a circle, whose radius was equal to the lateral distance, using a soil auger of 2 cm diameter (each experimental unit/tree was associated with a particular depth-lateral distance combination only). A PVC access tube was inserted into each soil hole leaving about 10 cm of the tube length above the soil surface, and the open end of the tube was closed with a plastic cap. Phosphorus-32 solution (4 ml per hole) at a carrier P level of 1000 mg 1-1 was dispensed into the access tube at the rate of 37 MBq (1.0 mCi) per tree on 23 November. 1993, using a device fabricated for the purpose (Wahid et al., 1988). After dispensing, the radioactivity adhering to the inner walls of the access tube was washed down with a jet of about 15 ml water. Carrier in the 32p solution was required to reduce the soil fixation of the applied radioactive label (IAEA, 1975). The radioactive label was applied in November, towards the end of north-east monsoon.

Leaf sampling and radioassay The most recently matured leaves were sampled from each treated tree for radioassay. Leaf samples were collected thrice: 15, 30 and 45 days after application of 32p. The samples were dried at 75 °C, wet digested ( H N O 3 and HC104) and the digests were radioassayed for 32p, employing the Cerenkov counting technique (Wahid et al., 1985). Count rates (counts per minute, cpm) were corrected for background and decay and subjected to log~0 transformation before statistical analysis (analysis of variance). Percentage root activity at a particular depth-lateral distance combination was computed from the equation: Root activity (%) 32p activity (cpm) recovered in the leaf for a particular depth and lateral distance x 100 Total cpm for all treatments

332

Results and discussion

32p uptake and root activity A b s o r p t i o n o f the a p p l i e d 32p l a b e l b y Artocarpus trees i n c r e a s e d with t i m e up to 45 d a y s after a p p l i c a t i o n (Table 1), a l t h o u g h the rate o f a c c u m u l a t i o n t e n d e d to d e c r e a s e with time. D i f f e r e n c e s in 32p r e c o v e r y b e t w e e n lateral dist a n c e s were s i g n i f i c a n t at all s t a g e s o f o b s e r v a t i o n (15, 30 and 45 d a y s after a p p l i c a t i o n ) . In g e n e r a l , 32p r e c o v e r y d e c l i n e d as lateral d i s t a n c e o f a p p l i c a tion i n c r e a s e d , with m a x i m u m r e c o v e r y f r o m p l a c e m e n t at 75 cm. R e c o v e r y o f 32p at 225 cm, t h o u g h low, i n d i c a t e d the p r e s e n c e o f p h y s i o l o g i c a l l y active roots at least 2.25 m from the trunk. E x c a v a t i o n studies for r o o t b i o m a s s e s t i m a t i o n c o n d u c t e d s i m u l t a n e o u s l y in the s a m e e x p e r i m e n t a l set up ( J a m a l u d h e e n , 1994) s u g g e s t e d that large c o a r s e r o o t s ( > 0 . 5 c m d i a m e t e r ) m a y e x t e n d o n l y u p t o 175 cm. S i g n i f i c a n t d i f f e r e n c e s w e r e also o b s e r v e d in the q u a n t i t y o f 32p a b s o r b e d f r o m v a r i o u s depths (Table 1). R e c o v e r y o f 32p in the l e a f d e c l i n e d with d e e p e r p l a c e m e n t o f the r a d i o a c t i v e label. It w a s g r e a t e s t f r o m a d e p t h o f 30 cm. H o w e v e r , the i n t e r a c t i o n b e t w e e n d i s t a n c e f r o m the trunk and d e p t h w a s n o t s t a t i s t i c a l l y significant.

Table 1. Radioactivity recovered from the leaves (log cpm g-~) of Artocarpus hirsutus trees as a function of lateral distances and depth of application in Kerala, India. Treatment

Lateral distance (cm) 75 150 225 P Depth (cm) 30 60 90 P

Days after application 15

30

45

1.400a (25.12) 0.942 b (8.75) 0.684 b (4.83)