These are the variables, both current and lagged, which are beyond the control of the smallholder. Those of particular interest in the
current study are age of trees, soil type, topography and climatic factors. 3.5.1 Age of Trees
Once the trees are planted, the age of the trees is beyond the control of the smallholder, except insofar as he may decide to replant or crown bud the particular block. This is unlikely to be an economic decision during the most productive phase of the trees (about years 2-12 of tapping), and is not a possibility considered here. Trees of different ages respond differently to various input factors and yield differently. Generally, rubber yield can be expected to rise during the early years of tapping, remain relatively stable for a short period, and then begin to decline rather sharply. Sometimes the age of rubber trees within a holding is different. This is the case where different portions of the holding
have been planted in different years, which usually occurs when a holding is too big for the owner to complete establishment during a single year. But in parcels of about one to two hectares, the age of the trees usually does not differ significantly.
There was no information on the difference in the age of rubber trees within a holding in this survey. For further analysis, it is assumed that there was no difference in the age of mature trees within each individual parcel.
In this study, the number of years in tapping (X^) and the number of years since planting used as independent variables which may affect rubber yields. The average values of these inputs can be seen in Table 1.2.
3.5.2 Soil Type
Different soil types will affect the level of nutrients available to the trees and also will affect the ability of the trees to absorb
fertilizer. Soils with different chemical and physical properties have different fertilizer requirements both in terms of quantity and type. For the purposes of this study a soil sample of each farm observed was taken, and soil fertility was classified as good, average and below average. From the soil analysis results, there are no significantly different soil types among farm samples within each region. Differences between regions are hard to quantify.
Since some differences were still suspected, however, in this study soil fertility (X^^) is used as a dLimmy variable. This was determined by the interviewer based on his observation on the field and the interview and also based on the interviewer's technical knowledge and experience. This variable is given a value one corresponding to all farms which have a good soil fertility and zero otherwise.
3.5.3 Topography
The effect of the topography of the land could change the number of trees per hectare in an area and also reduce the speed and ease of movement of the tapper during harvesting.
Teo (1976) pointed out that steep topography may reduce the number of trees per hectare. Lim (1976) has suggested that smallholders are likely to carry out only minimum maintenance if their holdings are hilly.
Sepien (1978) studied the variance analysis results and showed that
differences among the average yield on farms of various topography are not significant.
For the purpose of this study, holdings observed were grouped into three topographical catagories: (1) flat slope; (2) undulating; and (3) hilly. The grouping was simply based on a visual inspection of the holdings.
From the Table 3.2 it can bo seen that most of the farms studied belong to the first and second topographical categories. It is assumed that
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3 . 5 . 4 C l i m a t i c F a c t o r s
C l i m a t i c c o n d i t i o n s h a v e a s u b s t a n t i a l e f f e c t on y i e l d s . R a i n f a l l i n f l u e n c e s the q u a l i t y a n d q u a n t i t y o f r u b b e r p r o d u c e d , the loss o f
t a p p i n g d a y s d u r i n g t h e y e a r , l a t e t a p p i n g , e a r l y c o l l e c t i o n o r a t o t a l l o s s o f a d a y ' s c r o p d u e to i n a b i l i t y to c o l l e c t the l a t e x as a r e s u l t o f h e a v y r a i n . H i g h r a i n f a l l t e n d s to i n c r e a s e t h e l a t e x flows c o m p a r e d to low r a i n f a l l . R a i n f a l l f i g u r e s and m o n t h l y m a p s o f S o u t h S u m a t r a are p r e s e n t e d in T a b l e 2 . 1 . T h e r e w e r e n o r a i n f a l l r e c o r d s a v a i l a b l e from a l l t h e h o l d i n g s in e a c h r e g i o n . It is a s s u m e d t h a t r a i n f a l l d i d n o t d i f f e r s i g n i f i c a n t l y b e t w e e n r e g i o n s . W i n d d a m a g e is a n o t h e r i m p o r t a n t f a c t o r in a r e a s s u b j e c t t o r e g u l a r s t r o n g w i n d s and s t o r m s . A h i g h w i n d v e l o c i t y c a n d a m a g e b r a n c h e s and t r u n k s of r u b b e r t r e e s in t h e h o l d i n g s ; the n u m b e r of t r e e s a v a i l a b l e for t a p p i n g is c o n s e q u e n t l y r e d u c e d and r u b b e r y i e l d is d e c r e a s e d . It is a s s u m e d t h a t a l l t h e h o l d i n g s i n v e s t i g a t e d a r e e x p o s e d to the s a m e g e n e r a l w i n d c o n d i t i o n s . R a c h m a n ( 1 9 7 8 ) , b a s e d on the c l i m a t i c d a t a h e c o l l e c t e d , c o n c l u d e d t h a t in g e n e r a l the p r o v i n c e o f S o u t h S u m a t r a h a s a f a v o u r a b l e c l i m a t e for r u b b e r . A l l the h o l d i n g s s t u d i e d a r e a s s u m e d to h a v e s i m i l a r c l i m a t i c c o n d i t i o n s .
V A R I A B L E A V E R A G E S A N D C O R R E L A T I O N S 4 . 1 V a r i a b l e A v e r a g e s A n a n a l y s i s o f d a t a from s a m p l e f a r m s in K a b u p a t e n L I O T a n d K a b u p a t e n M U R A (Table 1.2) s h o w s t h a t a v e r a g e v a l u e s o f m o s t v a r i a b l e s i n c l u d e d in t h e p r e s e n t s t u d y w e r e s i g n i f i c a n t l y d i f f e r e n t b e t w e e n t h o s e t w o r e g i o n s . B a s e d o n t h i s r e s u l t , i t c o u l d b e e x p e c t e d t h a t t h e l e v e l o f the i n d e p e n d e n t v a r i a b l e s a f f e c t i n g t h e r u b b e r y i e l d s o f b o t h r e g i o n s w o u l d b e d i f f e r e n t . T h e s e d i f f e r e n c e s in a v e r a g e v a l u e s w e r e d i s c u s s e d e a r l i e r in C h a p t e r 2. 4.2 S i m p l e C o r r e l a t i o n s B e t w e e n V a r i a b l e s S i m p l e c o r r e l a t i o n b e t w e e n t h e v a r i a b l e s i n c l u d e d in t h e p r o d u c t i o n f u n c t i o n p r o v i d e s a n i m m e d i a t e p i c t u r e of the r e l a t i o n s h i p b e t w e e n t h e m . T h e c o r r e l a t i o n c o e f f i c i e n t i n d i c a t e s t h e d e g r e e t o w h i c h v a r i a t i o n in o n e v a r i a b l e is a s s o c i a t e d w i t h the v a r i a t i o n in a n o t h e r . S i g n i f i c a n t c o r r e l a t i o n b e t w e e n t w o v a r i a b l e s d o e s n o t n e c e s s a r i l y i m p l y t h a t o n e is c a s u a l l y r e l a t e d to the o t h e r , two v a r i a b l e s m a y m o v e t o g e t h e r b e c a u s e a t h i r d v a r i a b l e i n f l u e n c e s b o t h . O n the o t h e r h a n d , l a c k of c o r r e l a t i o n d o e s n o t n e c e s s a r i l y m e a n t h a t v a r i a b l e s are n o t a s s o c i a t e d w i t h e a c h o t h e r . T h e a s s o c i a t i o n m a y b e n o n - l i n e a r or m a r k e d b y v a r i a t i o n s in o t h e r v a r i a b l e s . T o c l a r i f y t h e p o s i t i o n , a p a r t i a l c o r r e l a t i o n is o f t e n c a l c u l a t e d . A p a r t i a l c o r r e l a t i o n c o e f f i c i e n t m e a s u r e s the r e l a t i o n s h i p b e t w e e n any two v a r i a b l e s , w h e n a l l o t h e r v a r i a b l e s c o n n e c t e d w i t h t h o s e two a r e k e p t c o n s t a n t . T h e p r i m e i n t e r e s t in t h i s s e c t i o n is t o e x p l a i n the s i g n i f i c a n t c o r r e l a t i o n b e t w e e n v a r i a b l e s w h i c h are l a t e r e x a m i n e d in t h e p r o d u c t i o n f u n c t i o n a n a l y s i s . T a b l e 4 . 1 s h o w s the r e s u l t s o f t h e s i m p l e c o r r e l a t i o n m a t r i x o f d e p e n d e n t v a r i a b l e and a l l the i n d e p e n d e n t v a r i a b l e s s e l e c t e d in t h e p r o d u c t i o n f u n c t i o n of K a b u p a t e n M U R A . T h e s i m p l e c o r r e l a t i o n m a t r i x
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between variables of Kabupaten LIOT is presented in Appendix Table 4.2. The specification discussions of all these variables h^ive been presented in Chapter 3. Partial correlation coefficients were not calculated owing to lack of time.
Table 4.1 shows that there is a strong positive correlation between yield (Y) and number of trees in tapping (X^) , (ryj^j^ = 0.4515*) and also between (Y) and total trees / (^yX14 0.3471***). The number of total
trees is highly correlated with number of trees in tapping (r = 0.8333*) X1a14
which, in turn, is also highly correlated with harvesting labour (X^),