A SIMULATION MODEL TO PREDICT THE YIELD OF CLEARWDOD FROM RADIATA PINE

Introduction

Clearwood production in radiata pine depends mainly upon the diameter of the pruned log, and the diameter of its knotty core. The first stq? in predicting clearwood yields is thus prediction of

these characteristics of pruned logs.

The Prediction of Increment and Yield

A growth model to predict the yield and increment of radiata pine in N e w Zealand has been constructed b y Beekhuis (1966). The predominant mean height of the stand is used as an independent

variable, instead of age, from which to predict basal area and volume increment. Predominant mean height is defined as the mean height of the ^0 tallest trees per acre selected one in each 1/4O th of an acre.

I n the model, gross basal area increment is given b y the equation:

where: I,

B

= I h x ( (B + 62.0) / (H + 15.2) ) (5) - gross basal area increment (m /ha) during the growth

p e r i o d ,

~ increment in predominant me a n height (m) during the growth period,

- stand basal area (m /ha) at the commencement of the growth period

H = p re d o m in a n t mean h e ig h t (m) a t th e commencement o f th e g ro w th p e r i o d .

I n th e B eek h u is y i e l d m odel, m o r t a l i t y i n term s o f s to c k in g i s d e term in e d from r e l a t i v e sp acin g * R e l a t i v e s p a c in g r e f e r s to th e a v e ra g e d i s t a n c e betw een t r e e s e x p re s s e d as a p e r c e n ta g e o f th e p re d o m in a n t mean h e ig h t o f th e s t a n d . M o r t a l i t y f i r s t o c c u rs a t a r e l a t i v e s p a c in g o f JO As p re d o m in a n t mean h e ig h t in c r e a s e s o v e r tim e , m o r t a l i t y g r a d u a lly re d u c e s r e l a t i v e s p a c in g u n t i l a minimum v a lu e o f r e l a t i v e sp ac in g i s re a c h e d . T h e r e a f te r r e l a t i v e sp a c in g te n d s to re m a in c o n s t a n t , w ith f u r t h e r in c r e a s e s i n p re d o m in a n t mean h e ig h t o f f s e t b y r e d u c tio n s i n s t o c k i n g .

M o r t a l i t y tr e n d s i n te rm s o f th e number o f stem s w hich d i e , a re p r e s e n t e d g r a p h i c a l l y i n B eekhuis (1 9 6 6 ) , b u t G ra n t (1 9 7 0 ) p ro v id e d

a m ethod o f c a l c u l a t i n g m o r t a l i t y a l g e b r a i c a l l y u s in g s to c k in g and h e ig h t as in d e p e n d e n t v a r i a b l e s . G r a n t 's m ethod was u sed

th ro u g h o u t t h i s s tu d y . The m o r t a l i t y tr e n d s w ere d e r iv e d i n th e a b se n c e o f abnorm al d ro u g h t o r i n s e c t o r f u n g a l a t t a c k .

M o r t a l i t y i n term s o f lo s s o f b a s a l a r e a in c re m e n t o v e r th e gro w th p e r io d i s g iv e n b y th e f o llo w in g e q u a tio n (B e e k h u is , 1966) :

M = 0.000268 x D .B .H .2 x N (6 ) w here: M - b a s a l a r e a lo s s (m /h e ) due to m o r t a l i t y , D.B.H. - d ia m e te r b r e a s t h e ig h t o v e r b a rk (cm) o f th e t r e e o f mean b a s a l a r e a a t t h e commencement o f th e grow th p e r i o d , N = lo s s e s i n stem s p e r h e c t a r e d u rin g th e g ro w th p e r io d . H e n c e fo rth s ta n d mean d ia m e te r r e f e r s to th e d ia m e te r b r e a s t h e ig h t o v e r b a r k o f th e t r e e o f mean b a s a l a r e a .

35

N et b a s a l a r e a in c re m e n t d u rin g th e g ro w th p e r io d i s g iv e n b y th e fo llo w in g e q u a tio n (B e e k h u is , 1966) :

h

-

h ~ M

(?)

2

w here 1^ e q u a ls n e t b a s a l a r e a in c re m e n t (m / h a ) and 1^ and M a r e a s d e fin e d above*

T o t a l s ta n d volume p e r u n i t o f b a s a l a r e a i s g iv e n b y th e fo llo w in g e q u a tio n (B e e k h u is, 1

966

) :

V/fe = 0 .9 1 4 + 0 .3 x H (8 )

w here: V - t o t a l s ta n d volume (w? /h a) , 2

B - s ta n d b a s a l a r e a (m / h a ) , H - p re d o m in a n t mean h e i g h t (m ).

E q u a tio n s 6 , 7 & 8 have b e e n w id e ly u sed to p r e d i c t th e g ro w th and y i e l d o f r a d i a t a p in e i n New Z e a lan d ( e . g . F e n to n and S u tt o n , 1968; F e n to n and T u a tin , 1972; F e n to n , 1972 a ) , b u t th e m odel h a s some l i m i t a t i o n s f o r p r e d i c t i n g clearw ood y ie l d s on

v a rio u s s i t e s , as f o llo w s :

( i ) th e m odel i s a p r o d u c t o f a v erag e d t r e n d s , and may b e b i a s e d i f u sed to p r e d i c t grow th o n s p e c i f i c s i t e s

(W hyte, 1 9 7 0 ),

( i i ) th e e f f e c t o f p r u n in g on b a s a l a r e a in c re m en t i s n o t s p e c i f i c a l l y in c o r p o r a te d i n th e m odel. The b u lk o f

th e d a ta on w hich th e model was b a s e d was ta k e n from s ta n d s aged 11 y e a r s o r more (B e e k h u is, 1966) and i t i s u n l i k e l y t h a t th e s e s ta n d s would have r e c e iv e d much p r u n in g ,

( i i i ) h e ig h t- a g e c u rv e s f o r s i t e in d ic e s o th e r th a n 29 m w ere n o t g iv e n i n th e o r i g i n a l p u b l i c a t i o n ,

( i v ) th e model may b e u n r e l i a b l e i f u sed to p r e d i c t grow th i n s ta n d s w here s ta n d b a s a l a r e a ( i n f t i s l e s s th a n p red o m in an t mean h e ig h t ( i n f t . ) (B e e k h u is , 1 9 6 6 ), (v ) i n th e m o d el, th in n in g i s r e s t r i c t e d to o c c u r a t a p red o m in an t mean h e ig h t g r e a t e r th a n 7» 9 m ( 2 6 .0 f t ) , ( v i ) th e e f f e c t o f i n i t i a l s to c k in g on b a s a l a r e a in c re m e n t i s n o t in c o r p o r a te d i n th e m o d el.

I n t h i s s tu d y th e m odel was u sed t o examine b ro a d gro w th tr e n d s so th e f a c t t h a t th e model i s th e p r o d u c t o f a v e ra g e d tre n d s was c o n s id e re d to b e o f no s i g n i f i c a n c e . However, th e e f f e c t s o f i n i t i a l s to c k in g and p ru n in g o n b a s a l a r e a in c re m e n t a r e im p o r ta n t i n t h i s s t u d y , b e c a u s e b a s a l a r e a in c r o n e n t a f f e c t s th e d ia m e te r o f th e k n o t t y c o re and th e lo g s i z e a t r o t a t i o n a g e , w hich a r e

im p o rta n t d e te r m in a n ts o f clearw ood y i e l d . F u rth e rm o re , i t

a p p ea re d t h a t th e th in n in g s w hich w ould y i e l d m o st clearw o o d would o c c u r b e f o r e p re d o m in a n t mean h e ig h t re a c h e d 7*9 m, and th e

r e s u l t a n t b a s a l a re a s ( i n f t . / a c . ) w ould t h e r e f o r e p r o b a b ly b e l e s s th a n p re d o m in a n t mean h e ig h t ( i n f t . ) f o r a p o r t i o n o f th e l i f e o f th e s t a n d s . I t was a ls o c o n s id e re d d e s i r a b l e t h a t th e e f f e c t o f s i t e in d e x on clearw o o d y i e l d b e in c o r p o r a te d i n th e m odel.

F u r t h e r r e s e a r c h and developm ent was t h e r e f o r e u n d e rta k e n to e n a b le th e B eekhuis m odel to b e u sed to p r e d i c t clearw o o d y i e l d s i n s ta n d s w hich c o v ered a ra n g e o f i n i t i a l s t o c k i n g , p ru n in g and

37

H eight-A ge C u rv es f o r O th e r S i t e I n d ic e s

S i t e in d e x i s e x p re sse d as mean top h e i g h t a t age 20 y e a r s , B eekhuis (1 9 6 6 ) c o n sid e re d mean top h e ig h t to b e synonymous -with p red o m in an t mean h e ig h t f o r a l l p r a c t i c a l p u rp o s e s and t h i s

assu m p tio n was ad o p ted f o r t h i s s tu d y ,

A s e t o f anamorphic h e ig h t- a g e c u rv e s (C a rro n , 1968) was c o n s tr u c te d f o r s i t e in d ic e s ra n g in g from 24*38 m (8 0 ,0 f t , ) to 33*53 m ( 1 1 0 ,0 f t . ) u s in g th e h e ig h t- a g e c u rv e f o r a s i t e in d e x o f 29 m p u b lis h e d b y B eekhuis (

1966

) as th e g u id in g curve* T h is allo w ed th e e f f e c t o f s i t e in d e x on grow th to b e in c o r p o r a te d i n th e m odel.

The E f f e c t o f I n i t i a l S to c k in g o n B a s a l A rea In c re m e n t

B o th s ta n d h e ig h t and s to c k in g ap p ear l i k e l y to a f f e c t e a r l y b a s a l a r e a gro w th . D a ta from B eek h u is (1 9 6 6 ), W rig h t, G-. (1 9 7 0 ), F e n to n and T u s tin (1 9 7 2 ) , F e n to n (1972 a) and from

tem porary p l o t s e s t a b l i s h e d a t K a in g a ro a F o r e s t were u sed to

In document Pruning radiata pine in New Zealand to produce clearwood (Page 46-70)