Forest Stand Dynamics and Ecological Factors in Relation to Dwarf

Dwarf mistletoes are markedly host specific, perennial, obligate parasites. The success of mistletoe populations is tied not only to the suitability of the environment, but also to the availability and conditions of the hosts they infect. Thus, the dynamics of forest stand development and change exert major influences on the capacity of dwarf mistletoes to spread, intensify, and cause damage to host trees. Silvicultural control of dwarf mistletoe damage rests mainly with manipulation of stand dynamics to minimize spread and intensification. Thorough understanding of dwarf mistletoe/forest stand interactions is therefore crucial to the development and application of controls. In North America there are some two dozen species of dwarf mistletoes associated with numerous host species growing in a variety of forest and habitat types distributed over different terrains under widely divergent climatic conditions. Detailed treatment of stand dynamics in each of these associations is beyond the scope of this discussion. Rather, I have attempted to highlight some stand and ecological relationships that appear to be general to dwarf mistletoes, to indicate how these relationships provide bases for dwarf mistletoe control, and to suggest where increased information could inprove control application. The literature on dwarf mistletoes is voluminous (Scharpf, et al. 1976). Citations were selected to confirm or to illustrate principles, but no attempt was made to cite all papers dealing with the material discussed here. DAMAGE, IMPACT, AND DWARF MISTLETOE POPULATIONS Land managers are concerned about dwarf mistletoes because mistletoes can reduce the Ñ~resente at Symposium on Dwarf Mistletoe Control Through Forest Management, Berkeley, Calif. April 11-13, 1978. Ñ'~rofessor Dept. of Plant Pathology, Univ. of Calif. Berkeley, California. number, size, quality, or longevity of trees, and such reductions can affect the productivity and value of trees or stands. Unfortunately, a standard terminology to express disease effects has not evolved. Such terms as effect, damage, loss, and impact are often used interchangeably to designate mistletoeinduced changes in trees or stands. It has been suggested that the term impact be restricted to "...the cumulative net effects of a given pest or pest complex on the productivity, usefulness and value of a tree species or forest type with respect to different resource uses and values (timber production, watershed protection, wildlife cover, recreation, etc.) and the management objectives . . . " ('Vaters, 1976). By this definition, various types of damage (growth reduction, quality reduction, mortality, etc.) may or may not have impact, depending upon whether such damage reduces the productivity or value of a stand. Investigators have seldom dealt with impact. Most available data involve various measures of damage either to individual trees or to stands. The following discussion therefore deals mainly with damage, with the recognition that land managers must ultimately convert damage data to impact data and that this conversion is dependent on the amount of damage to individual trees and the numbers and distribution of damaged trees in a stand. These data in turn are dependent on the rates at which mistletoes spread through stands and i n t e n s i f y w i t h i n t r e e s . Genera l ly , t h e amount o f damage is r e l a t e d t o t h e numbers o f i n f e c t i o n s and t h e i r d i s t r i b u t i o n w i t h i n t r e e crowns (Baranyay 1970; Baranyay and Saf rany ik 1970; Chi lds and Wilcox 1966; Dooling 1974; Dooling and Brown 1976; Hawksworth 1961a; Hawksworth and Lusher 1956; Kors t i an and Long 1922; P i e r c e 1960; Shea 1963; Shea and O r r 1963; Shea and Bel lus c h i 1965; Smith 1969) . A number o f systems have been dev i sed t o r a t e i n t e n s i t i e s o f dwarf m i s t l e t o e i n f e c t i o n (Hawksworth 1977; Dooling, t h i s v o l . ) and t o c o r r e l a t e damage w i t h i n t e n s i t i e s i n i n d i v i d u a l t r e e s o r wi th g e n e r a l c o n d i t i o n s o f s t a n d s . Damage on a s t a n d b a s i s i s u s u a l l y e s t i mated by comparing volumes, b a s a l a r e a s , o r m o r t a l i t y r a t e s i n s t a n d s without m i s t l e t o e t o t h a t i n s t a n d s wi th v a r i o u s amounts o f m i s t l e t o e . While a b s o l u t e volume, b a s a l a r e a o r m o r t a l i t y may d i f f e r wi th d i f f e r e n t h o s t / m i s t l e t o e combinat ions and wi th a v a r i e t y o f s i t e and s t a n d f a c t o r s , t h e a s s o c i a t i o n o f i n c r e a s i n g l o s s wi th i n c r e a s i n g m i s t l e t o e i n t e n s i t y i s w e l l e s t a b l i s h e d (Andrews and Danie l s 1960; Baranyay 1970; Graham 1960a; Graham and F r a z i e r 1962; Haglund and Dooling 1972; Hawksworth 1958a, 1968a; Hawksworth and Hinds 1964; Hawksworth and Lusher 1956; L i g h t l e 1966; Richardson and van d e r Kamp 1972; Wagener 1961) . In g e n e r a l , s t a n d l o s s e s below some c r i t i c a l l e v e l o f m i s t l e t o e i n t e n s i t y a r e n e g l i g i b l e (Baranyay 1970; Baranyay and S a f r a n y i k 1970; Dooling 1974; Richardson and van d e r Kamp 1972; Shea and Orr 1963) . Th i s l e v e l i s a f u n c t i o n o f numb e r s o f i n f e c t e d t r e e s i n a s t a n d , t h e i n t e n s i t i e s o f i n f e c t i o n i n t h e s e t r e e s , and t h e amount o f damage r e s u l t i n g from d i f f e r e n t i n t e n s i t i e s . Whether damage i s expressed a s decreased r a d i a l , h e i g h t , o r volume growth o r a s i n c r e a s e d m o r t a l i t y , s i g n i f i c a n t damage i s s e l dom observed u n t i l a t l e a s t t h e lower 1/3-1/2 o f a t r e e crown i s h e a v i l y i n f e c t e d (Baranyay 1970; Baranyay and Saf rany ik 1970; Dooling 1974; Dooling and Brown 1976; Hawksworth 1961a; Hawksworth and Lusher 1956; P i e r c e 1960; Shea 1963; Shea and O r r 1963; Shea and B e l l u s c h i 1965; Smith 1969) . Light i n f e c t i o n throughout t h e crown o r heavy i n f e c t i o n i n t h e lower crown u s u a l l y cause l i t t l e o r no damage, excep t where stem i n f e c t i o n s may l e a d t o h e a r t r o t (E the r idge 19731, where stem i n f e c t i o n s occur i n s e e d l i n g s and smal l sapl i n g s (Knutson and Toevs 1972; Roth 1971; Weir 1918) , o r pe rhaps where l a r g e brooms a r e produced. The e f f e c t s o f brooms on h o s t growth has n o t been i n v e s t i g a t e d thorough ly . P i e r c e (1960) sugges ted t h a t t h e growth o f t r e e s wi th brooms i n t h e lower crown was comparable t o t r e e s wi th h e a l t h y crowns o f a l e n g t h equal t o t h e unbroomed p o r t i o n o f i n f e c t e d t r e e s . Hawksworth ( l 9 6 l a ) , however, showed t h a t growth r e d u c t i o n was g r e a t e r i n broomed t r e e s than i n non-broomed t r e e s o f t h e same i n f e c t i o n c l a s s . F u r t h e r , L i g h t l e and Hawksworth (1973) i l l u s t r a t e d ramat ic improvement i n h o s t v i g o r fo l lowing p run ing o f brooms from t h e lower crown. S ince t h e numbers o f brooms i n c r e a s e wi th i n c r e a s i n g i n t e n s i t i e s o f i n f e c t i o n (Hawskworth 1961a) , t h e amount o f damage t o t r e e s can be expected t o i n c r e a s e wi th i n c r e a s i n g l e v e l s o f i n f e c t i o n . Rates o f i n c r e a s e i n dwarf m i s t l e t o e popu la t ion l e v e l s undoubtedly a r e a f f e c t e d by a number o f h o s t , s t a n d , and environmental f a c t o r s . Few d a t a a r e a v a i l a b l e on measured r a t e s o f i n c r e a s e . Hawksworth (1969) found t h a t t h e numbers o f i n f e c t i o n s on i n o c u l a t e d d i g g e r p i n e s i n c r e a s e d from 4 t o 240 p e r t r e e w i t h i n 16 y e a r s a f t e r i n o c u l a t i o n , i n d i c a t i n g a popu la t ion doub l ing r a t e o f about 2 .5 years . Doubling r a t e s were e s t i m a t e d t o be 1 .25 y e a r s i n lodgepole p i n e (Muir 1972) and 4 y e a r s i n hemlock (Richardson and van d e r Kamp 1972) . Scharpf and Parmeter (1976) found t h a t popu la t ion i n c r e a s e s i n i n o c u l a t e d r e d and whi te f i r s were h i g h l y v a r i a b l e . In two groups o f t r e e s , p o p u l a t i o n s doubled w i t h i n 12-15 y e a r s from i n o c u l a t i o n ; i n a t h i r d group t h e popu la t ion t r i p l e d ; i n a f o u r t h group t h e popu la t ion i n c r e a s e d 35 t imes (a doub l ing r a t e o f about 3 y e a r s ) , and i n one group t h e popu la t ion s c a r c e l y i n c r e a s e d . These extreme v a r i a t i o n s a r e p r e s e n t l y unexp l a i n e d , bu t t h e y sugges t t h a t t h e c o l l e c t i o n o f popu la t ion d a t a i n one a r e a may have l i m i t e d re levance t o o t h e r a r e a s . General ized e s t i m a t e s o f popu la t ion i n c r e a s e r a t e s have been made by r a t i n g t h e m i s t l e t o e i n t e n s i t y accord ing t o t h e system o f Hawksworth (1977) i n s t a n d s o f d i f f e r e n t ages o r i n s t a n d s i n f e c t e d f o r d i f f e r e n t p e r iods o f t ime . Hawksworth and Hinds (1964) es t ima ted t h a t t h e average r a t i n g (on a 0-6 s c a l e f i n lodgepole p i n e s t a n d s i n c r e a s e d 0.80.9 p e r decade f o r t h e f i r s t 4 decades o f i n f e c t i o n , wi th i n c r e a s e s o f 0 . 6 and 0 . 3 f o r t h e n e x t 2 decades . Th i s i n d i c a t e s an average i n c r e a s e o f 1 c l a s s each 14 y e a r s . F lo ra (1966) i n d i c a t e d t h a t i t t a k e s about 70 y e a r s f o r ponderosa p i n e s t o go from a 1 . 5 t o a 5 .5 r a t i n g , an i n c r e a s e o f 1 c l a s s each 17 .5 y e a r s . Wass (1976) s u g g e s t s t h a t s h o r e p i n e i n f e c t i o n i n t e n s i t i e s i n c r e a s e d by 1 c l a s s each 15 y e a r s . Myers, et_g.(1976) developed an equa t ion f o r c a l c u l a t i n g i n c r e a s e s i n i n f e c t i o n i n t e n s i t y f o r m i s t l e t o e i n southwestern ponderosa p i n e . By t h i s equa t ion , r a t e s o f i n c r e a s e vary accord ing t o t ime o f i n f e c t i o n , s t a n d d e n s i t y , and s i t e index. I t i s obvious t h a t equa t ions o r e s t i m a t e s f o r p o p u l a t i o n o r r a t i n g i n c r e a s e s can apply on ly t o s p e c i f i e d t ime p e r i o d s , h o s t / m i s t l e t o e combinat ions, and s t a n d / s i t e c o n d i t i o n s . With e s t i m a t e d doubling t imes o f 1.25-4 y e a r s , dwarf m i s t l e t o e popula t ions would reach i m p o s s i b l e numbers w i t h i n normal r o t a t i o n per i o d s f o r most h o s t s p e c i e s . On a s t a n d b a s i s , i f i n f e c t i o n r a t i n g s increased by 1 every 15 y e a r s (a l lowing 10 y e a r s f o r e s t a b l i s h m e n t ) , young i n f e c t e d s t a n d s would reach damaging 3-4 l e v e l s o f i n f e c t i o n w i t h i n 55-70 y e a r s , and we would be hard pressed t o j u s t i f y r o t a t i o n s beyond t h i s t ime. However, f o r each 5-year i n c r e a s e i n t h e number o f y e a r s i t t a k e s f o r r a t i n g s t o i n c r e a s e by 1, t h e r o t a t i o n i s extended by 30 y e a r s (F ig . 1 ) . Whethe r i n c r e a s e r a t e s a r e s t r a i g h t l i n e f u n c t i o n s o r whether t h e y change with s t a n d age and d u r a t i o n o f i n f e c t i o n ( a s t h e d a t a o f Hawksworth and Hinds, 1964, s u g g e s t ) , r educ t ion i n t h e r a t e o f i n c r e a s e may be c r i t i c a l t o sound management. Figure I--Ages a t which s t a n d dwarf m i s t l e t o e r a t i n g s would reach l e v e l s o f 3 o r more wi th i n f e c t i o n r a t i n g s i n c r e a s i n g by l e v e r y 10 y r (A), 15 y r @ I , 20 y r (C), 25 y r (Dl, o r 30 y r (E) , assuming i n f e c t i o n i s e s t a b l i s h e d a t about 10 y r . F o r t u n a t e l y , r a t e s o f dwarf m i s t l e t o e s p r e a d and i n c r e a s e a r e s u b j e c t t o a v a r i e t y o f l i m i t a t i o n s t h a t can be manipulated by land managers o r t h a t he can t a k e advantage o f . These involve t h e i n f l u e n c e s o f : (1) s i t e q u a l i t y and h o s t v i g o r , (2) s t a n d d e n s i t y , (3) h o s t age, (4) s t a n d s t r u c t u r e , and (5) s t a n d composition. These v a r i o u s elements o f s t a n d dynamics a r e i n e x t r i c a b l y interwoven and d i f f i c u l t t o d i s c u s s i n i s o l a t i o n . For convenience, however, t h e y w i l l be d i s c u s s e d i n d i v i d u a l l y . ELEMENTS OF STAND DYNAMICS AFFECTING SPREAD, INTENSIFICATION, AND DAMAGE S i t e Q u a l i t y and Host Vigor Reports o f t h e r e l a t i o n s h i p o f dwarf m i s t l e t o e preva lence , i n t e n s i t y o r damage t o s i t e q u a l i t y a r e d i f f i c u l t t o i n t e r p r e t . In ponderosa p i n e , increased m i s t l e t o e h a s been a s s o c i a t e d with poor s i t e q u a l i t y (Daubenmire 1961, 1969; Korst ian and Long 1922), wi th i n t e r m e d i a t e s i t e q u a l i t y (Larson e t a l . 1970), o r showed no s i g n i f i c a n t r e l a t i o n s h i p t o s i t e (Childs and Edgren 1967; Hawksworth 1961a, 1961b, 1968a) . In lodgepole p i n e , Alexander (1975) r e p o r t e d h e a v i e r i n f e c t i o n on poor s i t e s , Hawksworth and Graham (1963) found a h i g h e r percen tage o f i n f e c t e d t r e e s i n reproduc t ion on good s i t e s , and Hadfield (1977) found no r e l a t i o n s h i p o f s i t e q u a l i t y t o per cen t o f t r e e s i n f e c t e d . Baranyay and Saf rany i k (1970) found g r e a t e r growth l o s s on d r y than on wet s i t e s . I t has been suggested t h a t because m i s t l e t o e s a f f e c t h o s t growth, h e a v i l y i n f e c t e d s t a n d s may g ive t h e appearance o f low s i t e q u a l i t y (Childs and Edgren 1967; Hawksworth l 9 6 l a ) . S ince s i t e q u a l i t y i s measured by t r e e growth, and growth may be a f f e c t e d by a number o f f a c t o r s ( s o i l type and dep th , n u t r i e n t s , a v a i l a b l e wate r , d ra inage , l eng th o f growing season, e t c . ) , it i s n o t s u r p r i s i n g t h a t d a t a a r e d i f f i c u l t t o i n t e r p r e t . While s i t e qua l i t y p e r s e might have some d i r e c t e f f e c t s on dwarf m i s t l e t o e a c t i v i t y , it i s l i k e l y t h a t fundamental h o s t / s t a n d / p a r a s i t e i n t e r a c t i o n s a f f e c t e d by s i t e f a c t o r s a r e more important i n determining m i s t l e t o e spread and i n t e n s i -